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How to Measure Sunshine Duration?

How to measure sunshine duration? the Observatory started using a fully automatic CSD sunshine meter to record sunshine duration since 2005. The sunshine meter consists of three sensors. When sunlight is detected by the sensor, it will be transformed into electricity. Solar radiation can be calculated based on the generated voltage.

Since 1950s, the Observatory has been using conventional Campbell-Stokes sunshine recorder to record the sunshine duration at King's Park. Sunshine recorder essentially consists of a glass sphere mounted in a spherical bowl and a metallic groove which holds a record card. Sun's rays are refracted and focused sharply on the record card beneath the glass sphere, leaving burnt marks on the card. As the sun traverses, continuous burnt marks will appear on the card. Observers can measure the sunshine duration based on the length of the burnt marks.Although the design and operation of sunshine recorder are quite simple, it requires manual observations of burnt marks on record cards as well as manual change of cards. To streamline operations, the Observatory started using a fully automatic CSD sunshine meter to record sunshine duration since 2005. The sunshine meter consists of three sensors. When sunlight is detected by the sensor, it will be transformed into electricity. Solar radiation can be calculated based on the generated voltage. The sensor at the front, which is used for measuring global solar radiation, is not shaded and receives sunlight from all around. The sensors in the middle and to the rear are partly shaded. The purpose for the shading is to avoid direct sunshine for measurement of diffuse solar radiation. By using the values of global and diffuse solar radiation, the direct solar radiation can be computed and the sunshine duration can be determined according to the latest World Meteorological Organizations definition[1].

Sunshine and UV

March 2013

https://www.hko.gov.hk/en/education/weather/sunshine-and-uv/00118-how-to-measure-sunshine-duration.html

en

Solar Energy Resources in Hong Kong from a Climatological Point of View

Two major factors have to be considered when we evaluate solar energy resources available in Hong Kong - the geographical position, the temporal and seasonal variations of solar radiation.

Solar energy is one of the main renewable energy resources. Its application is also an important aspect in climate change mitigation. Along with the advances in science and technology, the use of solar energy in daily life (such as solar panels and solar water heaters) has gradually gained popular acceptance. According to a recent survey, Hong Kong people responded positively towards the increasing use of solar power [1]. The Hong Kong Observatory has also been using solar cells to power some of its automatic weather stations since the 1980s [2].The collection efficiency of solar energy has large seasonal and regional variations, depending on the local amount of solar radiation as well as other environmental factors. Located along the coast of southern China, Hong Kong has a relatively higher level of solar radiation compared to the central and eastern parts of China (Figure 1). As such, solar energy is considered a renewable energy resource worth exploring. The Observatory has been monitoring global solar radiation since 1958, and the measuring instrument was moved to King's Park in the following year. In 2008, another solar radiation station was set up in Kau Sai Chau. Currently, the two stations are monitoring both global and direct solar radiation in Hong Kong [3-4].Two major factors have to be considered when we evaluate solar energy resources available in Hong Kong. The first one is the geographical position: at a latitude of about 22°N, Hong Kong has the Sun shining usually from the south at noon.  In general, the amount of radiation increases with the elevation angle of the Sun. In the course of a year, the elevation angle during the Sun's transit over Hong Kong's meridian varies between a minimum value of about 44° at the Winter Solstice to a maximum of 90° [5]. The second factor is the temporal and seasonal variations of solar radiation.  Table 1 shows the climatological normals of monthly sunshine duration and global solar radiation (1981-2010) at King's Park. The amount of solar radiation in general peaks in July, and overall is more abundant in the second half of the year than the first half. Figure 2 shows that global radiation at King's Park has generally increased in recent years (2009-2013).  Location-wise, Kau Sai Chau at a more rural setting has a slightly higher amount of direct solar radiation in most months than King's Park in a more urban environment.  In actual applications, the effect of local ambient conditions on solar radiation should not be neglected, especially in a highly developed city like Hong Kong where many skyscrapers hinder the sky view and restrict the amount of sunshine received.Variation trends in solar radiation over the years also have implications for the long term application of solar energy resources. With an increasing trend in the mean cloud amount in the past few decades (Figure 3) and a rising trend in the number of hours of reduced visibility under 8 km (Figure 4), there is an overall decreasing trend in the total global solar radiation in Hong Kong (Figure 5). Although there seems to be a slight rebound in the solar radiation in recent years as illustrated in Figure 2, further monitoring is still required to affirm this observed trend.

Sunshine and UV

September 2014

https://www.hko.gov.hk/en/education/weather/sunshine-and-uv/00443-solar-energy-resources-in-hong-kong-from-a-climatological-point-of-view.html

en

Harmful and beneficial effects of UV radiation

Based on the wavelength, UV radiation can be broadly subdivided into UVA, UVB and UVC. What are the harmful health effects of UV radiation?

What is UV radiation?The sun emits electromagnetic radiation of different wavelengths (i.e. different energy). Some of the radiation, such as that making up the colours of rainbow, is visible which our eyes can see.  The part of radiation beyond the violet end of visible light is called UV radiation which our eyes cannot see.What are UVA, UVB and UVC?Based on the wavelength, UV radiation can be broadly subdivided into UVA, UVB and UVC.  Their main characteristics are shown in the following table.What are the harmful health effects of UV radiation?Excessive exposure to UV radiation from the sun may cause painful sunburns, wrinkling and premature ageing of the skin as well as increase the risk of skin cancers and cataracts. In respect of skin cancer, about 80% of all cases may be attributable to excessive exposure to sunlight. Specifically, studies suggested that UVB causes tanning of our skin slowly but significantly promotes the development of skin cancer, while UVA is responsible for the wrinkling, ageing and immediate tanning effects and may also enhance the development of skin cancer. In respect of cataracts, the World Health Organization estimates that up to 20% of cataracts may be due to UV overexposure. Therefore, one must not underestimate the potential harm UV overexposure may bring. What are the effects of UV radiation on plastics and dyes?Many polymers used in plastic consumer products are degradable by UV radiation.  When exposed to sunlight for a prolonged period, discoloration or even cracking of the product may occur. Many dyes and pigments may change colour after exposure to UV radiation.  Therefore, antique paintings need to be protected from sunlight.  For the same reason, taking photos of paintings with flash, which emits UV radiation, is normally prohibited in art galleries.Are there any beneficial effects of UV radiation on human health?A moderate exposure to the sun helps our body to produce vitamin D which could bring increased absorption and better utilization of calcium and phosphorus required for healthy bones and minimized risk of bone fracture.What are the applications of UV radiation?UV radiation has many applications in our daily life.  The following presents some examples. 

Sunshine and UV

https://www.hko.gov.hk/en/education/weather/sunshine-and-uv/00127-harmful-and-beneficial-effects-of-uv-radiation.html

en

Ultraviolet radiation at high altitude

At higher altitudes, the thinner atmosphere will filter out less UV radiation. Clinical evidence has shown that UVB is harmful to the crystalline lens of the eyes

While flying at 10,000 m aloft, have you ever experienced watching sunrise or sunset through the aeroplane's window? Though it might be exciting to have a view of the sun in the open sky at this high altitude, has it ever occurred to you that without proper protection, the level of ultraviolet (UV) radiation you are exposed to could be much higher than that at ground level.Ultraviolet radiation is the electromagnetic radiation of the sun beyond the violet end of visible light, which our eyes cannot see. UV radiation is commonly divided into UVA, UVB and UVC. At higher altitudes, the thinner atmosphere will filter out less UV radiation. For every 1000 m increase in altitude, the UV radiation level will increase about 12%.Clinical evidence has shown that UVB is harmful to the crystalline lens of the eyes and epidemiological studies have shown that certain types of cataract are associated with high exposure to UV radiation, especially UVB radiation. Fortunately, scientists are well aware of the intense UV at cruising heights so that protective actions have already been taken for the safety of the crew and travellers. For example, most aircraft windows use specialized material that effectively blocks out the majority of UV radiation. So, the next time when you travel on an aeroplane and the cabin crew ask you to lower the window shade, you should appreciate their extra effort in protecting you from UV radiation and ensuring the safety and comfort of all passengers.

Sunshine and UV

https://www.hko.gov.hk/en/education/weather/sunshine-and-uv/00120-ultraviolet-radiation-at-high-altitude.html

en

What is UVA?

During the day, the change in intensity of UVA was less than that of UVB. This is because UVA has a longer wavelength and therefore scattering by air and water particles will be less prominent. This phenomenon is more pronounced in the morning and during sunset when the sun's elevation angle is small.

UV radiation can be further classified as UVA, UVB and UVC respectively, corresponding to their different wavelengths.  UV radiation reaching the earth's surface consists of UVA and UVB only, as all UVC is absorbed by the atmosphere.  Excessive exposure to UVB may lead to sunburn and is one of the major causes of skin cancer.  Although the immediate effect of UVA to the skin is less prominent than that of UVB, over-exposure to UVA may lead to darkening, ageing and wrinkling of the skin, as well as potential development of skin cancer.  Therefore, when choosing sunscreen lotions, we should select the one that can effectively block both UVA and UVB. The Observatory has started measuring UVA at the King's Park Meteorological Station on an experimental basis since August 2009.  Preliminary analysis showed that during the day, the change in intensity of UVA was less than that of UVB.  This is because UVA has a longer wavelength and therefore scattering by air and water particles will be less prominent.  This phenomenon is more pronounced in the morning and during sunset when the sun's elevation angle is small.  For example, as shown in the following diagram, the intensity of UVB increased by more than 7 times as the suns elevation angle increased gradually from 8 a.m. to 10 a.m. The intensity of UVA only differed by about 3 times during the same period.  

Sunshine and UV

March 2010

https://www.hko.gov.hk/en/education/weather/sunshine-and-uv/00122-what-is-uva.html

en

Some Statistics on UV Index

What is UV index? What are the exposure categories for different UV index values? How are the daily maximum UV indices in different exposure categories distributed over different months? How can I obtain information about UV index?

What is UV index?UV index is an indicator that measures the effect of solar ultraviolet radiation on human skin. The higher the index, the greater the risk of skin damage due to ultraviolet radiation.  In Hong Kong, the UV index normally ranges between 0 and 15.What are the exposure categories for different UV index values?According to the World Health Organization, the UV index is classified as "low", "moderate", "high", "very high" and "extreme" categories based on the UV index values.When will the UV index attain its daily and monthly maximum values?Figure 1 shows the diurnal values of the hourly mean UV index, based on annual and seasonal averages for the past ten years (1 August 1999 to 31 July 2009).  The hourly mean UV index at 1 p.m. was highest in all seasons. The annual variation of the monthly mean values of UV index for the past ten years is shown in Figure 2.  It can be seen that the UV index is highest in July.  The diurnal and seasonal variations of the UV index are mainly due to the difference in solar elevation angle which determines the intensity of atmospheric attenuation and scattering of the solar UV radiation.How are the daily maximum UV indices in different exposure categories distributed over different months?An analysis of the UV index data from 1 August 1999 to 31 July 2009 shows that the daily maximum UV index reached the "high" or above categories for about 66% of the time. The "extreme" cases, occurring about 17% of the time, were mostly attained during the period between April and September.  The detailed frequency distribution of daily maximum UV index in different exposure categories and months is shown in the following table.How can I obtain information about UV index?The Hong Kong Observatory has been measuring and disseminating UV index since 1999.  Since 2006, the Observatory has started providing forecast of the maximum UV index for the next day.  When the index is measured or forecast to be 11 or above, the Observatory will also advise the public to avoid prolonged exposure under the sun. The public can check the latest UV index through radio, television, the Observatory's website and Dial-a-Weather system (1878200).

Sunshine and UV

December 2009

https://www.hko.gov.hk/en/education/weather/sunshine-and-uv/00123-some-statistics-on-uv-index.html

en

Tips for sunburn protection in Summer

To reduce the harm from UV radiation, the most important thing is to minimize direct exposure of the skin and the eyes to sunlight.

During the hot summer days, people will engage in various kinds of outdoor activities and water sports. When staying outdoors, we should take the necessary protective measures to reduce the harmful effect of ultraviolet (UV) radiation to our skin. What are the effects of UV radiation to us? What protective measures should we take?  Some tips for protection against sunburn are discussed below for your reference.What are the effects of UV radiation to the skin?UV radiation can be classified as UVA, UVB and UVC according to the energy or wavelength. UV radiation reaching the earth's surface consists mostly of UVA and some UVB, as all UVC and most of the UVB are absorbed by the atmosphere.  Studies suggested that UVA is responsible for the ageing, wrinkling and immediate tanning effects, as well as potential development of skin cancer. Over-exposure to UVB may cause sunburn and is one of the major causes of skin cancer.The World Health Organization estimates that about 80% of skin cancer and up to 20% of cataracts may be due to UV over-exposure. Therefore, one must not underestimate the potential harm UV over-exposure may bring.What protective measures should we take?To reduce the harm from UV radiation, the most important thing is to minimize direct exposure of the skin and the eyes to sunlight.  On days when the UV index is high, we should avoid staying outdoors for prolonged periods.  If you must be out in the sun, take the following precautions: - Apply sunscreen lotion - Use an umbrella - Wear long-sleeved and loose-fitting clothing - Wear a broad brim hat - Wear UV blocking sunglasses What should we pay attention to when choosing sunscreen lotion?Sun Protection Factor (SPF) is a measure of the degree of protection against UVB radiation provided by sunscreens. It relates to how long, on a sunny day, it takes to get burnt by the sun's UVB radiation. For instance, SPF 15 means that with sunscreen lotion on, it will take 150 minutes to produce a detectable burn on a person who gets sun burnt in 10 minutes.The SPF of a sunscreen lotion only tells us the time extension before a person gets burnt by UVB, but not UVA. An international standard has yet to be developed for protection against UVA, although the PA (Protection Grade of UVA) is usually used in Asia. There are 3 PA grades: PA+, PA++, and PA+++, with each additional plus (+) indicating a higher protection. So, when buying a sunscreen lotion, one should note the SPF as well as the PA.Do we need to take protective measures during a cloudy day?Up to 80% of solar UV radiation can penetrate thin cloud cover. There are also occasions when broken clouds enhance UV radiation by reflection from their sides. In case of doubt, it is advisable to check the latest UV index through radio, television, the Observatory's website and Dial-a-Weather system (1878200).

Sunshine and UV

June 2012

https://www.hko.gov.hk/en/education/weather/sunshine-and-uv/00121-tips-for-sunburn-protection-in-summer.html

en

Sun protective clothing for summer

Clothing provides the most convenient protection against UV radiation. In contrast to sunscreen lotion, clothing is relatively cheap and does not rub off. The Ultraviolet Protection Factor has been introduced as a measure of the protection against UV radiation provided by a piece of clothing.

Clothing provides the most convenient protection against UV radiation.  In contrast to sunscreen lotion, clothing is relatively cheap and does not rub off.  One may think that clothing can block UV radiation completely but in fact it may not.  The effectiveness of clothing in protection against UV radiation does depend on a number of factors.What factors affect the level of UV radiation protection provided by clothing?There are a number of factors: - Tightly woven or knitted fabrics have closer fibre structure and can block more UV radiation.  Thicker fabrics also block more UV radiation. - Many dyes can absorb UV radiation and therefore darker colour clothing provides more protection. - Over-stretched or worn out garments can allow more UV radiation to pass through their small pores. - Wet clothing offers less protection as scattering of light will become less with the presence of water. - Clothing treated with UV absorber additives during laundry can give better protection. What is the Ultraviolet Protection Factor (UPF) for clothing?In recent years, the Ultraviolet Protection Factor (UPF) has been introduced as a measure of the protection against UV radiation provided by a piece of clothing.  The higher the UPF value, the better the protection against UV radiation offered by the clothing.  A garment with a UPF of 50 allows 1/50 of the UV radiation falling on the surface of the garment to pass through it.  In other words, it blocks 49/50 or 98% of the UV radiation.  Although the UPF is a useful indicator for choosing sun protective clothing, one should note that it has not been adopted as an international standard yet.

Sunshine and UV

https://www.hko.gov.hk/en/education/weather/sunshine-and-uv/00125-sun-protective-clothing-for-summer.html

en

Sunburn and SPF

SPF stands for Sun Protection Factor. Put simply, it relates to how long, on a sunny day, it takes to get burnt by the sun's ultraviolet radiation. What SPF should I use for sunscreen lotion?

What is SPF?SPF stands for Sun Protection Factor. Put simply, it relates to how long, on a sunny day, it takes to get burnt by the sun's ultraviolet (uv) radiation (let's don't worry about the sun's intensity, time of day, skin colour etc. which obviously would affect the time duration to get sunburnt). For instance, SPF 15 means that with sunscreen lotion on, it will take 15 hours to produce a detectable burn on a person who gets sunburnt in an hour.Then, does it mean that an SPF 30 lotion offers twice as much protection as an SPF 15 one?No. The difference in protection is only 1/15 - 1/30, or roughly 3%. Likewise, the difference between SPF 15 and 50 is 1/15 - 1/50, or roughly 5%.What SPF should I use for sunscreen lotion?Sunscreen lotion with an SPF of 15 or above is recommended. Apply liberally and re-apply after swimming or sweating.What other precautions should I take?Small amounts of uv radiation are beneficial to people and essential in the production of vitamin D. However, excessive exposure to solar uv radiation may result in health effects on the skin, eye and immune system. You can check the latest uv level at: https://www.hko.gov.hk/en/wxinfo/uvinfo/uvinfo.htmlThe same website also provides uv forecast for the next day for planning purpose. Apart from the use of sunscreen lotion, other protection measures include: stay in the shade, use an umbrella, wear long-sleeved and loose-fitting clothing, wear a broad brim hat and wear uv blocking sunglasses. It is particularly important to protect babies and young children from the sun.Is it enough just to stay in the shade?Not enough. Shade can reduce uv by 50% or more. Indoor workers receive 10 to 20% of outdoor workers' yearly uv exposure. However, sand on the beach reflects up to 15% of uv radiation. Some people get burnt while skiing because snow reflects up to 80% of sunburning uv.Will the uv radiation be lower when there are clouds?uv radiation levels are highest under cloudless skies. But even with cloud cover, they can also be high even in the presence of clouds. There have been occasions when broken clouds enhance uv radiation by reflection from their sides.What else should I take note?At half a metre under water, the uv is still 40% as intense as at the surface. Hence the need for water-proof sun lotion. UV increases by 4% for every 300 metre rise in altitude. In Hong Kong, on average over 70% of uv is received between 10 a.m. and 3 p.m. on a sunny day.For further details on protection against uv radiation, please check out: https://www.hko.gov.hk/en/publica/gen_pub/files/uv_e.pdf.

Sunshine and UV

https://www.hko.gov.hk/en/education/weather/sunshine-and-uv/00130-sunburn-and-spf.html

en

More about Sunburn Protection (Part 2 of Sunburn and SPF)

How should we choose sunglasses? What clothing should we wear to get better protection from the sun?

How should we choose sunglasses?When sunglasses are worn, the pupils will dilate as the amount of light reaching the eyes is reduced. If the sunglasses do not filter out enough of the Ultraviolet (UV) radiation, the amount of UV radiation getting into the eyes may be even greater than not wearing sunglasses at all. So, when choosing sunglasses, we should note whether they have a UV filtering function. Good sunglasses should be able to filter out at least 98% of the UV radiation. Also, trendy 'wrap-around' sunglasses are good in preventing UV coming from the sides.What clothing should we wear to get better protection from the sun? Why is it better to wear dark and loose-fitting clothing?To avoid strong sunlight getting directly to the skin, long-sleeve clothing is recommended. Cloth woven in high fabric density can filter off more UV radiation. On the other hand, loose-fitting clothing is better than tight-fitting clothing. It is because tight-fitting clothing is stretched more easily by body movement. As a result, small pores are exposed in the fabric, thus allowing more UV to get through to the skin.Also, when the clothing is wet, there is less scattering of light because of the presence of water (check 'Why is wet sand darker than dry sand?'). This means that more UV may pass through and reach the skin. Lastly, dark clothing is more effective in filtering out UV than light-coloured clothing, giving better protection from the sun.What kind of hat should we choose to protect ourselves from UV?We should choose broad-brimmed hats as they shade our face, ears and neck. Although caps or visors are fashionable, they are not as effective.When buying a sunscreen lotion, what should we look for apart from SPF (Sun Protection Factor)?UV can be classified into 3 categories: UVA, UVB and UVC, according to the different wavelengths in the electromagnetic spectrum. Both UVA and UVB may affect our body. UVA can darken our skin and cause premature ageing while UVB can cause sunburn and swelling. The SPF on a sunscreen lotion only tells us the time extension before a person gets burnt by UVB. A standard has yet to be developed for protection against UVA, although the PA (Protection Grade of UVA) is usually used. There are 3 PA grades: PA+, PA++, and PA+++, with each additional plus (+) indicating a higher protection. So, when buying a sunscreen lotion, note the SPF as well as the PA.Is SPF (Sun Protection Factor) only applicable to sunscreen lotions?According to the definition of Sun Protection Factor (SPF), it is also applicable to other sunburn preventive measures. As a rough guide, the SPF of a broad-brimmed hat is 3 to 6, while that of ordinary summer clothing is 6 to 7. One can see from their SPF values that the degree of protection is not as high as sunscreen lotions (SPF 15 or above). Sunscreen lotions should still be used.

Sunshine and UV

https://www.hko.gov.hk/en/education/weather/sunshine-and-uv/00129-more-about-sunburn-protection-sunburn-and-spf-part-2.html

en

Do we get sunburned on a cloudy day?

You do get sunburned on a cloudy day, that is of course, if you are engaged in outdoor activities but not properly protected against ultraviolet radiation. There is a significant UV component in diffuse sunlight.

Yes, you do get sunburned on a cloudy day, that is of course, if you are engaged in outdoor activities but not properly protected against ultraviolet radiation (UV).  It is true that on a cloudy day, you won't get as much exposed to UV from direct sunlight as when it is a clear sunny day.  However, sunlight, including UV, are scattered by gases in the atmosphere, as well as by clouds, dust, haze and even fog.  This part of the sunlight that reaches the earth's surface is called diffuse sunlight.  Because atmospheric scattering is inversely proportional to the wavelength of radiation, therefore, by virtue of its short wavelength, UV is most severely scattered.  In other words, there is a significant UV component in diffuse sunlight.  From the time series of solar radiation on 20 May 2010 (Fig. 1), which is a cloudy day with 73 % cloud amount, one can notice that although the direct solar radiation has the highest peak values when direct sunlight reaches the ground through a break in the clouds, the average intensity of diffuse solar radiation is higher.  Around noon time, the highest value of diffuse solar radiation nearly reached 600W/m2, some 70% of the peak value of direct solar radiation in a clear day (Fig. 2).  In fact, in 27 days in April 2010 (which is a cloudier month than normal), the total diffuse solar radiation is higher than the direct solar radiation (see Fig. 3).  In some cases, the diffuse solar radiation is several times more intense.I had an unfortunate experience of getting quite nastily sunburned when I hiked in late March this year in Ma On Shan.  Although it was quite bright throughout, the fact that Ma On Shan was completely shrouded in hill fog gave me the wrong impression that I did not need to be protected against sunlight.  But the scattered UV did take its toll on me throughout my 5 hours of hiking in the fog. For real-time information of direct and diffuse solar radiation, please visit the Observatory's webpage at: https://www.hko.gov.hk/en/wxinfo/ts/display_element_solar.htm

Sunshine and UV

June 2010

https://www.hko.gov.hk/en/education/weather/sunshine-and-uv/00128-do-we-get-sunburned-on-a-cloudy-day.html

en

Special care for children in the sun

It has been estimated that the majority of a person's lifetime exposure to UV radiation is received before the age of eighteen. As the risk of adverse health effects from UV radiation exposure is cumulative, overexposure to sunlight during childhood will increase the chance of skin cancer occurring later in life.

Children love playing outdoors and spend more time in the sun than the average adult.  It has been estimated that the majority of a person's lifetime exposure to UV radiation is received before the age of eighteen.  As the risk of adverse health effects from UV radiation exposure is cumulative, overexposure to sunlight during childhood will increase the chance of skin cancer occurring later in life.  In other words, reducing the occurrence of sunburn and repeated UV radiation exposure during the early age can cause skin cancer rates to decline. Children themselves are usually not aware of the harmful effects of UV radiation.  Therefore, it is the parents' responsibility to protect their children from UV radiation.  Some common ways to protect children from the sun are given below: - When taking children outdoors, protect them from UV radiation by the use of hats, umbrellas, sunglasses and proper clothing. - Apply sunscreen lotion to protect those areas of the skin that cannot be adequately covered with clothing. - Do not leave children in the sun for extended periods of time, encourage them to play in the shade. - Teach children about sun safety measures and the UV index. - Whenever possible, schedule outdoor activities to avoid peak UV radiation periods. - Never allow children to use sunlamps.

Sunshine and UV

September 2009

https://www.hko.gov.hk/en/education/weather/sunshine-and-uv/00124-special-care-for-children-in-the-sun.html

en

Myths about protection against UV radiation

Although most people are aware of the need to take protective actions against sunburn, there are some common misconceptions about UV radiation and the ways of protection.

Human exposure to UV radiation may result in acute and chronic health effects on the skin, eye and immune system. Although most people are aware of the need to take protective actions against sunburn, there are some common misconceptions about UV radiation and the ways of protection:Myth 1: Darker sunglasses offer more protection from UV radiationThe most important thing to look for in sunglasses is how much UV radiation they filter out.  It should be noted that there is no relationship between the colour of sunglasses and their UV filtering action.  When one wears sunglasses the pupil widens as there is less light reaching the eye.  If the sunglasses have poor UV protection, the amount of UV radiation getting into the eyes may even be greater than not wearing sunglasses at all.  For adequate protection, one should wear sunglasses that are able to block at least 98% of the UV radiation.Myth 2: You can't get sunburn on a cloudy dayUp to 80% of solar UV radiation can penetrate thin cloud cover.  There are also occasions when broken clouds enhance UV radiation by reflection from their sides.  For example, on 23 February 2009, the UV index reached as high as 10 at around 1 p.m. even though the sky was covered by low level broken clouds.  Therefore, in case of doubt, it is advisable to check the latest UV index through radio, television, the Observatory's website and Dial-a-Weather system (1878200).Myth 3: You can't get sunburn while in the waterWater offers only minimal protection from UV radiation.  At half a metre under water, the UV radiation level is still 40% as intense as at the surface.  Also, the part of body above water is additionally exposed to ultraviolet rays reflected from the water surface.Myth 4: Sunscreen lotion protects me so I can sunbathe much longerSunscreen lotion should not be used to increase sun exposure time but to increase protection during unavoidable exposure. The protection provided by sunscreen lotion depends critically on their correct application.  For more about sunscreen lotion and sunburn protection, please check out "Sunburn and SPF".Myth 5: If you take regular breaks during sunbathing you won't get sunburnUV radiation exposure is cumulative.  The total health damage you get will be the sum of the effect of individual exposure.  Therefore, to protect yourself the objective is to reduce exposure to UV radiation as far as practicable. This could decrease the chance of skin cancer.Myth 6: If you don't feel the hot rays of the sun you won't get sunburnSunburn is caused by UV radiation which cannot be felt.  The heating effect is caused by the sun's infrared radiation and not by UV radiation.  Therefore, even if you don't feel the warmth, you may also get sunburn.

Sunshine and UV

https://www.hko.gov.hk/en/education/weather/sunshine-and-uv/00126-myths-about-protection-against-uv-radiation.html

en

Hong Kong Observatory Using Nowcasting to Support Drainage Services Department

The control mechanism of the penstock at Shuen Wan has been optimised through making reference to rainfall nowcast from SWIRLS. Based on the nowcast, the penstock will be closed automatically at lower tide level when severe weather is about to affect the catchment area, otherwise it will be closed at higher tide level.

The nowcasting system of the Hong Kong Observatory (HKO), "SWIRLS" (Short-range Warning of Intense Rainstorms in Localized Systems), is capable of forecasting heavy rain that may affect Hong Kong in the coming couple of hours. In addition to supporting the operation of rainstorm warnings and public weather nowcasting services, SWIRLS also provides the Drainage Services Department (DSD) with tailor-made products to enhance its operations and works projects.The control mechanism of the penstock at Shuen Wan has been optimised through making reference to rainfall nowcast from "SWIRLS". Based on the nowcast, the penstock will be closed automatically at lower tide level when severe weather is about to affect the catchment area, otherwise it will be closed at higher tide level. This design could fulfill the design flood protection standard, optimise seawater ingression in estuary and the nearby wetland area, and protect the wetland environment."SWIRLS" also supported the Kai Tak River Improvement Projects of DSD to enhance works efficiency. Using SWIRLS, the contractor would evacuate the workers, machines and materials in the nullah only when heavy rain was imminent over the catchment area, thus works could be arranged more effectively to avoid suspending work whole-day long merely based on rain forecast in weather bulletins. Compared with the work schedule before adopting SWIRLS, the workable hours had increased by around 20%.

Weather forecast

November 2019

https://www.hko.gov.hk/en/education/weather/forecasting/00531-hong-kong-observatory-using-nowcasting-to-support-drainage-services-department.html

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Conveying High-impact Weather Forecasts with Uncertainties — A New Challenge for Public Communication

With the rapid advent of computers and remote-sensing instruments, meteorologists nowadays can employ supercomputers to handle and assimilate massive observations and simulate extreme and rare weather phenomena by repeatedly running numerical prediction models.

Characteristics of High-impact WeatherUnder a warming world, different types of extreme weather are expected to occur at varying rates with an overall increasing trend. The associated high impacts that may cause enormous damages to the society and pose high risks to life and health could hardly be underestimated. Deadly heatwaves, chilling cold surges, tsunami-like storm surges, serious flooding arising from rainstorms, etc., are not uncommon to be noticed from the news every now and then.One common feature to the above mentioned high-impact weather types is their extremity and rarity. Their occurrence frequencies are so low that past observation data associated with such phenomena have been poorly available, making scientific studies difficult. With the rapid advent of computers and remote-sensing instruments, meteorologists nowadays can employ supercomputers to handle and assimilate massive observations and simulate extreme and rare weather phenomena by repeatedly running numerical prediction models. Before each computer simulation, a number of slightly different initial conditions from the actual atmospheric analysis are drawn to simulate various possible forecast scenarios. The simulation results are then collected and processed for calculating statistics about the distributions of all possible weather regimes, including the uncertainties associated with extreme high-impact events. The forecast system mentioned above, including the underlying analysis and simulation algorithms, is commonly known as Ensemble Prediction System (EPS).How to effectively convey to the general users the different forecast elements from an EPS, including their associated uncertainties, is of paramount importance. Yet, it has been a forbidden challenge to the meteorological community and the Observatory is of no exception. During the past few years, the Observatory has explored and developed some experimental extended forecast products based on EPS data. To unveil to the public all the possibilities and uncertainties depicted by EPS, we have abandoned the traditional worded forecasts and turned to drawing colourful pictures. Leaning on the mathematical language of probability, such a picture is worth a thousand words and better reflects the forecast distributions without much loss of fidelity as illustrated below.Extended Forecasts for Weather ElementsThe "Extended Outlook (Beta)" webpage provides the probability forecasts and evolution trends out to 14 days ahead for different weather elements, including maximum temperature, minimum temperature and mean-sea-level pressure (MSLP), in the form of coloured time series charts.Figure 1 shows the MSLP probability forecast associated with the case of Super Typhoon Mangkhut in September 2018. On the chart, different colours represent different chances of pressure occurring in the marked ranges. In general, the more reddish the colour, the higher the chance. Whereas more bluish the colour, the lower the chance. The colour chart shows a pronounced signal with a finite chance of pressure dropping drastically on 16 September way below the normal climate range (grey lines), hinting that Mangkhut might be closest to Hong Kong and bringing extreme low pressure to the territory by then. Having said that, it should well be noted that such a signal was weighted with a low probability, meaning that the actual occurrence of the corresponding extreme weather was still largely a variable. As the saying goes: “never tailgating”, particularly applicable to those forecast situations plagued with uncertainty.Tropical Cyclone Track Probability ForecastThe "Tropical Cyclone Track Probability Forecast" webpage provided by the Observatory displays the probability of a tropical cyclone (TC) passing within a certain region in the coming 9 days also in the form of a coloured diagram, enabling users to appraise the trend of TC movement in the near future.Figure 2 shows the track probability forecast for the case of TC Podul in late August 2019. The colour codes follow those of the Extended Forecasts. The probability distribution on the map indicated that Podul located at the seas east to the Philippines at that time would have a high chance to move across Luzon first. The subsequent track probabilities decreased generally to below 50%, meaning that the track of Podul upon entering the South China Sea would become more uncertain. Overall speaking, the chance of Podul tracking westward and edging closer to the vicinity of Hainan Island was still relatively high, while that of affecting the vicinity of the Pearl River Estuary region was low.In general, the pressure will drop significantly as a TC approaches. When intense cold surge affects Hong Kong, the pressure will rise markedly with temperatures dropping drastically. If members of the public can keep track of the evolution trends of the elements associated with the extreme weather using probability forecasts, they could make necessary preparation at the earliest instance possible to reduce potential impact or loss that may be incurred.

Weather forecast

November 2019

https://www.hko.gov.hk/en/education/weather/forecasting/00532-conveying-highimpact-weather-forecasts-with-uncertainties-a-new-challenge-for-public-communication.html

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Maintaining High Availability of Hong Kong Observatory Operational Computer Systems

The systems each has a set of backup, which, in case of an unexpected failure of its counterpart, can immediately takeover the operation.

The summer sun is blazing but a storm is creeping from afar, unseen beyond the horizon. The sky of Hong Kong, so blue and homogenous it almost resembles a waveless sea, gives nothing other than a hot sun hanging and a few clouds slacking off, and the blueness of the sky is showing no sign of the coming storm. Now let us be brave enough to imagine that the weather forecasters in the Central Forecasting Office (CFO) of the Hong Kong Observatory (HKO) are now suddenly deprived of weather data and forecast products. These data may include those from surface and upper-air weather observations automatic weather stations, weather satellites, weather radars and numerical weather prediction (NWP) models. Without these telescopic eyes and sensors, it is not hard to imagine how difficult it would be to predict whether a developing storm located far from human sight would affect Hong Kong in the upcoming few hours. Indeed, the weather data and forecast products are crucial to weather forecasting operations. One of the major tasks of the HKO staff is to ensure the computer systems that are used to process the data and generate products remain operational round the clock. Nevertheless, diligence does not promise flawless performance. If any one of the computer systems unexpectedly breaks down, it is just like blinding and handicapping the forecasters.CFO is not the only office operating 24-hour in HKO. In fact, colleagues in the Information Technology (IT) Management Division (ITMD) of HKO are also working 24-hour to monitor the operation of all critical computer systems and IT facilities in HKO. If any of the systems show a sign of problem, the Information Technology Officer (ITO) will follow the established procedures to handle the problem as soon as possible. In case an urgent situation occurs in non-office hours, even off-duty relevant officers have to assist the ITO to ensure that the computer systems at CFO can continue to operate smoothly. However, it may still need some time to recover from the problem if there is one. In that case, will the forecasters be blind to the sky during the period?There is an important concept called “redundancy” in engineering. Although it may sound negative, it is of extreme importance in keeping the availability of critical IT systems. Redundancy means that there is at least one duplicate of each critical component in the systems. In other words, each critical component has a set of backup that can immediately replace the original if the original malfunctions. In the human body, pair of organs like eyes, lungs and kidneys share a similar concept. If one in the pair fails, the other can still support the functioning of the body. Although these pairs of organs always work at the same time, this may not be the case for computer systems with redundancy. In general, only one set of these systems will be in operational mode. The backup system will switch to the operational mode (either automatically or manually) when the operating system shows a sign of problem.HKO strives to provide timely weather forecasts and warnings to the public and the operations of CFO must never be interrupted during any types of weather. Therefore, HKO designs all critical systems with the concept of redundancy in mind. The systems each has a set of backup, which, in case of an unexpected failure of its counterpart, can immediately takeover the operation. Upon acknowledging a system problem, ITMD will decide whether it is necessary to switch the backup system to the operational mode. If that turns out to be the case, ITMD will repair the malfunctioned system as soon as possible so that it can operate properly again as early as possible. There is thus no need for forecasters to worry about being blind to the sky.

Weather forecast

August 2018

https://www.hko.gov.hk/en/education/weather/forecasting/00516-maintaining-high-availability-of-hong-kong-observatory-operational-computer-systems.html

en

A New Milestone in Weather

SWIRLS can quickly analyse the motion of radar echoes and rainfall distribution, and then extrapolate radar echoes to forecast the rainfall amount and the risk of severe weather at various locations.

The Hong Kong Observatory was designated in June 2018 by the World Meteorological Organization (WMO) as the Regional Specialized Meteorological Centre (RSMC) for Nowcasting to provide support in nowcasting for meteorological services in the Asian region. On the Observatory’s dedicated website (https://rsmc.hko.gov.hk/), real-time products of location-specific rainfall, significant convection and ice crystal icing nowcasts for the next few hours are available (Figure 1) to enhance support in forecasting and alerting of severe weather and aviation meteorological services in the region."Nowcasting" refers to high-precision weather forecast for the next few hours. There are inherent uncertainties in the development of weather systems. In particular, the forecasting of the time and location of occurrence as well as the changes in movement and intensity of severe weather events are major challenges to the meteorology community. Heavy rain, thunderstorms, hail and gust can bring serious hazards within a very short time, thus requiring stringent forecast accuracy. To support rainstorm warning operation and to provide public nowcast services, the Observatory operates an in-house developed system called "SWIRLS - Short-range Warning of Intense Rainstorms in Localized Systems" that can quickly analyse the motion of radar echoes and rainfall distribution, and then extrapolate radar echoes to forecast the rainfall amount and the risk of severe weather at various locations.The Observatory collaborates with the Civil Engineering and Development Department, the Drainage Services Department and the Water Supplies Department in rainfall nowcasting to reduce the risk of landslides and flooding, improve the ecological environment, preserve fresh water resources and enhance efficiency of drainage works. Over the past few years, the Observatory has developed one-hour lightning nowcast for the public, and provided lightning nowcast services for the Airport Authority and public utilities such as power company, cable car and theme park, etc. to meet their needs in operation and emergency response. In research and development for further improving forecast techniques, the Observatory and the Hong Kong University of Science and Technology jointly developed a rainfall nowcast algorithm based on deep learning methods in artificial intelligence (AI). In 2018, the Observatory partnered with Shenzhen Meteorological Bureau to organise an AI challenge and gathered experts in the field to develop novel nowcast techniques.SWIRLS has supported numerous major international events, such as the Beijing Olympics Games, the Shanghai Expo, the Commonwealth Games in New Delhi, and the Universiade in Shenzhen. Built on these experience, the Observatory has developed a community version of SWIRLS (Com-SWIRLS) for sharing with other official meteorological agencies with an aim to building their capacity in weather nowcasting and provision of related warning services. To date, the meteorological services in the Mainland and overseas including Zhuhai, Hainan, the Philippines, Malaysia (Figure 2), India, Myanmar and South Africa have adopted Com-SWIRLS or engaged in active development of the applications of nowcasting products. The Observatory hosted workshops for the WMO, and organized the 4th International Symposium on Nowcasting and Very-short-range Forecast under the World Weather Research Programme in 2016, providing training on nowcasting technology and Com-SWIRLS to around a hundred meteorologists.Looking ahead, the Observatory will continue the development of advanced weather nowcasting techniques as well as promotion of Com-SWIRLS and data sharing to foster advances in severe weather forecast and warning services. This does not only contribute towards the capacity building in meteorology in the region, but also enables members of the public to be better prepared for potential natural disasters.

Weather forecast

September 2018

https://www.hko.gov.hk/en/education/weather/forecasting/00513-a-new-milestone-in-weather.html

en

Temperature Trend at Fingertips

Forecasters and meteorologists commonly utilize ensemble forecasting technique to assess the likely weather condition and other alternative scenarios. In "ensemble forecast", the computer weather model is executed simultaneously with a set of different initial conditions, or with slight modifications in atmospheric physical processes to forecast possible weather changes in the future and to generate "probabilistic forecast" of various weather scenarios.

Weather is a part of our everyday lives. People pay attention to weather changes to get better prepared. At present, the Observatory provides 9-Day Weather Forecast, including daily information on wind direction, wind speed, temperature, humidity and sky condition for the next 9 days. This is what we call "deterministic" weather prediction in which specific values or description of weather elements are given in the forecast for each day. The reader may have heard of the "Butterfly effect". It refers to a butterfly in Brazil flapping its wings causing tornadoes in Texas. Weather system is in fact a typical system in which "Butterfly effect" may occur. Weather prediction inevitably comes with uncertainty. As such, the certainty of the "deterministic" forecast generally decreases with increasing forecast time, making long period forecast usually less accurate. To deal with this problem, forecasters and meteorologists commonly utilize "ensemble forecasting" technique to assess the likely weather condition and other alternative scenarios[1].In "ensemble forecast", the computer weather model is executed simultaneously with a set of different initial conditions, or with slight modifications in atmospheric physical processes to forecast possible weather changes in the future and to generate "probabilistic forecast" of various weather scenarios. The key difference between probabilistic forecast and deterministic forecast is that deterministic forecast indicates a day with or without rain while probabilistic forecast gives the chance of rain occurrence, for example, 70% chance of rain reflecting higher likelihood of rain to occur. Figure 1 depicts forecast rainfall patterns from around 50 ensemble members. There were more than 70% of members showing occurrence of rain in Hong Kong. A particular member with less rainfall (Map ("A" as pointed by the arrow on the right side of Fig.1) forecasts local temperatures above 30 degrees in general, while a "rainy" member ((Map "B" in Fig.1) shows temperatures in parts of the territory ranging between 26 and 28 degrees. Based on the temperature forecast by different members of "ensemble forecast", we can likewise give the probabilistic forecast of temperatures[1].The Observatory provides "extended outlook" forecast service utilising ensemble forecast data. These data are automatically post-processed to generate probabilistic weather prediction which serves as the scientific basis for extending weather forecasts up to two weeks ahead. It enables the public to get hold of the trend of future weather. The 14-day probability forecast of daily minimum and maximum temperatures was launched in June 2017 (see samples in Fig. 2 and 3). The time series chart of probabilistic forecast of daily minimum (maximum) temperature shows the chance of minimum (maximum) temperature occurring within 1 degree of the marked temperature shown on the vertical axis of the graph in the next 14 days. Different colors represent different probabilities. For instance, if the probability of maximum temperature attaining 32 degrees on a certain day is 60%, it means there is about a proportion of 6 out of 10 cases with maximum temperatures ranging between 31 and 33 degrees. The "extended outlook" forecast product also provides a climatological average of minimum or maximum temperatures, as shown by the gray solid line in Fig.2, for reference. The upper and lower gray dotted lines on the chart represent the 85th and 15th percentiles of the climate respectively, meaning that there are 70% of the same period in the past with minimum or maximum temperatures falling within the range bounded by the upper and lower dotted lines.What are the benefits of using probabilistic forecasts? Taking temperature forecast probability as an example, a warmer colour (yellow to red) indicates a higher chance of occurrence. The trend of the higher probability region represents a higher chance of temperature scenarios. Figure 2 shows a case of the passage of a cold surge in Hong Kong. The daily minimum temperatures on the days before 22 February 2017 were more likely in between 15 and 17 degrees; while on 25 - 26 February the daily minimum temperature had a higher chance of falling to 12 to 14 degrees. In fact, the minimum temperatures recorded at the Observatory shown by the connected black dots in Fig.2 indicated a temperature fall after 22 February, which was in consistent with the "extended outlook". Another case in Fig. 3 shows the maximum temperature probability forecast in summer, where the variations in probabilistic forecast generally aligned well with the actual ups and downs in daily maximum temperatures recorded at the Observatory.Probabilistic forecast has another key application, that is users can make reference to the predicted probability for risk assessment in their planning of daily living and arrangement of holiday activities according to their acceptable risk levels under different weather conditions. By making an appropriate assessment, they can determine whether to continue with their activity or to take contingency measures in a timely manner. Here, we take an example of the probability forecast of minimum temperature to illustrate this concept. During the passage of a cold surge in winter, the public can make reference to the forecast probability of the daily minimum temperature falling below the cold level. As shown in Fig.2, the probability of minimum temperature reaching 12 degrees was around 30% on 24 February. For the elderlies and people with chronic medical conditions, a 30% chance of weather becoming cold may already need to call for attention. They can then prepare winter clothes earlier to reduce the risk of possible health impact arising from sudden temperature changes.The Observatory is exploring the development of other probabilistic forecast products including probability forecasts of tropical cyclone track, wind speed and rainfall intensity. We hope the probability forecast service can help users better understand the uncertainties in the changes of weather. If members of the public, public utilities and decision-makers could gradually utilise risk management concepts in their mode of decision-making, it is believed that the society and the vulnerable groups are able to better respond to and mitigate the effects of high-impact weather.

Weather forecast

August 2017

https://www.hko.gov.hk/en/education/weather/forecasting/00494-temperature-trend-at-fingertips.html

en

Does God play dice? – Randomness in Weather Evolution

Physical systems with sufficient complexity also exhibit randomness, which then limits the system's predictability. The motion of the Earth's atmosphere i.e. weather is a prime example. Tiny differences in initial conditions can lead to large discrepancies in the evolution of weather systems as represented by numerical weather prediction.

Does God play dice? – randomness in weather evolution HON Kai-kwong February 2017 "God does not play dice" is a saying often attributed to the renowned physicist Albert Einstein. Here "playing dice" refers to the randomness inherent in the motion of particles in quantum mechanics (Fig. 1), which makes a completely accurate prediction impossible even in a theoretical sense. Fig 1   Quantum mechanics utilise probability (given by the modulus of the "wave function") to describe possible locations of the electron.(Image taken from https://commons.wikimedia.org/w/index.php?curid=5854697) In fact, physical systems with sufficient complexity[1] also exhibit randomness, which then limits the system's predictability[2]. The motion of the Earth's atmosphere i.e. weather is a prime example. Tiny differences in initial conditions can lead to large discrepancies in the evolution of weather systems as represented by numerical weather prediction (Fig. 2), attesting to the famous "butterfly effect" or "chaos theory". Fig 2   Forecast satellite cloud imagery for Typhoon Nida by members of a mesoscale ensemble prediction system. Here members (1) and (2) suggest the possibility of Nida making landfall to the east and west of Hong Kong respectively. In order to address uncertainty in weather forecasts, the technique of ensemble prediction[3] is developed as a result. By performing computation using "perturbed" initial fields, ensemble prediction is capable of producing a series of possible atmospheric states, which then provide an objective basis for estimating the likelihood of weather events. Fig 3   Tropical cyclone strike probability forecast for Typhoon Nida by the ensemble prediction system of ECMWF[3]. It could be seen that, at the time, there was still considerable discrepancy between ensemble members regarding the movement and landfall position of Nida. Note: Normally, a non-linear system described by 3 or more differential equations is required. Strictly speaking, classical mechanical systems (i.e. non-quantum systems including the atmosphere) are not truly "random". Their perceived "randomness" arises from an extreme sensitivity to initial conditions, which makes it impossible in practice to predict or repeat the exact outcomes of the system's evolution. "Mark 6" is such an example. The European Center for Medium-range Weather Forecasts (ECMWF) began operating the first global ensemble prediction system since the 1990s. "God does not play dice" is a saying often attributed to the renowned physicist Albert Einstein. Here "playing dice" refers to the randomness inherent in the motion of particles in quantum mechanics (Fig. 1), which makes a completely accurate prediction impossible even in a theoretical sense.In fact, physical systems with sufficient complexity[1] also exhibit randomness, which then limits the system's predictability[2]. The motion of the Earth's atmosphere i.e. weather is a prime example. Tiny differences in initial conditions can lead to large discrepancies in the evolution of weather systems as represented by numerical weather prediction (Fig. 2), attesting to the famous "butterfly effect" or "chaos theory".In order to address uncertainty in weather forecasts, the technique of ensemble prediction[3] is developed as a result. By performing computation using "perturbed" initial fields, ensemble prediction is capable of producing a series of possible atmospheric states, which then provide an objective basis for estimating the likelihood of weather events.

Weather forecast

https://www.hko.gov.hk/en/education/weather/forecasting/00486-does-god-play-dice-v-randomness-in-weather-evolution.html

en

SWIRLS Nowcasting System

SWIRLS extrapolates the radar echoes according to the speed and direction of movement at each point, thereby projecting the future location of rain areas and hence the rainfall that they will bring to the region in the next few hours.

The Hong Kong Observatory (HKO) is responsible for the operation of rainstorm and related warning systems in Hong Kong. HKO also delivers rainfall nowcast products via smart phones and Internet web pages to the public, and provides tailor-made services to clients such as the airport and power companies. These services are made possible by an in-house developed nowcasting system called "SWIRLS". What exactly is "SWIRLS"? The answer is unveiled in this short article.The full name of "SWIRLS" is "Short-range Warning of Intense Rainstorms in Localized Systems". It is a nowcasting system developed and operated by HKO since 1999. "Nowcasting" refers to forecast of weather in the next few hours, primarily on severe weather such as heavy rain, lightning, hail and thunderstorm-induced gust. Compared to a more general description of local weather  in the "Local Weather Forecast" or "9-day Weather Forecast", SWIRLS focuses on severe weather that is localized and highly-changeable, thus demanding for a high level of accuracy and timeliness.Through analyzing radar echoes in two successive radar scans, SWIRLS computes the direction and speed of movement of rain areas to obtain information for the distribution of high-resolution radar echo motion distribution. This is similar to estimating the direction and speed of a moving object by observing it on two successively captured photos. After analying the motion of the radar echoes over the whole area (Figure 1), SWIRLS extrapolates the radar echoes according to the speed and direction of movement at each point, thereby projecting the future location of rain areas and hence the rainfall that they will bring to the region in the next few hours. Products in various forms are then generated to support the operation of the weather forecasting Office, other users and the public. The current publicly accessible products include the "Location-based Rain Forecast" in "My Observatory" (Figure 2) and the "Rainfall Nowcast for the Pearl River Delta Region" (Figure 3).At present, nowcasting is  primarily based on radar data. Radar is a type of remote sensing equipment that will inevitably  be contaminated by various types of noise. The topography around the radar site may also limit the detection capability of the radar which will affect the performance of nowcasting. Examples include false alarms arising from anomalous propagation of electromagnet waves in autumn (see Artifacts), and failure in detection due to precipitating cloud layer lying below the radar scan. In addition, there is  no reliable way to predict the growth and decay of rain areas for the time being. All these present challenges to nowcasting systems nowadays.SWIRLS has provided nowcast services in support of major international events, including the 2008 Olympic Games, the 2010 World EXPO and the 2010 Commonwealth Games. HKO also made SWIRLS available for trial use in meteorological services in other countries and regions. "SWIRLS" is one of the leading and renowned nowcasting systems in the world. Through continuous research and developments, we are committed to continue to enhance its performance in the future.

Weather forecast

December 2014

https://www.hko.gov.hk/en/education/weather/forecasting/00448-swirls-nowcasting-system.html

en

Butterfly Effect, Model Ensemble and Probabilistic Forecast

Model ensemble refers to the use of a number of models, each starting with a slightly different initial state, in an attempt to capture all possible scenarios of the future weather and to derive the chance of occurrence of each scenario.

"Does the flap of a butterfly's wings in Brazil set off a tornado in Texas?"--- Edward Lorenz, father of chaos theory and butterfly effectThis is one of the well-known quotes in the 20th century. The meaning of this quote is that for a complex system like our atmosphere, a small change in the initial stage (slight change in air flow due to a flap of butterfly's wings in Brazil) could result in a huge difference (setting off a tornado in Texas) in a later stage. This effect can be illustrated with the Lorenz Butterfly shown in figure 1. If the reader searches the keyword "Lorenz Butterfly" on the Internet, thousands of similar images will come up but it will be almost impossible to find two images that have exactly the same traces. The slight differences in the initial position of the traces that the authors used in generating the images have resulted in the subsequent differences that emerged in the path of the traces. In the simulation of weather using a computer model, slight inaccuracies in representing the initial condition in the model could result in completely different weather forecasts in the later stages. Such inaccuracies are, however, inevitable due to the inherent inaccuracies in taking weather observations to represent the initial condition of the atmosphere. In order to tackle this problem, the meteorologists have developed a forecast technique called the "model ensemble". Model ensemble refers to the use of a number of models, each starting with a slightly different initial state, in an attempt to capture all possible scenarios of the future weather and to derive the chance of occurrence of each scenario. One difficulty in operating a model ensemble system is the high computational cost involved. It takes a supercomputer to run a high-resolution model in an attempt to catch up with the evolution of weather in the real atmosphere. It is prohibitively expensive to run thousands of them to capture all the possible weather scenarios in the future. To overcome this difficulty, the resolution of the models has to be suitably reduced. Moreover, special mathematical techniques are applied to identify those initial variations that will more likely lead to different weather scenarios. By doing so, the number of model runs required for the model ensemble system can be reduced to a minimum. Nowadays, the model ensemble systems in operation at various big weather centres have dozens of ensemble members. Apart from the above, the interpretation of products from model ensemble is by no means straightforward. For a model ensemble with 50 ensemble members, it may give 50 different temperature forecasts for tomorrow (to illustrate how complicated it could be, imagine that one morning you switch on the TV and find that, as shown in figure 2, 50 weather presenters to report, with each of them presenting a different forecast). One common method to present the uncertainty in the weather forecast is by way of probabilities. For example, a probabilistic forecast for the rainfall tomorrow may be presented as something like the chance of having any rainfall tomorrow is 30%. Such a message tells you that if the weather is made to evolve 100 times from similar initial condition into the future, rainfall will actually occur the following day at around 30 times of them. When compared with "the chance of having any rainfall tomorrow is 70%", one will appreciate which is more likely to have rain in the following day. The probabilistic presentation of forecasts is gradually gaining popularity among the meteorological agencies around the world. Lets conclude with a joke: Because of the butterfly effect, reader of this article may have already accidentally seeded the formation of a typhoon somewhere on this planet by yawning during reading. Such effect may be offset by telling your friend about what you have learnt from this article and making him/her yawn too.  

Weather forecast

December 2012

https://www.hko.gov.hk/en/education/weather/forecasting/00136-butterfly-effect-model-ensemble-and-probabilistic-forecast.html

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The Dwarfs Behind Numerical Weather Prediction

Numerical weather prediction relies on the cooperation of these computing nodes to calculate the weather elements in each high-resolution grid of a numerical model.

How long is the time for a thought?  According to Buddhist Scripture, it is around 18 microseconds[1]. Within this tiny period, computing nodes[2] of a high performance computing (HPC) system exchanged data with each other some thousand times or more.  Numerical weather prediction (NWP) relies on the cooperation of these computing nodes (Figure 1) to calculate the weather elements in each high-resolution grid of a numerical model.During calculation, data are exchanged continuously among these nodes.  Some temporary data are stored in the Random Access Memory (RAM) of the nodes to support high speed input/output processes.  The multi-core Central Processing Units (CPU) of the nodes are operating in full steam to crunch the numbers to complete calculations within a short period of time.  The calculation results are fed into the "Parallel File System" of the HPC system for retrieval by other nodes running in parallel.  Lastly, the final results will be transformed into graphical charts or data listings for easy viewing.The dwarf-like nodes work closely with one another, making it possible to quickly generate and print the forecast charts for forecasters' consumption.  If any of the dwarfs slows down, possibly caused by hardware or software problems, the whole system may shut down like what happened to Snow White after she had eaten the poisoned apple.  At that time, the system administrator will take up the role of the Prince to identify the poisoned apple in the throat of Snow White, then isolate the problematic component and resume the system operation.The Moore's Law[3] forecasts that the number of transistors on chips of the same size would double every 18 months, thus increasing the computational power by one fold.  We can see that those dwarfs behind the new NWP systems will become little supermen after a few years.  So what kind of weather forecast charts can these little supermen compute and draw in future?

Weather forecast

December 2013

https://www.hko.gov.hk/en/education/weather/forecasting/00133-the-dwarfs-behind-numerical-weather-prediction.html

en

Simulating Atmosphere

The atmosphere is first divided into a three-dimensional grid with a large set of grid points; then the initial values of meteorological parameters are computed at the grid points based on observations.

Have you ever played computer simulation games?  In the virtual world, you can act as a train driver to drive a train, a mayor to run a city or even a king to rule an empire.  The experiences are definitely interesting and challenging.  Have you ever thought of going further to become the Mother Nature to control the weather?  If you are interested, you would certainly like to know more about numerical weather prediction (NWP).PrinciplesWeather condition reveals the state of the atmosphere that is governed by physical laws such as Newton's law of motion, gas law, thermodynamics equation and continuity equation.  To build a numerical weather model, the atmosphere is first divided into a three-dimensional grid with a large set of grid points; then the initial values of meteorological parameters such as pressure, temperature, wind, air density and humidity are computed at the grid points based on observations.  Finally, the future values can be computed according to the above equations and weather predictions can be made.HistoryNorwegian meteorologists Vilhelm Bjerknes proposed the above concept in 1904.  Later, British meteorologists Lewis Fry Richardson formulated the detailed procedures and tried to solve the problem numerically.  As general-purpose computers had not yet been invented at that time and the computation was highly complicated, Richardson spent almost six weeks to complete a weather forecast for six hours.  However, he predicted an air pressure change of 145 hPa in six hours, which was exceptionally erroneous.  As a result, when he published his method and findings in 1922, his work was not well recognised.  Meteorologist Jule Charney and mathematician John Von Neumann later simplified the mathematical model and ran it on the first general-purpose computer the Electronic Numerical Integrator And Computer (ENIAC).  The first computer-generated numerical weather forecast was eventually produced in 1950. In the early days of operational NWP, forecasters were skeptical about numerical models.  But nowadays NWP has become an indispensable companion of forecasters, as its skill has significantly improved due to the increase in computing power, better representation of atmospheric process by NWP models and enhancement in both quantity and quality of weather observations.Model Output Post-processingSometimes, systematic biases are observed in direct outputs of numerical models.  In addition, model forecasts are the same for locations within the same grid cell.  If the local terrain and land surface characteristics vary considerably among these grid point locations, large errors in surface temperature and winds could result.  To reduce these errors, model outputs are usually post-processed to add more values to the model outputs.LimitationsThough NWP has made huge progress since its birth, we do need to understand that there are still limitations.  Some atmospheric processes have to be simplified or even ignored in numerical models in order to reduce the model complexity.  Weather phenomenon with scales smaller than model grid resolutions cannot be resolved by models.  There are errors in observations and their data coverage are insufficient, which affect the accuracy of model forecasts.  And the most critical factor is the chaotic nature of the atmosphere itself, which means that a tiny error in the initial values could result in a large error in the future forecasts.  This really poses a real challenge in extended weather forecast.ApplicationsNumerical models are not only used in daily weather forecasts, it can also serve as an atmospheric laboratory.  Experiments which are difficult to be performed in real-life, such as study on the impact of rise in sea surface temperature on tropical cyclones intensification, could be easily done in numerical models.  Data output from NWP models are also utilised in driving other specialised forecast models to predict sea waves, storm-surge, etc.  Furthermore, NWP equations are used in climate models for simulating climate change and global warming.  If you are interested in numerical weather models, maybe you can try to think of other applications?

Weather forecast

December 2013

https://www.hko.gov.hk/en/education/weather/forecasting/00132-simulating-atmosphere.html

en

Your Weather "Crystal Ball"

Automatic Regional Weather Forecast webpage features hourly forecasts of temperature, relative humidity, wind direction and wind speed for various locations in Hong Kong for the next 9 days.

Will it be windy and chilly when you go jogging this evening? What is the chance of rain in the next few days? What the wind will be like when you go wind-surfing in the weekend? Weather forecasters of the Observatory make use of a large amount of observations and forecast data everyday to produce, based on their experience, weather forecasts of Hong Kong in the coming 9 days. As Hong Kong is located at the shore with rather complex terrain, the weather at different locations can often be different. Imagine that you now have a weather “crystal ball” showing the weather forecasts at various locations of Hong Kong. No doubt this would greatly help planning for various activities. This is what the "Automatic Regional Weather Forecast" webpage aims to serve. It features hourly forecasts of temperature, relative humidity, wind direction and wind speed for various locations in Hong Kong for the next 9 days. The webpage was enhanced in December 2015 with the addition of 3-hourly state-of-sky in the form of weather icons and daily probability of precipitation forecasts for certain stations in the territory. Like a "crystal ball", the "Automatic Regional Weather Forecast" webpage offers weather information at locations of your choice. Once you select a location on an interactive map on the webpage, the weather forecasts pertaining to the selected location will be presented in the form of a time series (Figure 1).Mother Nature is full of uncertainties, and hence weather forecasting cannot be totally accurate. While the wisdom and experience of a weather forecaster can hardly be emulated by computers, the number cruncher can nevertheless help consolidate various types of "big data" on weather and generate an objective "guess" of the future weather conditions. Computer models typically represent the atmosphere as stacks of grids (Figure 2). Within each grid, the changes in airflow, pressure, temperature and humidity are governed by physical laws (see this article: Simulating Atmosphere). Computer models first establish the current weather conditions based on observations collected from instruments such as ground weather stations, radar, weather balloons (radiosondes), aircraft, and meteorological satellites, etc., and then generate a forecast. Just as artists have different ways of depicting the world, different models also have different "characters" owing to different weather data input and physical processes represented in the model. For instance, one model may tend to forecast fewer clouds while another may tend to predict larger temperature variation in a day. Hence, the automatic regional weather forecast basically consolidates the forecasts from computer models of the Observatory and several major meteorological centres with adjustments to the direct model outputs to produce a more accurate weather forecast. It should be noted that the information on this one-stop-shop portal is generated automatically without any human intervention. What the weather forecasters of the Observatory issued represents a prediction of the overall weather conditions of Hong Kong, whereas the automatic regional weather forecast gives more detailed forecast information, usefully supplementing each other.

Weather forecast

December 2015

https://www.hko.gov.hk/en/education/weather/forecasting/00477-your-weather-crystal-ball.html

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Formulation of Weather Forecasts

There are three main steps in formulating a weather forecast: understand the current weather situation; forecast the evolution of weather systems; forecast the related weather elements.

We always hear some of our friends ask, "Isn't the Observatory using supercomputers to analyze weather data? Why can't they provide hundred percent accurate weather forecasts ?" In fact, a good weather forecast does not solely depend on the analysis of weather data by a supercomputer. A rigorous weather forecast has to be formulated based on knowledge in meteorology, accurate observations and simulation of weather processes. Moreover, the effect of other factors such as topography should also be taken into full account when making a weather forecast.The weather at a place is mainly determined by the type of weather systems in its vicinity. In general, there are three main steps in formulating a weather forecast: (1) understand the current weather situation; (2) forecast the evolution of weather systems; (3) forecast the related weather elements. These three steps are elaborated as follows:Understand current weather situationThe first step in making a weather forecast is the collection of meteorological observations. These include surface and upper air observation reports, readings from automatic weather stations such as air temperature, humidity, rainfall, etc from Hong Kong and other places in the world, as well as meteorological satellite and weather radar data. Forecasters need to analyze the latest weather data carefully and determine what weather systems and relevant mechanisms responsible for the current weather situation. During the analysis, the spatial and temporal distribution of various observed weather elements will be compared with those of typical weather systems. The purpose is to sieve through huge amount of data to determine the most likely weather system, such as cold front, trough or tropical cyclone, will affect the local weather.Forecast evolution of weather systemNumerical weather prediction (NWP) products are widely used nowadays (Figure 1) in forecasting the evolution of weather systems. NWP is performed by using a computer to simulate the motion of air in the atmosphere. An NWP model is a suite of computer programs to simulate the atmospheric motions. Such a model incorporates different laws of Physics, including Newtons Laws, gas law, thermodynamics equations, fluid mechanics, etc. Different types of observational data are collected and analyzed to provide the initial conditions for the simulation. The model then simulates the evolution of the atmosphere including the future location and intensity of weather systems for the following time steps according to the laws of Physics. The simulation results will be presented as graphical forecast products to facilitate forecasters analysis and forecast of the development, dissipation, movement and intensity change of the weather systems.Forecast related weather elements Based on the NWP model outputs, forecast products for various weather elements can be generated such as in the form of a time series (Figure 2) for forecasters reference. Yet, NWP model outputs often suffer from bias. In general, such bias can be reduced using statistical method based on past performance of the NWP model. Even so, the modified model forecasts can still be biased. Weather forecasters will adjust various model forecast weather elements based on their experience and understanding of the weather systems and model characteristics to produce the final forecast.It has to be emphasized that due to limitations of the current technology like insufficiency and errors of weather observations, incomplete understanding of atmospheric processes and limitations in computer simulation etc., it is impossible to have hundred percent accurate forecast of weather elements. Nevertheless, the Observatory will continue its research effort to further improve the accuracy of weather forecasts.

Weather forecast

December 2012

https://www.hko.gov.hk/en/education/weather/forecasting/00134-formulation-of-weather-forecasts.html

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Forecasting Extreme Weather

The ensemble prediction systems of computer models can help forecasters in forecasting extreme weather to a certain extent.

Extreme weather such as rainstorms, typhoons, very hot and cold weather very often causes a major threat to life and property. Providing accurate, reliable and timely forecasts of extreme weather is a subject of great importance for meteorological agencies around the world.Nowadays, numerical weather prediction models that simulate the evolution of the atmosphere using computers, are one of the main guidance tools for weather forecasting[1]. While the accuracy of computer models increases progressively with the advancement in technology, their forecast outputs are not perfectly accurate, and their reliability in forecasting extreme weather is still relatively low. Sometimes, a certain computer model might forecast extreme weather in Hong Kong in a few days' time, yet the next update of the same model might change its predictions in which the anticipated extreme weather would no longer occur. As such, to provide reliable forecasts of extreme weather, the forecasters cannot only make reference to the latest results of computer models, but also need to rely on other types of new forecast products.We discover that the ensemble prediction systems of computer models can help forecasters in forecasting extreme weather to a certain extent. Ensemble prediction refers to generation of multiple scenarios of the weather condition at a certain time by introducing small alterations to the initial conditions input to the same computer model. The spread of these multiple scenarios reflects the forecast uncertainties and also the probability of occurrence of specific weather condition[2].Let us consider an example. Assume the forecaster is forecasting the minimum temperature on a certain day in late January. The blue line in Figure 1(a) shows the frequency distribution of historical daily minimum temperatures in late January (also called the 'climate distribution'). We can see that the frequency of occurrence for 14 to 15 degrees was the highest, while minimum temperatures below 10 degrees or above 20 degrees were relatively rare. The red line in the same figure shows the distribution of forecast minimum temperatures output from the ensemble prediction for the day of interest. We can see that the most probable minimum temperature for that day is around 13 degrees, which is slightly lower than those in the same period of the year in the past. The chance of extremely low temperatures of below 10 degrees exists but is not particularly high.When the computer prediction models provide indications of the occurrence of extreme weather in a few days' time, forecasters can also refer to the forecast outputs from the ensemble prediction system. If the ensemble forecasts also suggest a high probability of such an event, it may be necessary to remind members of the public to get prepared. Taking an example from the intense cold snap in late January 2016, Figure 2 shows the "Extreme Forecast Index" (EFI) for the period from 8 a.m. on 24 January to 8 a.m. on the following day, based on the outputs on 18 January from the Ensemble Prediction System of the European Centre for Medium-Range Weather Forecasts (ECMWF). This index broadly indicates the discrepancies between the distribution of the ensemble forecast and the climate distribution. The larger the positive or negative value of the index, the further the ensemble forecast distribution deviates from the climate distribution[3]. We can see from Figure 2 that EFI was close to minus 1 over most parts of southern China, suggesting that most of the forecast minimum temperatures were lower than the minimum value of the entire climate distribution (a situation similar to that shown in Figure 1(b) rather than Figure 1(a)). In other words, the chance of occurrence of extremely low temperatures is rather high. In the end, a temperature of 3.1 degrees was recorded at the Hong Kong Observatory on 24 January, the lowest so far since 1957. Record-breaking low temperatures were also observed over many locations in southern China.Although in the above case the ensemble prediction system was able to indicate a forthcoming episode of extremely low temperatures a few days in advance, it should be noted that the current ensemble prediction systems are still unable to capture each and every extreme weather event effectively. Particularly, for extreme weather which evolves quickly or only affects a small area (such as rainstorm), the spread among ensemble forecasts is often very large with rather high forecast uncertainty. It remains a greatgreat challenge for forecasters to forecast different kinds of extreme weather.

Weather forecast

August 2017

https://www.hko.gov.hk/en/education/weather/forecasting/00496-forecasting-extreme-weather.html

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What Is the "Location-based Rain Forecast"?

Location-based Rain Forecast service was developed based on the combination of the rainfall nowcast technology and the positioning service of the mobile phone.

Have you ever pondered: "It's time for lunch. Will it be raining outside?"; "When will the rain stop?".  Maybe you have experienced: "It's raining outside the office.  Notified someone at home to shut the windows!  Yet, it turned out that there wasn't any rain at all near home.".  The "location-based rain forecast" of the mobile application "MyObservatory" is here to answer the above questions."MyObservatory" began to provide the "location-based rain forecast" service to the public in September 2012.  This service was developed based on the combination of the rainfall nowcast technology and the positioning service of the mobile phone.  What is rain nowcast?  It is a short-term forecast (in the next few hours) of rain and it is generated automatically by computer based on the data from weather radar.  Together with the positioning service of the mobile system, "location-based rain forecast" becomes a personalised version of rainfall nowcast!Because of the short-term nature of rainfall nowcast, "location-based rain forecast" only provides the rain forecast in the coming two hours and the forecast is presented in text, time series of weather icons and animated rainfall forecast maps (Figure 1).  According to the Observatory's forecast verification result, the accuracy of the rain forecast in the first half hour was about 75%.  For more details on rainfall nowcast, please refer to the related technical report (in Chinese only).When you are out for lunch next time, you may find out if there will be rain in the next couple of hours by using the "location-based rain forecast" service.  Apart from where you are, you may also set other locations in the "location-based rain forecast" (Figure 2) to get the rain forecast for other locations to enable better arrangement of daily activities or planning of outdoor activities.

Weather forecast

December 2013

https://www.hko.gov.hk/en/education/weather/forecasting/00137-what-is-the-locationbased-rain-forecast.html

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Weather and Drone Flying

A brief introduction to common weather conditions that impact drone flying.

Drone flying has become increasingly accessible to the general public. Aspiring to become a drone pilot? Let's see how different weather elements impact drone flying.WindControlling a drone can be particularly complicated under windy condition. The wind limit a drone can withstand is model-dependent and is usually indicated in its operation manual. More energy is consumed when flying against strong headwind that is approaching the limit. As a result, the drone might get sluggish since its radio receiver isn't getting enough power supply. If the wind speed is above the threshold, a pilot might lose control of the drone as the drone would not have enough forward momentum to keep flying.On the other hand, wind shear, the sudden change in wind speed and direction over a very short distance, negatively impacts drone stability. Flying behind an obstacle with constantly changing wind can make simple manoeuvres arduous.PrecipitationWhile some drones are waterproof, most can't tolerate water. Rainwater damages electrical components and renders a drone unusable. Consider rescheduling your flight if rain is coming.VisibilityFog, mist, haze and shower hampers visibility and blocks nearby drones from view. It is advisable to keep your drone in line of sight at all times. Don't want to lose your drone? Delay a flight when visibility is low!TemperatureAs most drones can operate between 0oC and 40oC, Hong Kong temperatures are favourable for drone flying almost all year round. Nevertheless, if you're to fly your drones through snowy landscapes during your travels, be sure to keep a close watch on battery level. Low temperatures reduce battery life and would, in turn, reduce flying time and drone range.When planning your next drone flight, on top of familiarizing yourself with your drone's operation manual and local regulations, don't forget to check the weather forecast. Re-route, delay or even postpone a flight if the weather is unfavourable. Happy flying!

January 2022

https://www.hko.gov.hk/en/education/weather/weather-and-life/00664-Weather-and-Drone-Flying.html

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Happy Hiking with "MyObservatory"

How can we protect ourselves from mishap while hiking or engaging in other outdoor activities? The "MyObservatory" mobile app will enable us to get better prepared for the journey.

You may still recall that there was news reporting three young men struck by lightning while hiking in Sai Kung on 24 May 2015 when Amber Rainstorm Warning Signal was in force. Fortunately, they did not get hurt seriously during the accident. So, how can we protect ourselves from mishap while hiking or engaging in other outdoor activities? The "MyObservatory" mobile app will enable us to get better prepared for the journey. 1. Make reference to 9-day weather forecast when planning for hiking or other outdoor activities. It would be better to conduct these activities on fine days than on hot days or days with potential thunderstorms.2. Before setting off, check "Local Weather Forecast" to get hold of the latest weather conditions. Also take note of the current weather situations along the route from the "Weather Photos".3. Enable the reception of "Special Weather Tips" and a series of weather warnings so as to receive timely notification of important weather warning messages.4.On reception of "Special Weather Tips" or weather warning messages, check out "Radar Images", "Lightning Location" and "Radar & Lightning Images" to monitor the location and movement of severe weather.5. Also, activate the "Location-based Rain Forecast" notification to have an idea of when rain will commence or cease normally one to two hours before the event.The "MyObservatory" app can facilitate hikers or people engaged in other outdoor activities to better plan for their journeys so as to reduce the impact of weather. Nevertheless, in case of thunderstorms, remember to switch off mobile phone immediately, remove all metallic objects from your body, and seek shelter in buildings or low-lying areas as soon as possible.

December 2015

https://www.hko.gov.hk/en/education/weather/weather-and-life/00475-happy-hiking-with-myobservatory.html

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Meteorologist x Artist

The public may think that weather forecasting is a pure science, but meteorological experts may say that it is a perfect fusion of science and art.

Elements of weather can be part of an artistic creation in daily life.I have written a seven-word ancient poem visualising scenes from the four seasons in a painting (see Figure 1).The poem is translated as follows: In summer, cicadas live happily on tree branches in the mountain. In winter, the migratory birds resting along the paths still have a long journey south. Would anyone bother the first leaf falling in autumn? It is not until the spring river water is no longer warm again when the lapse of one year is realised. The poem suggests that everything on Earth has its distinct wonderful moments, while the gone days are hard to be noticed.Have you ever imagined that artistic sense can also be incorporated into meteorological services?I have been working in the Observatory’s Forecast Systems Division for several years, while the gone days are also hard to be noticed. My work includes managing systems to generate tropical cyclone tracks and receive global computer model data. The raw data are processed to produce different types of charts for consumption by the public and the forecasting office. Some techniques in the use of colours in painting can be utilised, for example the use of cold and warm colours. Warm colours are used to represent high temperatures or high-impact weather while cold colours are used to represent low temperatures or relatively stable weather (see Figure 2). Colour contrast can be used to represent information with opposite natures, such as the convergence and divergence of airflow, while colour gradient can be adopted to reveal changes of coherent information, etc. With this technique, hard scientific information can be presented in a clear manner.One of the basic techniques in weather analysis is to learn how to draw weather charts. When there are insufficient observations, our excellent imaginative power in the right brain is essential to complete the task. As there is a certain degree of uncertainty in weather evolution, and the accuracy of computer model forecast generally decreases with longer forecast time, how to consistently provide accurate forecasts becomes one of the biggest challenges to forecasters. With the ever-changing weather, understanding the pulse and rhythms of weather changes caused by the alternating ridges and troughs, is not only a science but also an art.The public may think that weather forecasting is a pure science, but meteorological experts may say that it is a perfect fusion of science and art. I think much better results can be achieved by a good integration of one own skills.

September 2018

https://www.hko.gov.hk/en/education/weather/weather-and-life/00517-meteorologist-x-artist.html

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A brief introduction of the World Meteorological Organization

Being one of the specialized agencies of the United Nations, the World Meteorological Organization (WMO) is an intergovernmental organization exclusively focuses on the state and behaviour of the Earth’s atmosphere, its interaction with the land and oceans, the weather and climate it produces, and the resulting distribution of water resources.

You have probably come across the “World Health Organization (WHO)” in the light of the pandemic, but have you ever heard of the “World Meteorological Organization (WMO)”, another specialized agency under the United Nations? Now let us take this opportunity to introduce it to you briefly.Being one of the specialized agencies of the United Nations, the World Meteorological Organization (WMO) is an intergovernmental organization exclusively focuses on the state and behaviour of the Earth’s atmosphere, its interaction with the land and oceans, the weather and climate it produces, and the resulting distribution of water resources. One of its major roles is to facilitate world-wide co-operation in observations, research and services in meteorology and hydrology.The WMO was established in 1950 to succeed the International Meteorological Organization (IMO). The WMO now has 193 Member States and Territories (as of 23 March 2021). Hong Kong joined IMO in 1948 and became a member of the WMO upon its establishment. The membership was retained as "Hong Kong, China" from 1 July 1997 onwards. The Director of the Hong Kong Observatory (HKO) is the Permanent Representative of Hong Kong, China with the WMO. Many colleagues of HKO have participated in WMO activities, including international conferences, symposiums and workshops etc. Currently, there are 20 HKO colleagues and three university professors in Hong Kong serving as chair, vice-chair or member of various technical commissions, standing committees, study groups and expert teams under the WMO. HKO was also invited by the WMO to develop and operate the websites “World Weather Information Service (WWIS)”[1], “Severe Weather Information Centre (SWIC)”[2], “International Cloud Atlas (ICA)”[3] and etc.The World Meteorological Day takes place on the 23rd of March every year to commemorate the entry into force of the “Convention of World Meteorological Organization” on 23rd March 1950. World Meteorological Day has a theme each year and it is "The Ocean, our climate and weather” in 2021.Lastly, do you know anything about the logo of the WMO? It is in fact a compass rose on top of the United Nations logo. You may wish to think about its meaning and to visit the WMO’s official website to know more about WMO.

April 2021

https://www.hko.gov.hk/en/education/weather/weather-and-life/00563-A-brief-introduction-of-the-World-Meteorological-Organization.html

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More about the World Meteorological Organization

At the present time, the World Meteorological Organization (WMO) consists of 193 members, including 187 Member States and six Member Territories. WMO members (hereafter Members) are distributed in six geographical regions.

The World Meteorological Organization (WMO) was briefly introduced in Educational Resource earlier. At the present time, WMO consists of 193 members , including 187 Member States and six Member Territories.WMO members (hereafter Members) are distributed in six geographical regions (Figure 1): Region I (Africa), Region II (Asia), Region III (South America), Region IV (North America, Central America, the Caribbean), Region V (South-West Pacific) and Region VI (Europe).Hong Kong, China and Macao, China are the two Member Territories within Region II. And there are 33 Member States in the region including China, Japan, Republic of Korea, Democratic People's Republic of Korea, Mongolia, Viet Nam, Thailand, Myanmar, Lao People's Democratic Republic, Cambodia, India, Bangladesh, Bhutan, Nepal, Bahrain, Islamic Republic of Iran, Iraq, Pakistan, Sri Lanka, Afghanistan, Maldives, Saudi Arabia, Yemen, Oman, Qatar, Uzbekistan, United Arab Emirates, Kuwait, Kyrgyzstan, Turkmenistan, Tajikistan, Kazakhstan and the Russian Federation.Corresponding to the geographical divisions (or meteorological regions) above, there are six Regional Associations (RAs) composed of Members in the respective regions. These RAs are responsible for coordinating meteorological, hydrological and related activities in their respective regions. As the territories of the Russian Federation and Kazakhstan straddle over Region II and Region VI, both countries are also assumed Members in Region VI. In other words, individual Member may belong to more than one RA due to geographical consideration.As mentioned earlier, there are altogether six Member Territories in the world. Apart from Hong Kong, China and Macao, China, could you name the remaining four Member Territories? The answers can be found in the WMO website.By the way, the Hong Kong Observatory (HKO) and the WMO have a long history of co-operation and the Observatory has been supporting the WMO’s initiatives and fostering regional and international co-operation for many years. Besides developing and managing the two global weather information websites, namely World Weather Information Service, and Severe Weather Information Centre, which are the world’s first websites featuring global official city forecasts and weather warnings on behalf of WMO, the Observatory is also designated by WMO as one of the Regional Specialized Meteorological Centers for Nowcasting. HKO will continue to support the meteorological and hydrological services in the Asian region on nowcasting hazardous weather and capacity development activities for contributing to the Early Warnings for All initiative.

May 2024

https://www.hko.gov.hk/en/education/weather/weather-and-life/00718-More-about-the-World-Meteorological-Organization.html

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Get to Know more Channels for Acquiring Weather Information

You can know more about channels for acquiring weather information provided by the Hong Kong Observatory in this article.

Members of the public can obtain information such as weather report, forecast and warning provided by the Hong Kong Observatory via traditional media such as TV, radio, and newspaper. Yet, the weather information might not be updated in real-time on these media owing to their limitations. To provide a convenient way to obtain weather information, the Observatory established the "Dial-A-Weather" hotline in 1985 to enable the public to obtain the latest weather information anytime and anywhere. The hotline is user-friendly. You are welcome to try it at 1878200.As Internet became popular, the Observatory established its website in 1996 to provide a more convenient way to get access to weather information. In recent years, visitor statistics on the website continues to soar. The website becomes one of the main channels for delivery of weather information. With people-oriented mindset, the Observatory developed dedicated webpages for various communities such as Senior Citizens (Figure 1), Fishermen and Schools, etc. In addition, you may customize your own weather information website (Figure 2) to suit your needs and preferences by selecting color theme and information displayed on the personalized website. If you are a PC user and do not wish to miss any important weather information while working, you are strongly encouraged to try the "Weather Wizard"(Figure 3), a user-friendly software gadget developed by the Observatory. The "Weather Wizard" retrieves the latest weather information automatically from the Observatory's website. It will alert you when there is a change in the status of the weather warning signal and so you can concentrate on your work without missing any important weather messages. Currently, version 3.0 of the "Weather Wizard" has been launched with the installation of a number of popular services. These include UV indices, regional wind and rainfall data, satellite and radar imageries, location-based rain forecast, Observatory's tropical cyclone forecast track, and the special weather tips, etc.In recent years, smartphone has become highly popular and you may already have one. The Observatory launched its smartphone application "MyObservatory"(Figure 4) in 2010 to provide personalized location-based weather service. Currently, the "MyObservatory" supports iPhone, Android and Windows Phone platforms. If you are a smartphone user, you will be kept posted on the latest weather through receiving notifications of weather warnings and Special Weather Tips. If you need to go out for a while, you can learn whether it is going to rain in the next couple of hours by looking at the "Location-based Rain Forecast". If this service is turned on, you will be notified of any rain that may come. What a convenient tool! Many online users must have used the highly popular social media websites. If you are an experienced user, you may already know that weather warning information can be acquired by following the Observatory's accounts on Twitter and Sina Weibo(Figure 5). Since 2014, users may also obtain information such as special weather tips, weather forecasts, TV weather programmes, "My Location" weather, regional weather, tropical cyclone information, satellite imagery, and weather radar imagery, etc. through the official account "香港天文台 HKO" on WeChat. Although you may not have tried all of the channels mentioned above, it is hoped that you can know more about them from this article. The Observatory will continue to innovate with a view to developing more useful services for the convenience of the users. One example is the launch of high-resolution weather programmes produced by the "Weather On-Air" team established in late 2013. The programmes are available on free TV channels, "MyObservatory" mobile app and the Observatory's channel on Youtube website. After this introduction, you may have already decided to pick one or two favourites for acquiring weather information. Try them and be a wise user of weather information!

December 2014

https://www.hko.gov.hk/en/education/weather/weather-and-life/00447-get-to-know-more-channels-for-acquiring-weather-information.html

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Weather Feng Shui?

Why do people prefer their home facing south rather than north?

Why do people prefer their home facing south rather than north?There is a geographical reason and a meteorological reason. First, the geographical reason - remember the position of the Sun in different seasons?From the diagram above, it is obvious that in wintertime, the Sun stays to the south. This gives warmth to a south-facing apartment which lets sunshine in. On the other hand, in summertime, the Sun is nearly overhead in Hong Kong. Near Summer Solstice (June 21), the Sun will even be slightly north of us. A north-facing apartment tends to be warmer if sunshine gets in.For further details about the position of the Sun in different seasons, please check the webpage at :- https://www.hko.gov.hk/en/gts/astron2014/files/2014paths-sun.pdfSecond, the meteorological reason. In wintertime, a south-facing apartment with its back towards the north is less affected by the cold northeast monsoon, while in summertime it gets breezy under the southerly winds. This is also the reason for a windbreak north of a house in the countryside.So, overall, for a south-facing apartment it tends to be warmer in winter, and breezy and not as hot in summer.What about east and west, which is better?There is no clear-cut answer to this question. In terms of wind, it tends to get windier for an east-facing apartment because the prevailing wind in Hong Kong is from the east. For a west-facing apartment, it tends to be less breezy because westerly winds are normally light and relatively infrequent.In terms of sunshine, the amount of sunshine (solar radiation to be exact) is basically the same for an east- and west-facing apartment. For an east-facing apartment, the warm weather starts earlier as the Sun rises from the east. The warmth received by the apartment's concrete is released after midday, as soon as its temperature is higher than the surroundings. The same occurs on a west-facing apartment, but the process starts at a later time, i.e. in the afternoon. The overall effect is that for an east-facing apartment, warm weather sets in earlier and ends earlier. For a west-facing apartment, it starts later and ends later, resulting in warmer nights.In terms of the air temperature, the maximum temperature normally occurs at around 2 o'clock in the afternoon. At this time, it gets drier, i.e. the humidity becomes lower, because of higher temperatures. The combination of sunshine, higher temperatures and thus drier weather help disinfect the indoor environment. A west-facing apartment is therefore preferred if you do not like mouldy walls and clothes, which occur especially during springtime.Question for the readerIf you live in the Southern Hemisphere, e.g. in Australia or New Zealand, is it going to be the same?

https://www.hko.gov.hk/en/education/weather/weather-and-life/00143-weather-feng-shui.html

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Weather and Aedes Albopictus

The Observatory and FEHD worked together to develop a Climate Aedes albopictus Abundance Model based on the data collected at Ta Kwu Ling from July 2007 to June 2009.

In the past 30 years, there has been a worldwide surge of dengue fever in tropical and subtropical regions threatening over 100 countries and endangering the health of more than 2.5 billion people.  It is estimated that dengue fever, and associated dengue haemorraghic fevers are responsible for the loss of over 20,000 lives annually.  In Southeast Asia, dengue fever is endemic in many countries including Thailand, Malaysia, Singapore, China, Indonesia, Vietnam and the Philippines.  Dengue fever is an arboviral disease transmitted by Aedes aegypti and Aedes albopictus. The latter is one of the common mosquitoes in Hong Kong.  A total of 44 ovitrap sites over Hong Kong have been set up by the Food and Environmental Hygiene Department (FEHD) to monitor the abundance of Aedes albopictus in Hong Kong.  Ovitraps (Figure 1) were firstly used in the United States in an Aedes aegypti education project in 1969, and then in Malaysia in 1979 to study the abundance and distribution of Aedes species in Penang Island.  They have been commonly used in Aedes aegypti surveillance programmes in international airports worldwide.For a particular ovitrap site, the percentage of the collected ovitraps found positive with breeding of Aedes albopictus is recorded as the Ovitrap Index (Figure 2) and used to measure the abundance of Aedes albopictus in the region represented by the ovitrap site. The Hong Kong Government announces the recorded Ovitrap Indices to the public to promote their awareness on the abundance of Aedes albopictus and to encourage their participation in preventive and control measures.The Observatory and FEHD worked together to develop a Climate Aedes albopictus Abundance Model based on the data collected at Ta Kwu Ling from July 2007 to June 2009.  The model can be used to predict the level of Ovitrap Index for planning of preventive and control measures against Aedes albopictus in Hong Kong by making use of the total rainfall and the mean air temperature over a 15-day period prior to the setting up of ovitraps and the predicted mean air temperature of the 7-day period after setting up of ovitraps at the site.  Furthermore,the effectiveness of the preventive and control measures against Aedes albopictus taken before setting up of ovitraps can be assessed by comparing the recorded and predicted Ovitrap Indices.

September 2013

https://www.hko.gov.hk/en/education/weather/weather-and-life/00142-weather-and-aedes-albopictus.html

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Weather and Influenza

The Observatory collaborated with the Microbiology Department of the Chinese University of Hong Kong to conduct a study on the relationship between weather and influenza activity in Hong Kong. The study found that in general there were more influenza A than influenza B associated admissions in Hong Kong. The two types of influenza showed a different relationship with weather conditions.

Influenza epidemics, which can mainly be classified into A type and B type, occur throughout the world.  The activity of influenza has a clear and consistent seasonal distribution in the temperate regions with annual winter peaks.  In contrast, the temporal pattern of influenza is more variable in the tropical and subtropical regions, where multiple peaks can occur at different periods within a year.  To understand the seasonal variation of influenza activity in Hong Kong, the Observatory collaborated with the Microbiology Department of the Chinese University of Hong Kong to conduct a study on the relationship between weather and influenza activity in Hong Kong. The study analysed the information of the laboratory-confirmed influenza A and B cases admitted to the Prince of Wales Hospital and the Observatory's weather data recorded at Sha Tin from 1997 to 2006.The study found that in general there were more influenza A than influenza B associated admissions in Hong Kong. The two types of influenza showed a different relationship with weather conditions. Influenza A had two seasonal peaks in Hong Kong which occurred respectively in winter/spring months (February to March) and summer months (June to July). This observation is in contrast with the situation in temperate regions where only a single winter peak is observed annually.  Influenza B also had a clear winter/spring peak, but its activity during the summer months was more variable.It was also found that cold and humid conditions were associated with a higher level of activity of both influenza A and B in winter/spring months. In contrast, hot and humid conditions in summer months were associated with a higher level of activity of influenza A, but the association was not obvious for influenza B.Furthermore, for influenza A, a shift in the relative magnitude of the two seasonal peaks was observed.  In the early part of the study period (1998-2000), the magnitude of the winter/spring peak of Influenza A was higher than the summer peak. Towards the latter part of the study period (2004-2006), the magnitude of the summer peak had become larger than the winter/spring peak (see figure).

December 2009

https://www.hko.gov.hk/en/education/weather/weather-and-life/00141-weather-and-influenza.html

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Why does food taste bland on airplanes?

The environmental conditions like humidity and pressure in the cabin could affect our sense of taste and smell of food.

People having in-flight meals always find their food not particularly tasty. Is this a result of budget consideration of the airlines or some other causes?It may be interesting to note that some physical parameters of the environment would affect our sense of tastes. According to some foreign studies, under dry and low air pressure conditions, the sensitivity of our taste buds to sweet and salty food will be reduced by 30%. Charles Spence, a professor of Experimental Psychology at Oxford University, pointed out that food and drinks taste differently in the air as compared with that on the ground. There are several reasons for this. Among them, humidity, air pressure and environmental noise each plays a role.After the plane takes off and reaches the cruising altitude (usually at about 30,000 feet high), the flight attendants will start to serve meal to the passengers. It is noteworthy that as the plane climbs, the environmental conditions like humidity and pressure in the cabin also change, and these could affect our sense of taste and smell of food.At 30,000 feet, air in the cabin becomes very dry with the humidity drops significantly to the region of 20 percent or below. Due to lack of moisture, our sense of smell reduces and this affects our judgement on food taste. At the same time, lower air pressure will also affect the sensitivity of our taste buds. As a result, our perception of saltiness and sweetness of food also drops. All these make food taste blander inside the cabin of a flying aircraft. Of course, caterers of in-flight meals would endeavour to enrich the flavour of their food served aloft in response.In the atmosphere, pressure and temperature are changing all the time. It seems like ‘magic’ to generate various kinds of weather phenomena. Be it sunny, rainy, windy, and cold or hot, they all affect our outdoor activities. In fact, changes of these physical parameters in the environment also affect humans, at indoor venues or at some dozen thousand feet above the ground.

February 2018

https://www.hko.gov.hk/en/education/weather/weather-and-life/00503-why-does-food-taste-bland-on-airplanes.html

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"Sleeping in springtime, oblivious of dawn" explained

In the spring, you often feel tired when waking up in the morning and lose momentum during the day. What causes this?

You probably have the experience that in the spring, you often feel tired when waking up in the morning and lose momentum during the day. What causes this?Though the temperature starts to rise, spring is also a season with alternate cool and warm weather, brought about by the occasional passage of a cold front from the north. Because of the weather changes, our body has to constantly regulate the internal temperature. This requires more energy and makes us feel tired and sleepy. It is also humid and foggy in the spring. Fog forms when the moist and warm air from the south crosses the relatively cool waters in the South China Sea. The moisture blocks evaporation of sweat from our body, making things worse. Furthermore, the mixing of cold and warm air brings continuous rain and overcast skies. No wonder some people feel depressed and become inactive.After the Spring Equinox, the Sun originally overhead in the southern hemisphere moves north gradually. Days become longer and nights shorter in the northern hemisphere. For some people this may mean a shorter sleep and greater difficulty in getting up from bed.Therefore, it is important to get sufficient sleep and exercises in the spring, for a sound body to adjust to the march of seasons.

https://www.hko.gov.hk/en/education/weather/folklore/00078-sleeping-in-springtime-oblivious-of-dawn-explained.html

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"Morning thunder seldom lasts till noon" explained

Morning thunderstorms are generally related to the approach of a weather system. Weather systems are dynamic and thunderstorms will move with such systems.

The weather folklore "Morning thunder seldom lasts till noon" means that thunderstorms (Please click here to read "What are thunderstorms?") in the morning usually affect a place for only a few hours and seldom last till the afternoon. To understand this, we need to know how morning thunderstorms form.Thunderstorms are formed by the uplifting of warm and humid air. There are many factors that lead to the uplifting. Some main ones are solar heating, proximity of low pressure troughs and convergence of two different air streams.Thunderstorms caused by the uplifting of air under solar heating will only be formed when the temperature is high. Hence such "heat" thunderstorms usually appear in summer afternoon.In contrast, thunderstorms caused by weather systems, such as a front or trough, can occur at any time of the day. A front is a transition area between the cold and warm air masses. Where the two air masses meet, unstable weather results because of significant differences in temperature, humidity, wind direction and speed between them. Along the front, the warmer and lighter air will then ride on top of cooler air, resulting in rain and thunderstorms. A trough is an area of low pressure where air from higher pressure converges and is forced to lift upwards to form thunderstorms.From the above discussion, we can see that morning thunderstorms are generally related to the approach of a weather system (front or trough). Weather systems are dynamic and thunderstorms will move with such systems. When a weather system moves fast, the thunderstorms will move away too. Hence the phenomenon of "Morning thunder seldom lasts till noon" (Figure 1 and 2).Weather folklore usually reflects people's experience in a particular place. The reader should note that it may not be applicable to other places.

https://www.hko.gov.hk/en/education/weather/folklore/00077-morning-thunder-seldom-lasts-till-noon-explained.html

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"Mackerel sky, not twenty-four hours dry" explained

Mackerel sky, not twenty-four hours dry describes the deterioration of weather after the appearance of cirrocumulus clouds.

'Mackerel sky' refers to a state of sky with extensive clouds that look like fish scales in a bright day. These clouds are small and white, and usually line themselves up in groups in a regular pattern. They resemble the small ripples created by a gentle breeze on water surface. In meteorology, they are called 'cirrocumulus' clouds.'Cirrocumulus clouds' are high clouds, generally with a cloud base at above 6000 metres where the temperatures are several tens of degrees Celsius below zero. They are composed of tiny ice crystals and are thin and translucent. Since there is not much water vapour at that altitude, the growth of ice crystals in the cloud is limited and the crystals can remain suspended atop some ascending air. The ice crystals may occasionally fall from the clouds, but they tend to evaporate on the way down and seldom reach the ground.The occurrence of cirrocumulus clouds indicates the distant presence of a convergent zone such as a pressure tough or a cyclone in lower levels; or a frontal zone where cold air meets moist warm air. Vigorous convection and lifting motion bring water vapour up in the sky where the water vapour sublimes into ice crystals, forming the high clouds. Under the influence of atmospheric waves which are often present at high altitudes, the clouds line themselves up and move with the upper level winds, forming the cirrocumulus clouds having a fish-scale appearance.'Mackerel sky, not twenty-four hours dry' describes the deterioration of weather after the appearance of cirrocumulus clouds. Although it may be bright at the beginning, the weather will deteriorate in half a day's time as the cyclone or front approaches. As they pick up more moisture, the cirrocumulus clouds thicken and extend downward to become low-level clouds which bear rain.Observation of cloud change is an important tool in weather forecasting. The folklore of 'Mackerel sky, not twenty-four hours dry' reflects the experience and wisdom passed down by people over the years.

https://www.hko.gov.hk/en/education/weather/folklore/00080-mackerel-sky-not-twentyfour-hours-dry-explained.html

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"Red sky in the morning, sailors take warning. Red sky at night, sailors delight." explained

Reddish sky around sunrise means a lot of water vapour already present in the atmosphere. As clouds come in from the west, rainy weather is thus expected.

Sometimes the sky appears very colourful around sunrise and sunset. At these times, the sun shines from a low angle. Sun rays have to go through a thick atmosphere, resulting in scattering by water vapour, water droplets, dust particles etc. in the air. Light with shorter wavelength, e.g. cyan, blue or purple, is scattered away, leaving red, orange and yellow, turning the sky reddish.In mid-latitude region, storms and rains usually move from west to east. A reddish sky around sunrise means a lot of water vapour already present in the atmosphere. As clouds come in from the west, rainy weather is thus expected. This explains why "Red sky in the morning, sailors take warning".A reddish sky near dusk means that weather has improved in the upstream area to the west. Sunlight scattered by the atmosphere is reflected by clouds, making the sky red. As the clouds overhead have moved eastward, the weather is expected to improve. That explains why "Red sky at night, sailors delight".The relationship between red sky and weather is rather complicated in real life. One example is that near dusk, the sky can become reddish temporarily after rain. Besides, especially in low latitude areas such as Hong Kong, weather systems sometimes move from east to west, e.g. typhoons, and not from west to east. Therefore, one cannot fully understand how the weather changes simply by just looking at the colour of the sky. To make an accurate weather forecast, it is necessary to take account of other weather factors and carry out a full analysis.

https://www.hko.gov.hk/en/education/weather/folklore/00082-red-sky-in-the-morning-sailors-take-warning-red-sky-at-night-sailors-delight-explained.html

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"The higher you climb, the colder it gets" explained

Roughly, the temperature drops by about 6 degree Celsius for every 1000-metre rise in altitude.

A mountain covered with snow all the year round gives people a feeling of 'The higher you climb, the colder it gets'. Why is it colder at high ground? Let us first look at how the air gets heated up.The energy needed to heat up the Earth's atmosphere comes mainly from the Sun. It includes direct sunlight and the heat re-emitted from the ground after the latter has absorbed solar energy. In respect of direct sunlight, the air at high altitudes, being closer to the Sun, is supposed to receive more solar energy. However, it actually absorbs solar energy poorly. The reason why the air temperature rises is due mainly to the heat re-emitted from the ground through various physical processes (including radiation, convection and turbulence).So, the higher you go up, the colder it gets. But how cold would it get? Roughly, the temperature drops by about 6 degree Celsius for every 1000-metre rise in altitude. As an example, at Mount Everest, the highest mountain in the world, where the altitude is about 8 800 metres, the temperature drop will be about 50 degrees Celsius. Even with a summertime temperature of 19 to 28 degrees near the sea level, it will get down to 20-30 degrees below zero at the mountain top. It is even worse in winter, when the mercury could fall to minus 50!

https://www.hko.gov.hk/en/education/weather/folklore/00079-the-higher-you-climb-the-colder-it-gets-explained.html

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The kind of cloud brings the kind of rain

The kind of cloud brings the kind of rain.

The cumulus weather described by a Chinese poet as "the summer showers that develop over every other paddy field can only wet one of a crow's wings" is, in scientific terms, as follows. In day time, especially in mid-summer, the ground heats up under the sun and spurs convection in the air. This lifts moisture up the atmosphere aloft, which gradually develops into isolated, floating cumulus. Such clouds, with a flat base, a protruding top and the look of marshmallow, are commonly known in Chinese as "Cloud Dumplings".Cumulus clouds may not bring rain in their early stage, but if they are to develop further, rain will fall sooner or later. The kind of rain brought by the cumulus is called showers. Showers are distinguished by their limited rain area under the cloud, and their short duration. When encountering showers, you may as well find shelter and wait till the cumulus passes away, instead of getting yourself soaked. The literati dramatize the isolated nature of showers, describing it as "the summer showers that develop over every other paddy field can only wet one of a crow's wings."If the cumulus continues to grow, its top will become very bright in the late stage, looking like a cabbage and reaching a height beyond 10,000 m. By then it is called cumulonimbus. The rain at this stage will be torrential, possibly with thunderstorms.There is also a poetic description about the weather associated with stratiform clouds: "With dark clouds overshadowing the sun, there will be rain after rain at night." Stratiform clouds are layered clouds formed mainly by uplifting of air on a large scale, commonly seen near a front, which signifies the interface of warm, moist air climbing up cold air on encountering the latter. As the warm, moist air goes up, the moisture therein will condense into extensive stratiform clouds. The rain from thick stratiform clouds usually lasts longer than showers although the intensity is lighter.Winter in Hong Kong is often affected by cold air coming down from the north. When the cold air approaches Hong Kong, thick, extensive clouds obscuring the sky will rise from the horizon, moving over gradually. As the cold air arrives, there may be extensive rain lasting for a fairly long time. The phenomenon coincides with the folklore that says, "With dark clouds overshadowing the sun, there will be rain after rain at night."The kind of cloud brings the kind of rain. When you see these special clouds next time, watch how they evolve in shape and their areal coverage. You may as well do a weather prediction on the spot.The Hong Kong Observatory disseminates real-time weather pictures taken at various places in Hong Kong at fixed intervals. Those interested please surf the following website: https://www.hko.gov.hk/en/wxinfo/ts/index_webcam.htm

https://www.hko.gov.hk/en/education/weather/folklore/00081-the-kind-of-cloud-brings-the-kind-of-rain.html

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How Chatbot “Dr Tin” is Trained?

The article briefly describes the working principle behind the chatbot "Dr. Tin", thereby introducing the application of AI technology in public weather service.

The Hong Kong Observatory (HKO) launched in February 2020 the chatbot service "Dr Tin" which employs artificial intelligence (AI) to automatically answer a series of weather and astronomy related questions such as current weather, weather warnings, weather forecast, tidal information, Hong Kong standard time, weather forecasts of major world cities and sunrise or sunset time. The chatbot was well received with a monthly average of about 120 thousands of dialogues, and a rating of 4 or above out of 5 since launched. So, what is the working principle behind the chatbot? How does Dr Tin understand and respond to questions?First of all, Dr Tin will classify the questions. HKO will prepare a batch of sample questions and their respective intents. Then a computer program will generate a learning model by applying supervised learning on the sample questions.For example, HKO prepares a batch of questions related to temperature, like "What is the temperature in Causeway Bay?", "What is the temperature now?", "Is it hot", etc. and label them with "temperature" intent. Similarly, HKO also prepares questions related to other HKO’s services and labels them in advance to create a training data set.In order to enhance the training efficiency, tokenization will be applied to the question with the help of a tokenizer of Natural Language Understanding (NLU) engine.Obviously, we adopt the (v) tokenization as all tokens in this tokenization are meaningful. The technical detail of tokenization depends on the NLU engine chosen. Then, we have a set of tokenized questions and their respective intent, i.e. a well-prepared training data set. With the use of AI program, a learning model for classifying questions can be developed.When there is a question, Dr Tin will use the learning model to find out a number of intents that the enquiry most probably belongs to, and assign the question to the intent with the highest score. For example, if someone asks "Today’s temperature of Sha Tin?", Dr Tin will first find out the tokens of the question, and use the prepared learning model to find out the score of the question associated with each intent. The intent possessing the highest score would be taken as the intent of the question:Since the intent "temperature" has the highest score, Dr Tin will take "temperature" as the intent of the question.Finally, the chatbot will further extract entities from the questions to prepare an answer. In the example above, "Sha Tin", "today" and "temperature" are the values of entities "location", "time", and "temperature" respectively. As "today" is a  time period, Dr Tin will find out today’s maximum and minimum temperature of Sha Tin to prepare the answer for the user.

October 2021

https://www.hko.gov.hk/en/education/weather/data-and-technology/00569-How-Chatbot-Dr-Tin-is-Trained.html

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A Deep Learning Journey of Chatbot “Dr. Tin”

The article briefly describes the principles and training methods of the Chatbot Dr Tin's language model, thereby introducing how to optimise the Chatbot by deep learning.

Last time, we have talked about How Chatbot “Dr Tin” is Trained?.After joining in the HKO team, “Dr. Tin” has been bothered by some “mysteries”. Does “Your apple is broken!” mean the fruit is broken or an Apple phone is broken? What does “No. 8 on Monday, No. 10 on Wednesday!” actually mean? The field of deep learning is rapidly changing. “Dr. Tin” realises that it’s about time for a self-enhancement to improve his learning model.Right now, when “Dr. Tin” receives a question from user, he will first tokenise the sentence, and match each token with a vector called “embedding” according to the language model used. An embedding vector consists of solely numbers. These numbers represent the “features” of the token, just as how each token has a “meaning” in linguistics.Vectors can be visualised as arrows of different lengths and point at different directions. For tokens having similar meaning in linguistics, they should have similar vector representations. This means their arrows will be of similar length and direction. Vectors can be added or subtracted from each other. In ideal case, a perfect language model should be able to assign embedding vectors such that the below calculation is valid:<king>-<man>+<woman>=<queen>Due to polysemy, the same word or token can have different linguistics meanings, and therefore different possible vector representations. So how could we identify the correct one? This is where the concept “attention” comes in handy.Suppose a sentence is tokenised as the following:Will / typhoon signal / 8 / be / issued / on / June / 8 / ?The “8” at the end of the sentence has more relation with “June”, since they are both describing date and time. The “8” in the middle of the sentence, however, is more linked with “typhoon signal” and “issued”, because ... you know it.So, in order to train a language model that can take these connections into consideration, “Dr. Tin” chose a training method called “Masked Language Modelling”. As the name suggested, a portion of the tokens in the sentence will be masked during training. The model is required to learn and calculate which other token(s) they should “pay attention” to, in order to deduce correctly what is being masked. For the below training set:Will / typhoon signal / [MASK] / be / issued / on / June / 8 / ?When / will / typhoon signal / [MASK] / be / issued / ?Is / typhoon signal / [MASK] / issued / ?The model would be able to learn that “typhoon signal”, “issued” and the word being masked (“[MASK]”) seems more related. (As for which typhoon signal is being issued...hmm.)This is called “bidirectional encoder”, in which the tokens on both sides of the [MASK] token are used to determine the encoding of the [MASK] token. It is similar to how we read. To understand the meaning of a word in the sentence, you need to understand the sentence as a whole. After such training, the language model will be able to assign an embedding vector to each token that better represent its features and meaning in the sentence.Which also means that “Dr. Tin” will be able to identify the meaning of “No. 8” in different context!In other words, Masked Language Modelling is very similar to fill-in-the-blanks exercises. Deep learning is indeed computer algorithms developed by taking reference to human’s learning pattern. With a more localised or field-specific training dataset, “Dr. Tin” would be able to understand enquiries in localised or field-specific context better.However, building a language model from scratch requires tens of millions of training sentences. The whole training process would take up to a week’s time to finish. In order to be cost-effective, “Dr. Tin” chose to do “fine-tuning” instead. There are plenty of open source pre-trained models online nowadays. These open source language models are trained with some big enough datasets beforehand, which give the models general knowledge on sentence structures, grammar or common vocabulary. “Fine-tuning” means to use these language models as basis to a train a fine-tuned model. Only a small amount of new training data of a specific field, such as Cantonese or meteorological terms, is needed for the model to learn some specific knowledge in a few hours. For example, building a new language model from scratch is like teaching an illiterate baby to understand this passage. “Fine-tuning” is like how you are reading this passage now.In the journey, “Dr. Tin” also discovered many more deep learning methods that may train him to be an interactive voicebot. It would seem that using techniques similar to Masked Language Modelling may help to identify homophones or homographs too. Keep an eye on a smarter “Dr. Tin” on his continuous journey!

October 2022

https://www.hko.gov.hk/en/education/weather/data-and-technology/00682-A-Deep-Learning-Journey-of-Chatbot-Dr-Tin.html

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How is the Hiking Trail Weather Service made?

In early 2020, the Observatory collaborated with the Agriculture, Fisheries and Conservation Department (AFCD) to provide weather information at various popular hiking hotspots. Upon scanning the weather information QR code located at the information board of the country parks, hikers can obtain the latest weather information, including 2-hour rainfall and 1-hour lightning nowcast of nearby hiking routes.

In early 2020, the Observatory collaborated with the Agriculture, Fisheries and Conservation Department (AFCD) to provide weather information at various popular hiking hotspots. Upon scanning the weather information QR code located at the information board of the country parks, hikers can obtain the latest weather information, including 2-hour rainfall and 1-hour lightning nowcast of nearby hiking routes (Figure 1). This service is different from the conventional 9-day weather forecast that provides a summary of the daily weather conditions. The new Hiking Trail Weather Service provides users with hourly automated weather forecast during the hiking trip, and the forecasts are refined to more than 60 major hiking trails in Hong Kong.To provide the latest weather information for hiking routes near the notice board with QR codes in country parks, we have to find out all the possible hiking routes nearby reachable in two hours. To find these routes, we need to first understand the hiking route data. There are more than 60 hiking routes in the dataset, and each route consists of a number of short straight line segments. Each segment has the coordinates of the start and end points; and each route contains information such as its length and the approximate hiking time required. However, there is no information on the relation between different routes in the dataset like overlapping sections, which is important for locating all possible hiking routes. Taking the segment of the hiking routes on Lantau Island near Pak Kung Au as an example, we find that there is an overlap between the Phoenix Trail and the South Lantau Country Trail on map (Figure 2(a)). As there is no relationship between these two routes as seen from the raw data of the hiking route dataset, the possible routes could only be two. However, if the overlapping sections are considered, there will be more than two possible routes (For instance from the South Lantau Country Trail to the Phoenix Trail in Figure 2(b)).With the information of overlapping sections, we are able to identify all possible hiking routes which is reachable within two hours near the QR code. To find all possible routes, one of the methods is to draw all the routes and the location of the QR code on the map, identify all the routes which are reachable within two hours near the QR code, and then count the number of the possible routes while considering the overlapping sections. The aforementioned method is feasible, yet not efficient. In particular, when there are changes or new additions to the hiking routes, it needs manpower to re-calculate. To automate this procedure, we can employ some tools in Graph Theory to find all possible hiking routes reachable within two hours. Let’s take the aforementioned Phoenix Trail and the South Lantau Country Trail near Pak Kung Au as an example again. Since there is no relationship between the two routes in the raw data, they are expressed in Graph by two non-intersecting lines (Figure 3(a)). If overlap is considered, there will be more than two possible routes (Figure 3(b)). When we draw the Phoenix Trail and the South Lantau Country Trail near Pak Kung Au on a map (Figure 4), it will take some time to find out the possible routes manually. However, it is easy to figure out the number of possible routes automatically if we use a graph to express (Figure 5). Therefore, after converting the data into a graph and using algorithm in graph theory, all the routes that are reachable within two hours can be calculated automatically.After identifying the possible routes, the "Hiking Trail Weather Service" can be made by putting together the automatically generated rainfall and lightning forecast data.Through the above brief introduction, we hope one can understand more about how the "Hiking Trail Weather Service" is prepared. Before hiking, please visit the "Hong Kong Hiking Trail Weather Service" webpage to plan your trip, and scan the weather information QR code on the information board of the country parks during your hiking trip to get the latest weather information.

September 2020

https://www.hko.gov.hk/en/education/weather/data-and-technology/00551-how-is-the-hiking-trail-weather-service-made.html

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Application of 3-Dimensional Animation Technology in Weather Observation

The Hong Kong Observatory has recently set up a number of high-resolution cameras at the Hong Kong International Airport with a view to automating cloud observations with the application of computer stereo vision technology.

Many weather elements can now be measured by advanced meteorological instruments, but a few of these elements such as identification of cloud types have still not yet been automated. Recently, the Hong Kong Observatory has conducted a trial to apply computer stereo vision technology in assisting weather observers in routine cloud observation.Humans, through their left and right eyes, simultaneously capture two visual images of an object. The brain, through processing the small difference between the two images, generates a 3-dimensional and spatial perception (e.g. distance and depth) of that object. This is referred to as parallax (Figure 1).Readers can appreciate parallax via the following method: Place your finger in front of your nose, and take turns to open and close your left and right eyes. You should be able to notice the small difference between the two images seen by the two eyes. Furthermore, the closer the finger from the nose, the larger the parallax is (Figure 2).The computer stereo vision technology is based on the same parallax principle as in the case of human eyes. By using two or more cameras to capture multiple images of an object from different viewpoints and computing the parallax between the corresponding points of the object, its 3-dimentional information can then be extracted. 3D films also make use of the same principle. Two cameras are used to record information when shooting. 3D glasses are then used to separate the screenshots into two, showing the 3D effect. Since parallax increases with the separation between the two cameras, cameras separated over a larger distance can more accurately analyse the 3-dimentional structure of distant objects.Weather observers usually infer the approximate height of cloud base by comparing it with reference landmarks with known heights. Although ceilometers can help, they are point measurements and thus the observations are very localized and not representative of the cloud base beyond the observing point. Besides, observers are not able to make observations all the time. Even if real-time weather photos are used, they are 2-dimensional and cannot provide important 3-dimentional information such as cloud base height. Cloud base height is extremely important for classifying cloud types, especially for identifying cloud types associated with high-impact weather (e.g. cumulonimbus clouds). Moreover, when the cloud base is very low, it may directly affect whether the pilot can see the runway of the airport. As such, cloud base height is very important to aviation safety. The Hong Kong Observatory has recently set up a number of high-resolution cameras at the Hong Kong International Airport with a view to automating cloud observations with the application of computer stereo vision technology. Preliminary trial shows that this technology can be used to construct the 3-dimensional image of the mountains and cumulonimbus clouds in the vicinity of the airport (Figure 3). However, development of this technique is still at an initial stage, and further research and evaluation are needed before it can be applied operationally in the future.

January 2021

https://www.hko.gov.hk/en/education/weather/data-and-technology/00558-Application-of-3-dimensional-animation-technology-in-weather-observation.html

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Artificial Intelligence Overlooks the Storm

The HKO utilises artificial intelligence technique to simulate radar-like satellite image, namely the satellite-derived radar reflectivity, and extended the monitoring area for significant convection. The Observatory also produces satellite-generated radar reflectivity images covering the Hong Kong Flight Information Region (FIR) for reference by the Air Traffic Control Centre of the Civil Aviation Department, facilitating the air traffic controllers to understand the impact of severe convective weather on key areas over the FIR.

Significant convective weather such as thunderstorms will bring not only lightning and heavy showers, but can also bring squalls, turbulence and hailstones, affecting flight safety. Weather radar is a very effective tool for monitoring severe weather such as tropical cyclones, thunderstorms and heavy rain near Hong Kong (Figure 1).However, a radar could typically cover ranges up to 500 km only, while aviation users would need weather information over a much larger area. Therefore, there is a need to extend the area coverage of radar networks, in particular over the seas. Another useful tool for weather monitoring is the geostationary meteorological satellites that could provide continuous high-resolution optical image covering a large range of the Earth (Figure 2).The HKO utilises artificial intelligence technique to simulate radar-like satellite image, namely the satellite-derived radar reflectivity. An artificial neural network (ANN) model (Figure 3) is developed on composite radar image over China to retrieve equivalent radar reflectivity from the Himawari-9 satellite data from JMA with a view to enhancing significant convection nowcasting services. HKO has also applied the same algorithm to GEO-KOMPSAT-2A (GK2A), the new generation geostationary meteorological satellite of the Korea Meteorological Administration (KMA) for even larger coverage of satellite-generated radar reflectivity (Figure 4).In support of air traffic flow management, HKO also produces satellite-generated radar reflectivity images covering the Hong Kong Flight Information Region (FIR) (Figure 5) for reference by the Air Traffic Control Centre of the Civil Aviation Department, facilitating the air traffic controllers to understand the impact of severe convective weather on key areas over the FIR.

March 2024

https://www.hko.gov.hk/en/education/weather/data-and-technology/00712-Artificial-intelligence-overlooks-the-storm.html

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From Machine Learning to Nowcasting

Artificial intelligence (AI), machine learning and deep learning have been evolving rapidly in various fields during the recent years. For heavy rain forecasts, is it possible for AI to replace weather forecasters?

Artificial intelligence (AI), machine learning and deep learning have been evolving rapidly in various fields during the recent years. For heavy rain forecasts, is it possible for AI to replace weather forecasters?Weather changes are governed by the laws of physics. Meteorological agencies around the world use a variety of instruments to monitor weather elements – such as wind, temperature, humidity and pressure measured directly from surface weather stations, radiosondes and aircraft; rainfall intensity and locations from Doppler weather radar; and cloud images from multi-spectral imagers on-board of meteorological satellites. Forecasters make use of massive real-time weather data to analyse and monitor current weather condition and its trend. Meanwhile, the observational data are input to the numerical weather prediction models running on computers to project weather changes. The computer model outputs are provided to forecasters for reference or utilized directly to generate automatic forecast products. However, the accuracy of the computer model forecast is limited by many factors. In particular, there remains considerable room for improvement in forecast of fast-changing and small-scale weather phenomena such as heavy rain and thunderstorms[1].The meteorological communities have been applying AI for decades in support of analysis and forecast of weather. In SWIRLS nowcasting system developed and operated by the Observatory, the core component is indeed based on object tracking method from computer vision technique of AI. Radar echoes are forecast by extrapolating rain areas and thunderstorms for the next few hours. However, the intensity of the rain area and motion field are assumed unchanged in this method[2].In reality, development of rainstorm can be extremely rapid, where movement and intensity change of the thunderstorms and rain areas vary significantly within a few minutes or so. If the characteristics of the variations in thunderstorms and rain areas can be obtained from radar images during the past heavy rain situations, they will be useful to improve forecasting the growth and decay of rainstorm. The deep learning method makes use of radar image data over past many years in training to analyse the characteristics of the rainfall in space and time. The deep learning method also looks for optimized algorithms to establish an effective neural network to register features about the change of radar rainfall intensity and movement during the past weather processes. When new radar image is available, the deep learning model will identify rain areas from this current image and compare their changes over the recent radarscopes. The features stored in neural network are then applied to generate rainfall distribution and intensity forecast for the next couple of hours[3] ( Figure 1).In recent years, the Observatory has been cooperating with universities to develop deep learning technique to enhance rainfall nowcasting. From the results of real-time trial, the deep learning model has considerable impact on improving the performance of heavy rain forecast in the next one to two hours. At present, the accuracy of machine learning in forecasting rainstorms or other weather phenomena relies on appropriate computation models to identify the characteristics or features of weather changes. The training process requires a large amount of past data. More abundant and diversified training datasets can improve the capability of neural network in identifying different weather scenarios from the data. The latest research on deep learning models has also actively used or incorporated physical laws governing weather changes to improve heavy rain forecasts, or to explore applications in forecasting extreme weather. The reliability and stability in performance of deep learning forecast products remain to be verified. Moreover, AI, machine learning and deep learning can undoubtedly provide more valuable reference and forecasting tools derived from meteorological big data that can support forecasters to enhance or develop meteorological forecasts and warning services.

September 2020

https://www.hko.gov.hk/en/education/weather/data-and-technology/00552-from-machine-learning-to-nowcasting.html

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Meteorological Information Sharing in the 21

Meteorological observations, forecast products and warnings, climatological and hydrological as well as earthquake related information are exchanged in real-time over the globe under the Global Telecommunication System (GTS) of the World Meteorological Organization (WMO).The article gives an overview of the global data exchange infrastructure in the World Meteorological Organization.

Meteorological observations, forecast products and warnings, climatological and hydrological as well as earthquake related information are exchanged in real-time over the globe under the Global Telecommunication System (GTS) of the World Meteorological Organization (WMO). The GTS was established under the World Weather Watch Programme of WMO in 1950s for the free exchange of meteorological information. It is a coordinated global system of point-to-point telecommunication facilities and standardised arrangements for timely collection and exchange of the aforementioned information. Nowadays, through the GTS, meteorological services can exchange data and information using various telecommunication methods, such as satellite communication, dedicated circuits and the Internet.By late 1990s, as technology advanced over the decades, the meteorological community recognised the need for a modernised and integrated information system to enable the seamless exchange of weather, climate and water information among the community, which is of top importance to weather hazard management, disaster risk reduction, and climate monitoring and research. The development of the WMO Information System (WIS) began in the early 2000s under the collaboration, consultation and innovation of WMO Member countries. WIS is a coordinated global infrastructure for all WMO Programmes to meet the requirements for routine collection and automated dissemination of data and products, as well as data discovery, access and retrieval services for all weather, climate, water and related data produced by meteorological centres and Member countries in the framework of WMO. It has revolutionized the way data and information are shared, leading to improved weather forecasts, better support for early weather warning systems, and climate monitoring and research activities.Alongside with the continuous growth of observational data and data created by numerical weather prediction systems, the constant increase in the flow of data to exchange and information to be disseminated to various users poses impact to the telecommunications networks. At the same time, meteorological related data was more common in cooperated use with other information in the society. More users wish to combine mobile, cloud and social media platforms to access a wider range of information sources and to collaborate in new and different ways. On the contrary, assessing the impact of weather-related hazards requires meteorological data to be combined with other socio-economic data, particularly those from crowd-sourcing. With the advancements in IT and communication in recent years, the second generation of WIS (WIS 2.0) builds upon the foundations of its predecessor while embracing the latest advancements in information technology and data management.In WIS 2.0, user-centric is the key objective that takes into account the variety of different aspects of meteorology and responds to the growing demands for weather information in the society. The use of Web or cloud-based infrastructure as a data sharing platform greatly enhances data resilience and ensures uninterrupted access to critical weather information. Interfacing with popular web search engines makes the discovery of information much easier, and is particularly handy for the general public. The adoption of application programming interfaces and web services powered by industry standard messaging protocols provide effective means for process and exchange of data between machines over the web (Figure 1). All the above not only enables the seamless discovery and exchange of weather, climate and water information, but also promotes global collaboration on meteorological and climate research and analysis.The Hong Kong Observatory (HKO) has recently set up a WIS 2.0 system on a public cloud and interfaced with the WIS 2.0 system operated by the China Meteorological Administration (CMA). Collaboration and partnership with CMA to develop and implement WIS 2.0 was underway.

December 2023

https://www.hko.gov.hk/en/education/weather/data-and-technology/00704-Meteorological-information-sharing-in-the-21st-century.html

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Exploring Potential Applications of Cloud Technology in Weather Services

The article discusses the potential applications of cloud technology in the processing of weather big data and generation of weather products.

The Hong Kong Observatory (HKO) exchanges weather data with meteorological authorities over the world through the Global Telecommunications System (GTS) of the World Meteorological Organization. In 1969, HKO worked with Japan Meteorological Agency to establish the Hong Kong-Tokyo dedicated regional telecommunication circuit for real-time exchange of weather data. In subsequent years, HKO collaborated with more meteorological authorities, including the China Meteorological Administration (CMA), the Thai Meteorological Department and the Macao Meteorological and Geophysical Bureau to exchange meteorological data. This further developed the connections between HKO and other meteorological authorities to support the growing needs of international meteorological data exchange. In the 1990s, HKO took advantage of the advance in information technology and started to exchange meteorological data via the internet. With the development of internet technologies, the transmission of meteorological data becomes faster and more convenient, the network capacity is also upgraded incrementally. All these bring remarkable benefits to the transmission of a large volume of data, such as numerical weather prediction products and data as well as the data from meteorological satellites and weather radars.In the era of big data, HKO receives more and more meteorological and non-meteorological data from different sources with the types and volume of data surged significantly in recent years. At present, more than 20TB of meteorological data are processed by HKO every day using numerous computer systems to generate different types of reports, charts, and weather maps for forecasters’ reference (Figure 1 and 2). In response to the rapid growth of meteorological big data, limitations on the bandwidth of telecommunication leased lines, stability of internet, scalability and availability of computer systems, and etc., we are looking for a more effective way to process the vast amount of meteorological data.With cloud computing services and technology becoming more developed in recent years, HKO actively explores the potential applications of cloud technology in our operations. One of the areas is to explore the feasibility of applying cloud technology in data exchange. In late 2020, HKO and CMA jointly set up virtual machines on the cloud platform in Hong Kong and Beijing respectively, and connected with each other via high performance internal network to test data exchange between the two places. After testing, the results revealed that cloud technology can effectively support data exchange between Hong Kong and Beijing. Apart from efficiency, the high scalability and availability of cloud platforms fulfill the fundamental requirements of operational systems. Furthermore, one of the development directions that HKO is working on is to process and produce weather data and products directly on cloud which could be transferred back to HKO upon completion.

October 2021

https://www.hko.gov.hk/en/education/weather/data-and-technology/00660-Exploring-Potential-Applications-of-Cloud-Technology-on-Weather-Services.html

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Exploring Big Data for Weather-Related Societal Impact Assessment

The Hong Kong Observatory has been utilising big data to relate weather to its impact. This enables the weather forecasters to assess the society impact of inclement weather.

The Hong Kong Observatory (HKO) has been utilising big data to relate weather to its impact. This enables the weather forecasters to assess the society impact of inclement weather. One case in question is the approach of super typhoon Mangkhut in September 2018. Impact information such as fallen trees or flooding from incident reports disseminated by the government was automatically decoded and overlaid with weather information on a map. This provides a more comprehensive picture for impact analysis of weather by forecasters.Tropical cyclone Mangkhut formed over the western North Pacific on 7 September 2018 and intensified into a super typhoon four days later. It moved across the northern part of Luzon and entered the South China Sea on 15 September. Mangkhut edged closer to the coast of Guangdong rapidly on 16 September. Mangkhut came very close to Hong Kong, with its centre just about 100 km from the HKO. As Mangkhut made landfall near Taishan, Guangdong, local winds gradually veered from northeasterlies to southeasterlies.The hilly terrain of Hong Kong acted as an effective shelter. When north to northeasterly winds prevailed in Hong Kong on the morning of 16 September, winds in the urban areas on the lee side of the hills were relatively weak (the red ellipse in Figure 1). Having said that, high winds at gale force or above were affecting high ground and offshore areas. Reports of fallen trees were received. When local winds veered gradually to the east and strengthened further in the afternoon, most of the previously sheltered places became fully exposed to the ferocious winds. The cumulative number of fallen tree reports increased sharply in only 4 hours (Figure 2). Figure 3 shows the time series of the number of reports of fallen trees at hourly intervals.Comparison of Figure 1 and Figure 2 indicates clearly that the hilly terrain of Hong Kong is an effective shelter under north to northeasterly wind regimes, and the sharp increase in the number of the fallen trees was due to the strengthening of winds while veering to the east. It is important to note that the highest and the broader peak in the number of the fallen trees (around 08 HKT on 17 September) actually occurred later when winds weakened. Hence the public should stay on the alert of the danger of high wind impact even when a typhoon has departed.

September 2018

https://www.hko.gov.hk/en/education/weather/data-and-technology/00515-exploring-big-data-for-weatherrelated-societal-impact-assessment.html

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Weather Services and IT Security

Secured IT systems are the building blocks and enablers for timely dissemination of weather forecasts and warnings, which is essential to ensuring quality services.

The mission of the Hong Kong Observatory is to provide people-oriented quality services in meteorology and related fields, and to enhance the society's capability in natural disaster prevention and response, through science, innovation and partnership. To fulfill this mission, the Observatory has made use of computing systems since 1968 to provide solid platform in support of the delivery of these services. Advanced information technology (IT) systems, say high performance computers as an example, were introduced as early as 1999 to process vast amount of meteorological and geophysical data for provision of weather and related services.People using mobile devices to acquire up-to-date information from the Internet have become very popular nowadays. The Observatory has introduced different web services such as MyObservatory mobile app, Weather Wizard, Weather Banner, Web Clock and SWIdget. Weather and geophysical information is also disseminated to the public via Twitter, Sina Weibo and RSS, etc. . The Observatory's website was one of the top ten ".hk" websites in years 2010, 2011 and 2013. The "Weather Services on Geographic Information System" of the Observatory also won the "Best Lifestyle (Learning & Living) Bronze Award" in the "Hong Kong Information and Communication Technology (ICT) Awards" in 2014.IT is a fast growing industry, particularly in emerging areas, such as cloud technology, social media and related services. IT improves the quality of living, but it also introduces security issues to IT developers. The Observatory faces the same challenge when utilizing advanced IT in the development of applications and the provision of services. In fact, secured IT systems are the building blocks and enablers for timely dissemination of weather forecasts and warnings, which is essential to ensuring quality services. The fundamental concept of information security includes "Confidentiality", "Integrity" and "Availability" of information. "Confidentiality" is a set of rules that limits access or places restrictions on the use of certain types of information, such as access control over classified information like restricted or confidential document. "Integrity" means maintaining and assuring the accuracy and consistency of information over its entire life-cycle. The information has to be well secured such that altering or destroy of data caused by any system faults or human maliciousness will not occur. "Availability" is to ensure that the information should be available when it is needed. Systems that provide information have to be protected from service disruptions due to power outages, hardware failures, system upgrades and malicious attacks, etc.The Hong Kong Observatory commits to complying with the Government IT security standards. IT security risk assessments and audits for computing systems are conducted regularly. The Observatory also adopts the Government information security incident response procedures to coordinate with other Government departments to protect the information in response to any information security incidents.

December 2014

https://www.hko.gov.hk/en/education/weather/data-and-technology/00450-weather-services-and-it-security.html

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Intelligent Meteorological Monitoring Assistant

The HKO initiated an in-house project to develop the Intelligent Meteorological Monitoring Assistant for alerting forecasters of emerging weather conditions.

With the advent of the Big Data era, we can all experience the pressure of being bombarded by huge amount of information every day. Being an active user on the internet, you may receive hundreds of emails, WhatsApp messages, Twitter tweets, photos on the Instagram, in addition to countless interesting video clips that you want to view on YouTube, and the many Facebook posts waiting for you to comment or "like" each day. The weather forecasters in the Central Forecasting Office of the Hong Kong Observatory (HKO) are also facing with similar challenges of information overload, for example, high-density automatic weather station data, high-resolution Numerical Weather Prediction model products, frequent scans from meteorological radars and satellites, etc. Currently, the data processed by HKO amount to over 150 million pages of text each day!In 2013, the HKO initiated an in-house project to develop the Intelligent Meteorological Monitoring Assistant (IMMA) for alerting forecasters of emerging weather conditions. The system has since been put into operational. Being a knowledge-based expert system, IMMA is a computer program designed to simulate the problem-solving behaviour of an expert in a specific domain or discipline, in our case, weather forecasting. The system applies expert knowledge to real world problems based on given facts and well-defined rules. Facts are pieces of information but on their own are of limited use (for example, a lightning signal detected over the waters to the south of Hong Kong). It is necessary to apply rules to reveal the usefulness and value of all the facts as a whole (for instance, prompt the forecaster to consider issuing the thunderstorm warning if the lightning is close enough to Hong Kong).At present, IMMA processes more than 530,000 data points every minute. It does not merely gather data and display them as necessary, but also turns them into actionable advice to aid the forecasters in performing weather monitoring and forecasting duties. It serves as a tireless assistant to the forecasters working in a busy and information overwhelming environment, such that the amount of time spent on weather watch and the associated decision-making processes can be reduced considerably.With the help of IMMA, the knowledge and experience in weather forecasting can be retained and managed in a more effective manner. In the future, when combining with location-based and social networking services on mobile devices, the technology may be utilized to develop new services for people on the go by providing them with personalized advice at different locations under different weather conditions.

August 2017

https://www.hko.gov.hk/en/education/weather/data-and-technology/00497-intelligent-meteorological-monitoring-assistant.html

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Characteristics of Meteorological Databases

Weather database thus demands for more proficient operational performance than general office database in order to avoid data loss and delay.

Hong Kong has a very dense weather observation network despite its relatively small geographical area (Figure 1). More than 300 automatic weather and rainfall stations were set up by the Hong Kong Observatory (HKO) and other government departments over some 1,100 square kilometers of land in Hong Kong [1]. As automatic weather stations generally measure a number of weather elements simultaneously, more than 2,500 weather observation data are transmitted from different parts of the territory to the Observatory via communication networks at one-minute intervals. These raw data are stored in one of Observatory’s databases for handling weather observations from local and neighbouring areas after pre-processing. They are one of the vital sources for the compilation of weather reports and generation of forecast products.Perhaps you may ask: Why a sophisticated database system is required to handle data with an incoming rate of less than 3,000 records per minute? In fact, these records will be extracted from the database and modified a number of times during the analysis and quality assurance processes. Some 20,000 sets of derived data and products so generated will also be stored in the database. As a result, the workload to process these data multiplies rapidly in a short period of time. Moreover, data and products are extracted from the database by dozens of applications to generate meteorological reports and forecast bulletins. Other than local data, the database also processes and stores huge amounts of meteorological data from nearby areas. Hence, the Observatory needs to employ highly efficient databases to handle the vast volume of weather data.To facilitate weather analysis and diagnosis by meteorological personnel, technical support staff will strive to provide them with meteorological data with the same observation time for reference. The weather elements collected from all weather stations are thus transmitted to the Observatory’s database in real time, requiring the database for handling a large volume of raw, derived and product data within tens of seconds. After the peak, the workload of the database will reduce significantly until the next data collection cycle. In other words, prominent differences in the workload can be seen during different time periods in a data collection cycle (Figure 2). Meteorological database needs to deal with this explosive data surges, which is rather different from the daily operation of general office database. Weather database thus demands for more proficient operational performance than general office database in order to avoid data loss and delay.Fluctuations in the workload of a meteorological database are in fact not uncommon for a database of financial market. The major difference between the two probably is that weather is non-stop and round-the-clock, whereas financial markets have non-operating hours. Global financial markets operated in different time zones are tightly connected, but still not comparable to the systems handling global weather data. In the meteorological world, data are running “7 x 24”. Identifying a certain time interval for database maintenance is not an easy task. Therefore, the hardware and software requirements of a meteorological database are much more stringent in terms of their availability and capability in disaster recovery. With rapid development of science and technology, more and more advanced database technologies become available. The Observatory will continue to strive to deploy advanced and appropriate technologies to provide reliable services to the public.

August 2017

https://www.hko.gov.hk/en/education/weather/data-and-technology/00498-characteristics-of-meteorological-databases.html

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ABC of Meteorological Data Encoding

Most meteorological observations are being produced and transmitted in special formats known as Traditional Alphanumeric Codes and Table-Driven Code Forms as promulgated by WMO.

Meteorological data around the globe are being exchanged among meteorological centres under the framework of the World Meteorological Organization (WMO) in a timely, reliable and cost-effective manner. They are of vital importance to the forecasting and warning operations of the Hong Kong Observatory. Currently, most meteorological observations are being produced and transmitted in special formats known as Traditional Alphanumeric Codes (TAC) and Table-Driven Code Forms (TDCF) as promulgated by WMO. Both TAC and TDCF are ways of representing meteorological data intended for international exchange in real-time.Owing to limited bandwidth of telecommunication lines, meteorological data need to be encoded to reduce size. TAC is a combination of human readable alphanumeric characters developed many decades ago for transmission on low-bandwidth telecommunication lines, such as Telex. In order to reduce bandwidth consumption, abbreviated coding and translation by code rather than the exact value are two key features of TAC. Loss of precision in the data is therefore inevitable when using TAC. For example, surface observations encoded in TAC can only hold the visibility information in steps of 1 km for values above 5 km (see Figure 1).Binary Universal Form for the Representation of meteorological data (BUFR) is a kind of TDCF developed by WMO as an intended replacement for TAC to encode meteorological observations in the late 80’s when the use of computers for meteorological data processing became more commonplace. BUFR is binary that makes it compact, flexible and expandable, and hence excellent for transmitting and storing huge amount of meteorological information. Nowadays, new types of meteorological information with higher resolution and measurement accuracy are emerging and exchanged among meteorological centres. The traditional TAC encoding method becomes incapable of accommodating the new data to meet the expansion and growing needs. BUFR, on the other hand, provides effective support for the representation of new data types, metadata, higher temporal or spatial resolution data and higher precision data such as meteorological satellite data, aircraft observations and also tropical cyclone information (Figure 2).BUFR is now an important component in meteorological information exchange. The benefits of using BUFR are becoming more well-known and recognized by the meteorological centres. The Hong Kong Observatory started to produce daily observations in BUFR format in 2010 and all essential observation reports are being disseminated to other meteorological centres in both TAC and BUFR formats.

December 2015

https://www.hko.gov.hk/en/education/weather/data-and-technology/00479-abc-of-meteorological-data-encoding.html

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Processing and Preservation of Meteorological Observation Data

After several generations of enhancements, the HKO currently has four main databases for different business areas. The database employs partitioning technology to segregate the huge amount of data to facilitate processing.

Have you ever wondered about the weather of the day you were born or other important days in the past? Were they cold, windy or stormy? The webpage for "Weather on a Special Day" on the Hong Kong Observatory’s website provides information on the weather conditions of the days you are interested. In fact, the webpage is supported by the weather data accumulated bit by bit by the Hong Kong Observatory since its inauguration more than a hundred years ago.Since 1884, the HKO has been taking regular weather observations. In the old days, all weather observation data were recorded on paper manually due to the limited resources and technologies. As a result, the amount of data recorded were relatively meagre.With technology advancements, volume of weather observation data is ever increasing. The Observatory started to use computers to record and store these observation data in plain text files. Due to establishment of an Automatic Weather Station Network in 1984, and more frequent data exchange with other weather centres, the volume of data to be handled increased rapidly. Storing data in plain text files became inappropriate. In response, relational database management system was first introduced in the mid 90's and used up to now.After several generations of enhancements, the HKO currently has four main databases for different business areas. They are: - Automatic Weather Station Database - handling observation data from automatic weather stations over the territory and adjacent areas. - Global Telecommunications System Database - handling bulletins and data exchanged with weather centres in other parts of the world via the Global Telecommunications System. - Meteorological Database - handling observation data other than those from automatic weather stations. - Climate Database - handling local climate data. The existing relational database management system handles millions of weather observation data per day to support weather forecast and warning services, as well as research activities. The database employs partitioning technology to segregate the huge amount of data to facilitate processing. Older data are archived on magnetic tapes or other electronic media.Information technology is changing fast. The ever increasing amount of weather observation data is bringing new challenges for the HKO. We are exploring and introducing new data processing and preservation methods and techniques to embrace challenges and seize opportunities in the imminent Big Data era.

December 2015

https://www.hko.gov.hk/en/education/weather/data-and-technology/00476-processing-and-preservation-of-meteorological-observation-data.html

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Tropical Cyclone Classification

How many categories are there in the classification of tropical cyclones? Why is typhoon classification introduced?

How many categories are there in the classification of tropical cyclones?Tropical cyclones of different intensity are given different names. Tropical cyclones are classified in accordance with the World Meteorological Organization's recommendation by their maximum sustained wind speeds near the centre.  In Hong Kong, the classification is defined in terms of wind speeds averaged over 10 minutes.  With effect from 2009, a new classification is used and consists of 6 categories as follows:  What is the difference between the new and old classifications?The first three categories, namely TD, TS and STS, remain the same as before. The old typhoon category will be further divided into three levels, viz T, ST and SuperT.With the new typhoon classification, what will be the frequency of occurrence of different categories of typhoons?For all typhoons requiring the Signal No.8 or above from 1950 to 2008, 75% were typhoons, 15% severe typhoons and 10% super typhoons.How often will Hong Kong have a direct hit of a super typhoon or severe typhoon?In the 53 years from 1956 to 2008, 12 tropical cyclones necessitated the issue of Signal No.10 in Hong Kong. At a certain point in their lifetime, four of them belonged to the super typhoon category (Wanda 1962; Ruby 1964; Rose 1971 and Hope 1979) while two belonged to the severe typhoon category ( Gloria 1957 and Ellen 1983).  Why is typhoon classification introduced?Typhoon classification is introduced to facilitate the public in recognizing more intense typhoons, prompting people to be extra vigilant with the approach of more intense tropical cyclones.How does the new classification affect the Tropical Cyclone Warning System in Hong Kong?The Tropical Cyclone Warning System in Hong Kong will remain unchanged. The Observatory does not anticipate that the introduction of typhoon classification will affect the existing contingency plans for tropical cyclones. 

Classification, Naming, Characteristics

https://www.hko.gov.hk/en/education/tropical-cyclone/classification-naming-characteristics/00145-tropical-cyclone-classification.html

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Typhoon and Hurricane

The terms typhoon and hurricane are often mentioned in the news or weather bulletins. What are the differences?

The terms 'typhoon' and 'hurricane' are often mentioned in the news or weather bulletins. What are the differences? Cyclones that form over the ocean in the tropical regions are generically called 'tropical cyclones', although they are also known by different names in different ocean basins (Table 1), such as 'typhoon' in the western North Pacific, 'hurricane' in the North Atlantic and 'cyclonic storm' in the North Indian Ocean.The origin of the Chinese term 'typhoon' remains a subject of much speculation, and a brief description is provided at the end of this article. Interestingly, the English term 'typhoon' sounds like the Chinese pronunciation of 'Da Feng', which literally means 'great winds' or 'a wind strike'. According to the definition in the Oxford English Dictionaries, the word 'typhoon', originating in the late 16th century and brought in by the Portuguese, probably stemmed from the Arabic 'ṭūfān' or the Greek 'tuphōn' for 'whirlwind', and might also come from the Chinese dialect 'tai fung' for 'big wind' (Ref. 1).According to the World Meteorological Organization recommendations, tropical cyclones are classified by the maximum sustained wind speeds near the centre. Over the western North Pacific and the South China Sea, tropical cyclones having maximum sustained winds reaching 118 km/h or more are known as 'typhoons'. Tropical cyclones of lesser intensity are categorized as 'severe tropical storms', 'tropical storms' and 'tropical depressions'. To heighten people's alertness of more intense typhoons, the Observatory further categorizes typhoons into 'Typhoon', 'Severe Typhoon' and 'Super Typhoon' starting from the 2009 tropical cyclone season. Over the North Atlantic, central and eastern North Pacific, tropical cyclones are known as 'hurricanes' (Table 1), linked to the native central American word 'huracan' which means 'god of evil'. Hurricanes are classified further into Category 1 to 5 according to the Saffir-Simpson Hurricane Wind Scale (SSHS). According to SSHS, the maximum 1-minute mean wind speed ranges from 119 – 153 km/h for a Category 1 hurricane, whereas a Category 5 hurricane has a maximum 1-minute wind speed of at least 252 km/h. Examples of well-known hurricanes are Hurricane Sandy of 2012 and Hurricane Katrina of 2005, with peak intensities at Category 3 and Category 5 respectively. In another context, the term 'hurricane' refers to 'hurricane-force wind', wind at a certain location has a speed equal to or greater than 118 km/h or Force 12 in the Beaufort wind scale (Table 2 and Ref. 2; note that operationally, the Hong Kong Observatory adopts the 10-minute mean wind speeds as the reference). In Hong Kong, such sustained wind speeds are rare over land and are usually experienced during the close passage of a mature typhoon. The Observatory will consider issuing the Hurricane Signal No. 10 whenever winds of such strength are expected near the sea level (Figure 1). Gusts may be much higher than the mean wind speed and can exceed 220 km/h. For example, a maximum 60-minute mean wind speed of 133 km/h and maximum gust of 259 km/h were recorded at the Observatory's Headquarters during the passage of Typhoon Wanda in 1962. Squall lines or intense thunderstorms may also bring very high gusts, but the occurrence of sustained hurricane-force winds are rare in these relatively short-lived weather systems.In other parts of the world, hurricane-force winds often occur on high mountains, and may occur from time to time during the passage of extratropical cyclones or depressions. On 8 December 2011, a deep Atlantic low pressure system brought gale to storm-force winds across the northern part of the United Kingdom, with the highest hourly mean wind speed reaching hurricane force at the mountain summits (Ref. 3). In Australia, the summit of Mount Wellington in Tasmania is often affected by high winds, with the maximum gust reaching 200 km/h (Ref. 4). Over the east coast of the United States and Canada, the 'northeaster' associated with low pressure areas may give rise to hurricane-force winds and flooding (Ref. 5).Historically, the distinction between 'typhoon' and 'hurricane' was not clear-cut. With reference to the book on the 'History of Meteorology in China' (Ref. 6), when the term 'typhoon' started to be used in the meteorological history of China remained uncertain. The term 'hurricane' was adopted in literature prior to or during the Ming Dynasty and the term 'typhoon' had not been used. Starting from the Qing Dynasty, some literature described transient squalls in spring as 'hurricanes' and the more persistent strong winds in summer as 'typhoons'.In Hong Kong, historical literature suggested that the terms 'typhoon' and 'hurricane' had been used interchangeably. For example, in the Local Storm Signal Code published by the Observatory in 1950 (Figure 2), the No. 10 Signal was known as the 'Typhoon' signal, meaning hurricane-force or typhoon winds of 64 knots (118 km/h) or above. Starting from the mid-1960s to 1970s, the Observatory started to conform to the practice of using 'typhoon' in the intensity classification of tropical cyclones and 'hurricane' for describing wind strength at a certain location.

Classification, Naming, Characteristics

September 2014

https://www.hko.gov.hk/en/education/tropical-cyclone/classification-naming-characteristics/00445-typhoon-and-hurricane.html

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Extra-tropical Cyclone vs Tropical Cyclone

Extra-tropical Cyclone vs Tropical Cyclone

We always talk about tropical cyclones.  As you are probably aware, there are other kinds of cyclones, one of the most common being extra-tropical cyclones.  What are extra-tropical cyclones? How are they different from tropical cyclones?An extra-tropical cyclone is a low pressure system that primarily gets its energy from the temperature difference in the horizontal direction across the cyclone (known as temperature gradient in meteorology) . Extra-tropical cyclones have frontal features, i.e. they are associated with cold fronts, warm fronts, and occluded fronts.  Structurally, extra-tropical cyclones are "cold-core".  "Cold-core" means that the center is colder than the surroundings at the same height in the troposphere.  Extra-tropical cyclones or their associated fronts may affect Hong Kong in the cool season (Figure 1).Tropical cyclones, in contrast, typically have little or no significant temperature differences across the storm.  Their energy are derived from the release of heat due to cloud/rain formation from the warm moist air of the tropics.  Structurally, tropical cyclones are "warm-core".  Tropical cyclones usually affect Hong Kong during the hot months (Figure 2).Often, a tropical cyclone will transform into an extra-tropical cyclone as it recurves poleward. Once in a while, an extra-tropical cyclone may lose its frontal features, develop convection near the centre of the cyclone and turn into a tropical cyclone.

Classification, Naming, Characteristics

https://www.hko.gov.hk/en/education/tropical-cyclone/classification-naming-characteristics/00146-extratropical-cyclone-vs-tropical-cyclone.html

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Structure of Typhoons

Based on the cloud structure, a mature typhoon can roughly be divided into three regions, namely the eye, the eye wall and the spiral rain bands.

When a tropical cyclone is affecting Hong Kong, the Hong Kong Observatory will issue Tropical Cyclone Bulletins and we will come across terms such as the eye of a typhoon, its spiral rain bands, etc.  Actually what do these terms mean?Based on the cloud structure, a mature typhoon can roughly be divided into three regions, namely the eye, the eye wall and the spiral rain bands (Figure 1).The "eye" is roughly circular and is found at the centre of a typhoon. Its size varies and can range from below 10 km to over 200 km across, but most are about 30 to 60 km in diameter. Winds are comparatively light and the weather is fair inside the eye.  There is little or no rain and one can sometimes see the blue sky or stars. The eye is the region of lowest surface pressure (Figure 2 & 3) and highest temperatures.  Compared with its surroundings, the eye may be up to 2℃ warmer at the surface and 10℃ or more at an altitude of 12 km. The eye is surrounded by the "eye wall", a roughly circular ring of thick clouds.  Inside the eye, the air is sinking.  On the other hand, the eye wall has very deep convection (i.e., ascent) and is the area of highest surface winds and heavy rain in the typhoon. Figure 4 shows how the winds at Lau Fau Shan changed during the passage of Typhoon Dujuan in 2003.  They increased significantly as the eye wall of Dujuan approached and decreased rapidly as the storm moved away.Convection outside the eye is organized into long, narrow rain bands which are oriented roughly in the same direction as the surface wind. Because these bands appear to spiral into the centre of a tropical cyclone, they are sometimes called "spiral rain bands".

Classification, Naming, Characteristics

https://www.hko.gov.hk/en/education/tropical-cyclone/classification-naming-characteristics/00147-structure-of-typhoons.html

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Tropical Cyclone Names in the western North Pacific

Starting from 1 January 2000, tropical cyclones in the western North Pacific are named from a new list of names. The new names are words originating in Asia or Pacific islands, and were contributed by members of the World Meteorological Organization's Typhoon Committee.

For over a half-century, the US military weather forecasters have named tropical cyclones forming in the western North Pacific. The names used were almost exclusively English feminine names through 1978.  But beginning in 1979 men's names were used with women's names in an alternating manner. Starting from 1 January 2000, tropical cyclones in the western North Pacific are named from a new list of names. The new names are words originating in Asia or Pacific islands, and were contributed by members of the World Meteorological Organization's Typhoon Committee. Fourteen nations or regions contributed ten names each to make up a total of 140 names. The new names will be allotted to tropical cyclones reaching tropical storm strength by the Tokyo Typhoon Centre of the Japanese Meteorological Agency. These new names have two major differences from the past name lists. Firstly, the names are mostly not names of people.  The majority are names of flowers, animals, birds, trees, or even foods, etc, while some are descriptive adjectives. Secondly, the names are not allotted to the tropical cyclones in alphabetical order of the names, but rather, in alphabetical order of the contributing nations or regions.According to the convention of the Typhoon Committee, the country or region which has suffered serious human casualties and economic losses from a tropical cyclone can propose to remove the name of the tropical cyclone from the name list. Sometimes names are removed for other reasons, such as cultural considerations. For example, "Durian" was once a tropical cyclone name but was removed from the list because of the severe damage it incurred. Typhoon Durian crossed the central part of the Philippines in November 2006. Heavy rain associated with Durian triggered widespread mudslides in the region. Over 570 people were killed and 746 missing.  "Durian" was later removed from the list and replaced by the new name "Mangkhut". 

Classification, Naming, Characteristics

September 2009

https://www.hko.gov.hk/en/education/tropical-cyclone/classification-naming-characteristics/00144-tropical-cyclone-names-in-the-western-north-pacific.html

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Will we find gales under Strong Wind Signal, No. 3?

This article illustrates that there could be regional difference in wind speeds under Strong Wind Signal, No.3. In particular, gale winds would be possible over offshore waters, on high ground or places that are relatively closer to the centre of a Tropical Cyclone.

Tropical Cyclones (TC) may affect Hong Kong generally from May to November (Super Typhoon Rai in December 2021 was an exception) with highest occurrence frequency during July-September. Whenever there is a TC locating within about 800 kilometers of Hong Kong and may affect the territory, the Hong Kong Observatory will assess its impact on the local winds and issue the corresponding TC Warning Signals in a timely manner based on the forecast intensities and tracks of that TC. In general, when strong winds (a sustained wind speed of 41-62 kilometres per hour) are blowing or expected to blow generally in Hong Kong near sea level, and the wind condition is expected to persist, the Hong Kong Observatory will consider issuing the Strong Wind Signal, No.3.Take TC Chaba as an example. The Observatory issued the Strong Wind Signal, No.3 at 10:40 pm on 30 June 2022. Figure 1 shows the 10-min mean wind direction and speed at 01:00 pm on 1 July 2022. There were strong winds blowing generally over the territory when Strong Wind Signal, No. 3 was in force. Furthermore, gale winds (a sustained wind speed of 63-87 kilometres per hour) or above were recorded in Cheung Chau Beach and Ngong Ping. Why did we find gales under Strong Wind Signal, No.3 over parts of the territory?"Winds" refer to the air flow from a region of higher pressure to a region of lower pressure. Movement speed of air flow (i.e. wind speed) would be slowed down due to friction. The friction is generally smaller over sea surface than that over land surface. Together with the fact that there is no obstruction from building nor complicated topography over the sea surface, offshore winds are generally higher than that over land surface, especially in urban areas. Besides, the friction over high grounds are also smaller as they are far away from buildings. In general, the higher the places, the stronger the winds. Apart from “high ground” and “offshore”, the complexity of Hong Kong’s topography (Figure 2), the orientation and density of buildings will also exert local influence to the wind conditions over the landmass of Hong Kong leading up to significant regional differences.Furthermore, the high winds associated with a TC are usually distributed close to its circulation centre (except for the “eye” of a typhoon) and wind speeds decrease with increasing distance from the centre. Referring to Figure 1, Chaba was still centred at about 400 km south-southwest of Hong Kong at that time and the southern parts of the territory, e.g. Cheung Chau and Lantau Island, were situated relatively closer to Chaba’s centre. As such, the wind speeds recorded over there were higher than those over the northern parts of the territory.Therefore, even if local winds are generally strong under Strong Wind Signal, No. 3, there could still be gale winds over individual locations over offshore waters, on high ground or places that are relatively closer to the centre of a TC. The public should take note of the regional wind information mentioned in TC Warning Bulletin (or make reference directly to the Regional Weather webpage for real-time information on mean wind and maximum gust) and take appropriate response actions, including staying away from the shoreline, not to engage in water sports and avoid visiting high grounds or locations exposing to high winds.

Intensity

October 2022

https://www.hko.gov.hk/en/education/tropical-cyclone/intensity/00681-Will-we-find-gales-under-Strong-Wind-Signal-No-3.html

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How does vertical wind shear affect the development of Tropical Cyclone?

The article introduces the effect of vertical wind shear on tropical cyclone development.

The formation and development of tropical cyclone normally require to tie in with various favourable atmospheric and oceanic conditions, e.g. warm sea surface temperatures[1] and being over areas 5 degrees outside the equator[2] etc. Meanwhile, the strength of vertical wind shear in the atmosphere is also a critical factor to the development of a tropical cyclone. So what is vertical wind shear and how does it affect the development of tropical cyclone?Vertical wind shear is the change in wind speed and direction with the altitude in the atmosphere (Figure 1). The larger the change in wind speed and direction with the altitude, the stronger the vertical wind shear. On the opposite, the smaller the change, the weaker the vertical wind shear.The immense energy required for the development of a tropical cyclone are mainly derived from the release of latent heat energy during condensation of water vapour. Those heat energy generally concentrate in the core region of a tropical cyclone, causing the temperature at the storm centre to be higher than the outer region, and this structure is known as “warm-core”. If the warm-core structure is damaged, the development of the tropical cyclone will be impacted.Rather strong vertical wind shear could tilt the convective clusters near the centre of the tropical cyclone. Not only will this weaken vertical activities of the convections, but will also shift the heat and moisture away from it the core region, resulting an adverse impact on the warm-core structure which could eventually affect the development of tropical cyclone. If a tropical cyclone is being affected by rather strong vertical wind shear for a sustain period of time, the storm structure will be damaged and its low level circulation could be separated from the main convective cluster (figure 2), further weakening the tropical cyclone and even possibly causing it to dissipate.

Intensity

October 2022

https://www.hko.gov.hk/en/education/tropical-cyclone/intensity/00685-How-does-vertical-wind-shear-affect-the-development-of-Tropical-Cyclone.html

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How to Assess the Intensity of Tropical Cyclones?

In the weather bulletins issued by the Hong Kong Observatory, intensity changes of tropical cyclones are sometimes mentioned (for example: Tropical Storm Nock-ten has intensified into a severe tropical storm). However, how do forecasters estimate the intensity of tropical cyclones in real operations?

In the weather bulletins issued by the Hong Kong Observatory, intensity changes of tropical cyclones are sometimes mentioned (for example: Tropical Storm Nock-ten has intensified into a severe tropical storm). However, how do forecasters estimate the intensity of tropical cyclones in real operations?Use of surface observation dataTo assess the intensity of a tropical cyclone accurately, the best way is to collect and analyze the wind speed information near the center of the storm. However, since tropical cyclones develop over the ocean, observations from meteorological stations and ships are sparse. It is not easy for forecasters to obtain reliable surface wind observations. Generally speaking, when a tropical cyclone edges closer to an island or the coastline, surface observations can play a more important role. Tropical cyclone Pabuk is one of the examples. Pabuk was located over the western part of Hong Kong on 10 August 2007. The observations of the Observatory’s meteorological stations in Cheung Chau and Waglan indicated that the Hong Kong waters were generally affected by sustained gale force winds. This revealed that Pabuk had reached tropical storm intensity (Figure 1). Use of satellite observation dataOwing to the limitation of surface observations, an important tool for the forecasters to assess the intensity of tropical cyclones is to make use of satellite information, which has the advantage of excellent availability and extensive coverage. One of the methods still commonly used by meteorological services in the world nowadays is the technique developed by D'vorak. In simple words, D'vorak technique analyzes the distribution and patterns of cloud top temperatures of a tropical cyclone. Incorporating with the statistical data summarized by D'vorak through a number of years, the intensity of a tropical cyclone can be deduced indirectly. One example is illustrated in Figures 2 and 3. On 15 September 2010, tropical cyclone Fanapi was still loosely organized. On 18 September 2010, its center was noticeably surrounded by tight spiral cloud bands, forming a discernible eye. D'vorak technique can quantitatively estimate the intensification of Fanapi from tropical depression to severe typhoon strength through analyzing the evolution of its cloud patterns.The advantage of this technique is that there are no geographical limitations on the location of the tropical cyclone. Forecasters can adopt this technique to assess the intensity of tropical cyclones as far as a few thousand kilometers away. Its disadvantage lies on the subjectivity arising from the judgment of cloud patterns by forecasters. As D'vorak technique is based on statistical analysis of past tropical cyclones, larger errors are expected for some extreme cases (for example: cyclones with exceptionally rapid rate of intensification). Use of radar observation dataWhen a tropical cyclone enters the surveillance range of Doppler radar of the Hong Kong Observatory, forecasters can assess the intensity of the storm by making reference to the maximum wind speed derived from movement of rain echoes. From Figure 4, winds reaching hurricane force (wind speed of 33 m/s or above) can be detected by the radar near the eye of tropical cyclone Molave at 21H. on 18 July 2009. However, based on such wind data, one cannot determine whether Molave had reached typhoon strength immediately. The main reason is that wind speed measured by the Doppler radar is that at a distance above the ground, and such data should be appropriately transformed to reflect the true surface wind speed. Studies indicated that there is a proportionate relationship between the maximum wind speed detected by the Doppler radar and the intensity of the tropical cyclone. Such result is useful in applying the radar observation data to analyze the intensity of tropical cyclones.Aircraft monitoringThe United States arranged aircraft regularly to monitor the structure of tropical cyclones in the northwestern Pacific on or before 1987. After the project has ceased for more than 10 years, another project known as "Dropsonde Observation for Typhoon Surveillance near the TAiwan Region" (DOTSTAR) has been launched by Taiwan since 2003. Its aim is to observe the structure of selected tropical cyclones in the northwestern Pacific using aircrafts. Although the application of such data in determining the intensity of cyclones is still under study, the additional information is, to some extent, useful reference for the forecasters. ConclusionThe methods mentioned above all have their respective pros and cons in assessing the intensity of tropical cyclones. Forecasters need to analyze the reliability of various kinds of data carefully and then make a timely and professional judgment.

Intensity

September 2011

https://www.hko.gov.hk/en/education/tropical-cyclone/intensity/00152-how-to-assess-the-intensity-of-tropical-cyclones.html

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Why do Tropical Cyclones always Form more than 5 degrees of Latitude away from the Equator?

As evident from past observations, tropical cyclones are difficult to form over a region within 5 degrees of latitude from the Equator because the Coriolis force there is too small to generate a vortex. That said, the formation of tropical cyclone still hinges on other environmental factors.

The formation of a tropical cyclone requires a number of favourable environmental conditions and one of them is a suitable latitude.  Almost all tropical cyclones form over regions more than 5 degrees of latitude away from the Equator and very few occur near the Equator.  To explain why this happens, we need to first understand which forces in the atmosphere are critical to the formation of tropical cyclones.  There are three such forces including the frictional force, pressure gradient force and Coriolis force due to the Earth's rotation.  The characteristics of these three forces are introduced below:Frictional force: generated when the air molecules move and rub against ambient substances (for example, the ground surface).  Frictional force causes the air molecules to lose kinetic energy and slow down.Pressure gradient force: arising from the pressure difference between two places.  The air molecules are pushed from an area of higher pressure towards an area of lower pressure.  The air flow will be faster when the pressure difference is larger.Coriolis force: the self-rotation of the Earth can cause a deflection in the air motion.  Simply put, any place on the Earth's surface moves from west to east as the Earth rotates.  A point on Earth will traverse a shorter distance in a day, or in other words, at a slower moving speed if it is further away from the Equator.  When air moves towards the north in the Northern Hemisphere, it maintains its initial eastward moving speed due to inertia while the Earth's surface underneath is slower.  Relative to the Earth's surface, the air will appear to be deflected to the right in the Northern Hemisphere (see Figure 1).  The situation reverses in the Southern Hemisphere and the moving air will appear to be deflected to the left (see Figure 2).  In effect, it looks like some kind of force is in play to push the air to the right (Northern Hemisphere) or to the left (Southern Hemisphere).  This fictitious force is named the Coriolis force in Meteorology.  The Coriolis force increases with an increase in latitude.  Or, the closer to the Equator, the smaller the Coriolis force.  As a result, the deflection caused by the Coriolis force will become increasingly prominent in higher latitudes.We all know that the air pressure decreases as we move towards the centre of an area of low pressure.  If there were only pressure gradient force and frictional force in the atmosphere, the air would just flow from the surroundings of higher pressure to the centre of the low pressure without any rotation (see Figure 3).  However, in the presence of Coriolis force, the air will not just move from the surroundings towards the centre of the low pressure, but will also be deflected leading to the formation of a vortex (see Figure 4), which is favourable to tropical cyclone formation. As evident from past observations, tropical cyclones are difficult to form over a region within 5 degrees of latitude from the Equator because the Coriolis force there is too small to generate a vortex.  That said, the formation of tropical cyclone  still hinges on other environmental factors.  Under other very favourable conditions, tropical cyclones may still form in the vicinity of the Equator.  Tropical Cyclone "Vamei" in December 2001[1] was the first tropical cyclone on record formed within 1.5 degrees latitude of the Equator[2].

Intensity

December 2013

https://www.hko.gov.hk/en/education/tropical-cyclone/intensity/00150-why-do-tropical-cyclones-always-form-more-than-5-degrees-of-latitude-away-from-the-equator.html

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Why do Tropical Cyclones Require Sea Surface Temperatures of 26°C to Form?

It is a well known observation that sea surface temperatures of at least 26°C would favour the formation of tropical cyclones. Studies showed that this warm water needs to be a relatively deep layer of around 50 metres. The 26°C value is tied to the thermal properties of the atmosphere in the tropical and subtropical areas.

It is a well known observation that sea surface temperatures of at least 26°C would favour the formation of tropical cyclones. Studies showed that this warm water needs to be a relatively deep layer of around 50 metres.The sea surface temperature requirement is related to how tropical cyclones derive their energy. A tropical cyclone can be thought of as an engine that requires warm, moist air as fuel. This warm, moist air cools as it rises in convective clouds in the rainbands and eyewall of the typhoon. The water vapour in the cloud condenses into water droplets and releases heat (known as latent heat). This heat provides the energy for the tropical cyclone. The 26°C value is tied to the thermal properties of the atmosphere in the tropical and subtropical areas. Above this temperature deep convection can occur, but below this value little or no convection can be found. 

Intensity

https://www.hko.gov.hk/en/education/tropical-cyclone/intensity/00151-why-do-tropical-cyclones-require-sea-surface-temperatures-of-26suposupc-to-form.html

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The use of Ship Reports in Determining the Intensity of a Tropical Cyclone

Despite the advance in the satellite remote sensing technology, the weather observations taken by the VOS will continue to be vital to the safety of the shipping community over the oceans against hazardous weather phenomena.

The intensity of a tropical cyclone is determined by the strength of the surface winds near its centre.  Amongst other meteorological observations, ship reports in the vicinity of a tropical cyclone are very important as they provide surface wind speed observations which are directly related to the intensity of the tropical cyclone.  With the scarcity of direct surface wind speed observations especially over the oceans, analysis of remote sensing data collected by weather satellites for estimating tropical cyclone intensity has been extensively used since the availability of satellite imageries in late 1970's.However, satellite imageries may not be helpful sometimes in estimation of tropical cyclone intensity, especially in the initial development phase of the tropical cyclone when the cloud feature has yet to be better organized.  In these cases, ship reports in the vicinity of the tropical cyclone once again become most valuable in safeguarding the marine community. An example is given below.On the early morning of 27 July 2004, a low pressure area over the South China Sea about 200 km east-southeast of Hong Kong was developing.  Its development was closely monitored by the Hong Kong Observatory.  Due to the poorly organized cloud feature, analysis of the satellite imagery at about 18 UTC on 26 July 2004 could not justify a tropical depression over the region (Figure 1).  Fortunately, a ship nearby reported a strong wind of 29 knots in the vicinity of the low pressure area at almost the same time (Figure 2).  Based on this ship report, the duty forecaster upgraded the low pressure area to a tropical depression (a tropical cyclone with maximum sustained wind speed of 22-33 knots near the centre) and issued the Tropical Cyclone Warning for Shipping to warn the shipping community about the storm.Ship weather observations are made by Voluntary Observing Ships (VOS), which are merchant ships equipped with meteorological instruments.  Ship officers on board make weather observations regularly and transmit them via satellite to meteorological centres during their voyages.  Despite the advance in the satellite remote sensing technology, the weather observations taken by the VOS will continue to be vital to the safety of the shipping community over the oceans against hazardous weather phenomena.

Intensity

September 2011

https://www.hko.gov.hk/en/education/tropical-cyclone/intensity/00153-the-use-of-ship-reports-in-determining-the-intensity-of-a-tropical-cyclone.html

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Aircraft Meteorological Observation for Tropical Cyclones

The Observatory has been in close collaboration with the aviation community to collect meteorological observations for enhancing our weather services, for instance the investigation flights were conducted to study the low-level windshear and turbulence effects around the Airport.

The Observatory has been in close collaboration with the aviation community to collect meteorological observations for enhancing our weather services, for instance the investigation flights were conducted to study the low-level windshear and turbulence effects around the Airport.  In 2009, the Observatory commenced the regular flight data collection with the Government Flying Service(GFS).  A fixed-wing aircraft Jetstream-41 of GFS was equipped with a dedicated meteorological measuring system to provide horizontal and vertical winds, temperature, pressure and humidity at a high frequency of 20 times per second.  These high quality data are used to verify the windshear and turbulence alerts provided by the ground-based meteorological systems and to enhance the turbulence alerting algorithms of the Windshear and Turbulence Warning System (WTWS).In 2011, our cooperation with GFS extended to reconnaissance flights to capture weather data for tropical cyclones (TCs) over the South China Sea (Figure 1).  This greatly enhances our capacity in monitoring the location and intensity of TC that have been based on limited observations or satellite pictures.To further enhance meteorological data collection in TC situation, HKO and GFS have planned to implement a launcher on the replacement GFS aircraft to release a measuring instrument called dropsonde.  Similar dropsonde measurement missions have been in real-time experimental running. A dropsonde consists of a set of weather sensors hanged under a mini-parachute.  As the dropsonde descends, it measures weather data for transmission to the Observatory via the aircraft.  This would provide the vertical meteorological profile of TCs, and more importantly the near sea surface wind information for better determination of the storm intensity.  It would be safer to do the measurements as the aircraft can release the dropsondes on a higher altitudes and leave immediately without confronting the severe convection associated with the tropical cyclones.

Intensity

January 2013

https://www.hko.gov.hk/en/education/tropical-cyclone/intensity/00115-aircraft-meteorological-observation-for-tropical-cyclones.html

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Data for Tracking Tropical Cyclones

Use of satellite observations is one of the tools that forecasters can rely on for estimating the location and intensity of a tropical cyclone. Physical quantities like sea surface temperatures and derived tropical cyclone heat potential can provide useful guidance to forecasters on the potential development of a tropical cyclone.

The devastating scenes brought about by Super Typhoon Mangkhut during its passage in September 2018 were still vivid in our mind. Its accompanied damaging winds, heavy downpours, storm surge and high seas remind us of the power of nature. Whenever tropical cyclone season comes, weather forecasters will monitor closely the formation and development of tropical cyclones over the western North Pacific and the South China Sea, especially those possibly posing a threat to Hong Kong. As such, collecting and processing all tropical cyclone-related observational data and information is the first key step for tracking and forecasting tropical cyclones.Weather is changeable, yet there are signs of its evolution. Noting the chaotic nature of weather systems, numerical weather prediction (NWP) models cannot produce accurate forecasts all the time, and sometimes the predictions can be very different from the actual situation. This makes weather observations even more important and valuable for monitoring purpose.Over the vast ocean of the western North Pacific and the South China Sea, use of satellite observations is one of the tools that forecasters can rely on for estimating the location and intensity of a tropical cyclone. Physical quantities like sea surface temperatures and derived tropical cyclone heat potential can provide useful guidance to forecasters on the potential development of a tropical cyclone. Meanwhile, forecasters also make reference to other observational data acquired from various communication channels and stored in the meteorological database, including: • Data via the Global Telecommunication System (GTS), such as - Ship reports; - Surface synoptic observations (SYNOP); - Radar observations (RADOB); - Tropical cyclone-related bulletins from different meteorological centres; • Data from automatic weather stations in the territory and adjacent areas; • Radar imagery; • Drifting buoys; and • Dropsonde system. After consolidating various meteorological observational data and information, forecasters can analyse and monitor the development of a tropical cyclone on the in-house developed a GIS-based Integrated Meteorological Information Display (MET-GIS) platform. Together with the Intelligent Meteorological Monitoring Assistant (IMMA) system, forecasters are able to keep a close weather watch.Monitoring is the first step in the forecast process. Next time, we will talk more on the forecasting techniques for tropical cyclones.

Tracking

November 2019

https://www.hko.gov.hk/en/education/tropical-cyclone/tracking/00527-data-for-tracking-tropical-cyclones.html

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Weather Systems Mix and Match – Tropical Cyclone Track

The track of a tropical cyclone is affected by many weather systems such as the subtropical ridge, the westerly trough and the monsoon trough. Conceptual models of these systems provide a guidance to the forecaster when forecasting the track of a tropical cyclone.

May to November is generally the tropical cyclone season of Hong Kong. But why do some tropical cyclones that formed over the western North Pacific come close to Hong Kong while some turn northwards?The track of a tropical cyclone is affected by many different factors. Whether it moves close to Hong Kong depends on the coordination of different weather systems. Among them, the steering flow is an important factor in determining how a tropical cyclone moves. The steering flow can generally be represented by the wind field at a certain height or the mean wind field of a number of layers.Major weather systems that affect the track of tropical cyclone over the western North Pacific include:(1)   Subtropical ridge (abbr. STR): Located over the subtropical region with the largest area of coverage.(2)   Westerly trough: A disturbance in the mid-latitude westerlies covering a smaller area.(3)  Monsoon trough: Extending from the intertropical convergence zone. The monsoon trough over the southeast Asia covers an area ranging between a subtropical ridge and a westerly trough.Generally speaking, the subtropical ridge steers a tropical cyclone along its periphery. If the subtropical ridge is covering an area towards the east, the tropical cyclone may recurve over the western North Pacific towards the vicinity of Japan. On the contrary, if the subtropical ridge covers East China, there is a chance that the tropical cyclone enters the South China Sea (abbr. SCS) (Conceptual model 1 in Figure 1). However, the strength, position and orientation of the subtropical ridge will be affected by other weather systems. If there is a deep westerly trough moving towards the northwestern side of the subtropical ridge, the latter may split into two cells and the tropical cyclone will be steered northwards[1](Conceptual model 2 in Figure 1). On the other hand, the monsoon trough also affects the track of a tropical cyclone. If the monsoon trough is relatively strong and extends eastwards forming a large low pressure gyre with the tropical cyclone, the subtropical ridge will be north-south oriented[2], and the tropical cyclone may move northwards at a position farther to the east (Conceptual model 3 in Figure 3).It is worth noting that tropical cyclones can exhibit a variety of tracks, sometimes making a loop or a sharp turn (for example, Bebinca and Son-Tinh in 2018, Vongfong in 2020). The aforementioned conceptual models serve only as a reference. Forecasters have to combine the observations, experience and outputs from numerical weather prediction to predict the challenging tropical cyclone track.

Tracking

December 2023

https://www.hko.gov.hk/en/education/tropical-cyclone/tracking/00707-Weather-Systems-Mix-and-Match-Tropical-Cyclone-Track.html

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Tropical Cyclone Forecast Track Information

During typhoon season, information on tropical cyclone forecast track is one of the items that the public and the media are most concerned with. This information is provided through various ways to meet different needs of the public and the media. This article introduces the various channels to obtain such tropical cyclone track information.

During typhoon season, information on tropical cyclone forecast track is one of the items that the public and the media are most concerned with. This information is provided through various ways to meet different needs of the public and the media. This article introduces the various channels to obtain such tropical cyclone track information.Tropical Cyclone Track Information webpageThere are two versions of the webpage, namely the "Fixed-area map version" (Figure 1) and the "Geographical Information System (GIS) version" (Figure 2). Apart from desktop browsers, the two versions are also accessible on the "MyObservatory" mobile app. The "fixed-area version" shows the forecast track of a tropical cyclone with its intensity indicated by different coloured symbols, facilitating viewing and sharing of the latest storm information. If one needs to view detailed geographical information, for example the cities or areas likely to be affected by the tropical cyclone, one may use the interactive map on the "GIS version" to pan and zoom into the area of interest. Furthermore, on desktop browsers, one can also overlay the latest satellite or radar imagery to know about the extent of clouds and rain bands associated with the tropical cyclone at the time."Earth Weather""Earth Weather" was recently launched on the Observatory's website and "MyObservatory" App. In addition to the latest tropical cyclone forecast track issued by the Observatory, computer model forecasts of surface wind direction shown in the form of animated streamlines and overlaid with a coloured map of wind speed or rainfall forecast facilitates people getting hold of the extent of winds and rain associated with the tropical cyclone at different forecast times (Figure 3). It should be noted that there is no manual adjustment to computer model forecast display. It explains why sometimes it may not be entirely consistent with the Observatory's tropical cyclone forecast track.Tropical cyclone track data setApart from directly accessing images of tropical cyclone forecast tracks, you may be interested in downloading the corresponding data. The Observatory has recently made available the machine-readable tropical cyclone track data files in XML format on its website and the DATA.GOV.HK portal. The content of the data files includes past and forecast tropical cyclone positions, intensity categories and the maximum sustained wind speed near the centre of the tropical cyclone (Figure 4). Please refer to the data dictionary on the DATA.GOV.HK portal for details of the data files.No matter how you obtain tropical cyclone forecast track information, please be reminded that very often there will be errors[Note] in the forecast track and intensity, and such errors generally increase with forecast lead time. You can also browse the Tropical Cyclone Track Probability webpage to appreciate the trend of its movement and the chance of different track scenarios. Please also take note of the latest tropical cyclone warning issued by the Observatory.

Tracking

November 2019

https://www.hko.gov.hk/en/education/tropical-cyclone/tracking/00528-tropical-cyclone-forecast-track-information.html

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Probabilistic Forecast for Tropical Cyclone Tracks

The Observatory launched The Tropical Cyclone Track Probability Forecast in August 2017. It is a post-processing product utilising ensemble forecast data from computer weather models.

There are around 30 tropical cyclones in the western North Pacific annually, normally around 6 of which will affect Hong Kong and necessitate the issuance of tropical cyclone warning signal. Tropical cyclones very often bring severe impact such as high wind, rainstorm and storm surge. As such, accurate forecast of tropical cyclone track is one of the major challenges to meteorological services. With ever-increasing computational power and continued advancement in the numerical weather prediction models, many people nowadays may just make use of direct model outputs to predict tropical cyclone track and discuss whether a tropical cyclone will directly hit Hong Kong. However, owing to the chaotic nature of weather systems, a small change in the initial condition may result in a substantial difference in the weather forecast for the next few days. Even though major meteorological services use high performance computers to simulate the weather evolution, their forecast tracks may differ from each other significantly because of the use of different weather observation data and computer weather models.Weather forecasters will analyse all the observation data, current synoptic pattern, forecast outputs from different computer weather models and formulate a forecast track of tropical cyclone based on an integrated approach and their experience. At present, the Observatory provides 5-Day Tropical Cyclone Track and Intensity forecast on its Tropical Cyclone Track Information webpage. This type of forecast with explicit information on the forecast track and intensity is known as deterministic forecast. However, it is inevitable that the errors of forecast track and intensity generally increase with increasing in forecast time. If we rely solely on the deterministic forecast, other potential alternative scenarios would be ignored.Probabilistic forecast could fill the gap of such deficiency in the deterministic forecast. The Observatory launched The Tropical Cyclone Track Probability Forecast in August 2017. It is a post-processing product utilising ensemble forecast data from computer weather models. In ensemble forecast, the computer weather model is run simultaneously for many times to simulate the tropical cyclone tracks for the next nine days under various weather scenarios. The probability of tropical cyclone passing within 120 kilometres of each location on a map will be calculated. For instance, Figure 1 depicts an example of forecast tracks of tropical cyclone Mangkhut in September 2018 from different ensemble members of computer weather models on 11 September. From the possible scenarios shown in the figure, Mangkhut was forecast to move towards the vicinity of Luzon Strait and then quickly edged close to the coastal areas of southern China, posing a serious threat to Hong Kong and Guangdong region. Ensemble members in Figure 1 indicated the potential landfall positions of Mangkhut covering the whole south China coast and even the northern part of Vietnam, showing that there was a certain extent of uncertainty in the overall forecast trend. From the probability forecast map in Figure 2, Mangkhut had a higher probability of making landfall near the Pearl River Estuary. During the daytime on 16 September, Mangkhut skirted within about 100 kilometres to the south-southwest of the Hong Kong Observatory and made landfall near Taishan, Guangdong, which was consistent with the track probability forecast given a few days ago as described above.In addition to showing the trend of movement and possible scenarios of a tropical cyclone, how can we make better use of the information on the track probability forecast? Taking Severe Typhoon Soulik in 2018 as an example, the probability forecast map on 19 August 2018 in Figure 3 shows a 50% chance of posing a threat to the vicinity of Korea. Therefore, travellers who planned to visit or had already been there could adjust their itinerary or leave earlier depending on their own situation and needs. Meanwhile, the chance for Soulik to affect eastern China like Shanghai or Kyushu of Japan was relatively low. However, if their plan to visit these places was vulnerable to weather, the corresponding backup or contingency arrangement should be prepared for the sake of safety. Besides Tropical Cyclone Track Probability Forecast, the Observatory also launched the Extended Outlook to provide the 14-day probability forecast of daily minimum and maximum temperatures. The Observatory is developing other probabilistic forecasts with a view to supporting the public, the government and the public utilities to better prepare for the weather changes and manage their risks as early as possible in order to mitigate and reduce the impact of natural disasters.

Tracking

September 2018

https://www.hko.gov.hk/en/education/tropical-cyclone/tracking/00518-tropical-cyclone-track-probability-forecast.html

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Super Computer Super Accurate?

For the same weather system, observational data gathered by different meteorological agencies can differ, depending on the types of observation instruments used, area of coverage of observations, data collection methodology, analysis and prediction algorithms etc. Such differences in the initial data ingested by the numerical models, even though the differences may only be small, could lead to vastly different and contrasting prediction outcomes.

In recent years, you may see from time to time headlines such as "Typhoon may hit Hong Kong next week". Yet checking the Hong Kong Observatory's website, there is no sign of any "typhoon" at all. So where does the typhoon come from?Supercomputers are now used by many meteorological agencies around the world to run numerical models for weather forecasting. Some mobile applications with the function of providing numerical model predictions have also emerged. Users can easily make reference to such apps on their smartphones to obtain information on changes in the weather; and the media also like to "whip up the storm" to make news. But are these predictions always reliable?For the same weather system, observational data gathered by different meteorological agencies can differ, depending on the types of observation instruments used, area of coverage of observations, data collection methodology, analysis and prediction algorithms etc. Such differences in the initial data ingested by the numerical models, even though the differences may only be small, could lead to vastly different and contrasting prediction outcomes. In general, the longer the forecast period, the larger is the uncertainty. The deviation may also increase accordingly. Take, for example, the case of Super Typhoon "Talim" in September 2017. Some numerical models at the early stage predicted that "Talim" would cross the Luzon Strait or Taiwan and head towards the coastal areas of Guangdong and Fujian. However, "Talim" eventually made a turn east of Taiwan, crossed the East China Sea and hit Japan.For more complex and intricate weather systems such as rainstorms of a volatile and changeable nature, forecast results from numerical models may vary even over short forecast periods. Therefore, it is never clever to over-rely on predictions based on just a couple of individual computer model simulations. It would be better to consult official meteorological agencies that provide authoritative forecasts based on a comprehensive assessment of local and overall situations, and regularly check for the latest updates in the forecast and warning messages issued.

Tracking

April 2018

https://www.hko.gov.hk/en/education/tropical-cyclone/tracking/00508-super-computer-super-accurate.html

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Tracking by Satellite Observations

Satellite pictures are now received at half-hourly intervals by the Hong Kong Observatory and form a near real-time observational basis for tracking tropical cyclones. Combining with other methods in locating the tropical cyclone, the centre of a tropical cyclone are plotted on weather map to show its movement.

Tropical cyclones develop over the ocean and spend most of their lives over water.  As conventional observations over the ocean are sparse, satellite analysis is by far the most important method in locating the centres of tropical cyclones. The appearance of a well-formed eye is a definitive indicator of where the tropical cyclone is (Figure 1). If an eye is absent, it is still possible to obtain a reasonable estimate of the centre by tracing the spiral rain bands of the storm (Figure 2) or by studying the relative motion of cloud features near its centre (Figure 3). Satellite pictures are now received at half-hourly intervals by the Hong Kong Observatory and form a near real-time observational basis for tracking tropical cyclones.  Combining with other methods in locating the tropical cyclone, the centre of a tropical cyclone are plotted on weather map to show its movement (Figure 4).

Tracking

June 2010

https://www.hko.gov.hk/en/education/tropical-cyclone/tracking/00159-tracking-by-satellite-observations.html

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Tracking by Radar Observations

Different radars have different effective ranges for detecting tropical cyclones depending on the wavelength, power, and the height of the location of the radar antenna. For Hong Kong, the centre of a tropical cyclone can normally be tracked on the radar if it is located within 500 kilometres of the territory.

Tracking tropical cyclones using satellite pictures was discussed in a previous article.  A similar approach can be used to locate the centre of a tropical cyclone if it comes within the tracking range of a weather radar.  A weather radar detects rain in the atmosphere by emitting pulses of microwave and measuring the reflected signals from the raindrops.  In general, the more intense the reflected signals, the higher will be the rain intensity. The distance of the rain is determined from the time it takes for the microwave to travel to and from the rain. Again, an eye on the radar screen is a clear indicator of where the tropical cyclone is (Figure 1).  If an eye is absent, it is still possible to obtain a reasonable estimate of the centre by studying the relative motion of rain echoes near its centre (Figure 2). Different radars have different effective ranges for detecting tropical cyclones depending on the wavelength, power, and the height of the location of the radar antenna. For Hong Kong, the centre of a tropical cyclone can normally be tracked on the radar if it is located within 500 kilometres of the territory. Doppler weather radar has become increasingly popular in recent years.  Apart from rain intensity, it is also capable of measuring the approach (or departing) speed of raindrops, commonly called the radial speed. The principle is that the faster the raindrops move towards the radar, the higher will be the frequency of the microwave reflected from raindrops. The raindrops' approach speed is a good estimation of the winds which carry the raindrops. For a tropical cyclone, the radial speed is zero (i.e. the raindrop is neither moving towards nor away from the radar) along the line joining the centre of the cyclone and the radar (Figure 3).  This line is called the zero isodop.  As such, the centre of a tropical cyclone always lies on the zero isodop.  In Figure 4, the white line is the zero isodop. 

Tracking

September 2010

https://www.hko.gov.hk/en/education/tropical-cyclone/tracking/00158-tracking-by-radar-observations.html

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Tracking by Actual Observations

Apart from satellite and radar, surface observations are also useful information in locating tropical cyclones, especially for those weaker ones whose centres are not well defined on satellite pictures. Both ship and land observations in the vicinity of a tropical cyclone are useful.

Apart from satellite and radar, surface observations are also useful information in locating tropical cyclones, especially for those weaker ones whose centres are not well defined on satellite pictures.Both ship and land observations in the vicinity of a tropical cyclone are useful.  The inflow angle method is the most commonly used method.  Here, the basic assumption is that the tropical cyclone is a circularly symmetric weather system.  The principle is that because of friction with surface, the air inside the circulation of the storm flows at an inflow angle towards the centre of the tropical cyclone.  The inflow angle ranges from 15 to 30 degrees and is loosely taken to be 20 degrees most of the time.The method is, with a minimum of three surface wind observations, (1) to draw a line making an inflow angle with the direction of the surface wind observation, (2) to draw a line perpendicular to the line in (1), and the centre of the tropical cyclone is at the centre of the polygon bounded by lines constructed by (2).However, this method cannot always be used as ship observations are scarce over the ocean and may not be available in the vicinity of tropical cyclones.  In applying this method, the wind observations must be used with care as they may be affected by other pressure systems such as the northeast or southwest monsoon. 

Tracking

December 2010

https://www.hko.gov.hk/en/education/tropical-cyclone/tracking/00157-tracking-by-actual-observations.html

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Why Tropical Cyclone Recurves?

TC motion can be thought of as the cumulative effect of many factors, including the environmental wind and the presence of other tropical cyclones nearby. The environmental wind is usually of primary importance to determining TC motion and it can be represented by the winds at a certain altitude, or the mean of the winds at different altitudes.

In Hong Kong, tropical cyclone (TC) attracts immense public concern during TC season every year.  Although the intensity of a TC is of significant interest, the impact of a TC to Hong Kong depends much on where it is going, or its track.  In Northern Hemisphere, recurvature of a TC is defined as the situation when a TC transits from a mainly westward track to a northward and sometimes even an eastward track.  In order to forecast TC track and in particular, whether it will recurve, one must understand how a TC moves and how different factors influence its motion.TC motion can be thought of as the cumulative effect of many factors, including the environmental wind and the presence of other tropical cyclones nearby.  The environmental wind is usually of primary importance to determining TC motion and it can be represented by the winds at a certain altitude, or the mean of the winds at different altitudes.What determines the environmental wind?  Synoptic-scale weather systems are very important in influencing the environmental wind.  The most prominent ones are upper level trough at mid-latitude and subtropical ridge.  A trough can alter the environmental wind to carry a component flowing more towards the north or northeast direction such that the cyclone recurves, or at least takes a more northerly track.  A strong subtropical ridge usually steers a TC to move along its periphery.  If the ridge retreats eastward, there is a high chance for the TC to move along the southwestern or western flank of the ridge and track northward.Besides the above, there are also other factors affecting the TC motion, for example, the interaction of TC with terrain and the presence of other tropical cyclones nearby.  All these factors affect each other, making the forecast of TC motion a challenging task.

Tracking

September 2011

https://www.hko.gov.hk/en/education/tropical-cyclone/tracking/00155-why-tropical-cyclone-recurves.html

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Interactions between Tropical Cyclones

Whenever there are interactions between two or more tropical cyclones, they will drag each other, rotate, cause one of them weakening, merge together or escape from each other, etc. These are superimposed on the steering flow of the synoptic environment.

Entering July this year, tropical cyclone activities over the western North Pacific have been increasing. The satellite imagery in Figure 1 shows that there are a total of three tropical cyclones over the northeastern part of the South China Sea and the western North Pacific. You may wonder: If Chan-hom continues to edge closer to Linfa, what will be the impacts on the latter? Concurrent occurrences of three tropical cyclones happened in the past, but were not many. Some examples in recent years can be found in the HKO's educational material. Dating back to the 20's to 30's of the last century, Dr. Sakuhei Fujiwhara (1884 - 1950) already discovered that when two tropical cyclones approach each other, they tend to rotate anti-clockwise about a point between them. This phenomenon is usually known as the Fujiwhara effect. Research studies showed that tropical cyclones begin to interact more prominently when their centers come within around 1200 km of each other. The degree of interaction increases as the separation distance decreases[1], while the separation distance where interaction commences depends on the sizes of the tropical cyclones[2]. Other research studies also pointed out that the interaction between two tropical cyclones depends on the sizes and intensities of the tropical cyclones as well as the environmental steering flow; whereas tropical cyclones of unequal sizes likely to have greater interaction than two of similar sizes[3]When two tropical cyclones come into proximity, it may bring about the following situations: 1. The two tropical cyclones rotate in a stable orbit (Fujiwhara effect), followed by a release and escape (Figure 2). For example in 2009, tropical cyclone Parma near the Philippines interacted with another tropical cyclone Melor, and Parma underwent a looping motion during 5-7 October (Figure 3 and 4).2. One tropical cyclone is captured and "swallowed up" by another tropical cyclone, or the two tropical cyclones undergo a merger (Figure 5). This scenario is most likely to occur when one tropical cyclone is much larger and stronger than the other[4]. Examples are Zeb and Alex in 1998 (Figure 6); Namtheun in 2010 near the Taiwan Strait interacting with Lionrock over the northeastern part of the South China Sea, weakening and dissipating afterwards (Figure 7 and 8).3. The two tropical cyclones only exhibit "semi-direct" interaction, and their tracks are largely determined by the steering flow associated with other synoptic weather systems[4][5] (Figure 9).Whenever there are interactions between two or more tropical cyclones, they will drag each other, rotate, cause one of them weakening, merge together or escape from each other, etc. These are superimposed on the steering flow of the synoptic environment. The tracks of the tropical cyclones will then become rather complex, making forecasts more difficult. Nowadays, we have a grasp of the basic conceptual models, and computer numerical models can also generally capture the interaction processes between tropical cyclones. However, as many factors come into play (including changes in intensities, sizes and relative positions, etc. of the tropical cyclones), there will be discrepancies between different model forecasts, posing great challenges to forecasters. In any case, members of the public are advised to pay close attention to the latest tropical cyclone information and weather forecasts issued by the Observatory.

Tracking

July 2015

https://www.hko.gov.hk/en/education/tropical-cyclone/tracking/00460-interactions-between-tropical-cyclones.html

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What is Fujiwhara Effect?

'Fujiwhara effect' applies to the interaction of two tropical cyclones. In general, two tropical cyclones may start to affect each other when the distance between them is around 12 latitude degrees (about 1350 kilometers).

'Fujiwhara effect' is named after Dr. Fujiwhara of Japan who performed a series of experiments and observations on water vortices from 1921 to 1923. He discovered interaction between two cyclonic vortices when they were close to each other. The vortices would rotate cyclonically about an axis connecting their centres. The two vortices also had a tendency to approach each other and then merge together.In meteorology, 'Fujiwhara effect' applies to the interaction of two tropical cyclones. In general, two tropical cyclones may start to affect each other when the distance between them is around 12 latitude degrees (about 1350 kilometers). When this occurs, The tropical cyclones will rotate around one another in a counter-clockwise direction (in Northern Hemisphere) about their geometric centre. The centre needs not be the middle of the axis. The stronger cyclone tends to have a dominant effect on the track of the weaker one. The interaction will end when: there is a stronger influence of a large scale weather system from outside, one of the tropical cyclones weakens or the two cyclones merge. Research by the Observatory shows that if the distance between the two tropical cyclones is 1200 kilometers or more, the chance of merging is slim.Figure 2 and 3 show the tracks of tropical cyclones Wukong and Sonamu in August 2006. It is evident that Wukong had a counter-clockwise movement during 14 and 15 August under the influence of Sonamu.Tropical cyclones appear in the Northwestern Pacific every summer. It is not rare to have two tropical cyclones there at the same time. When 'Fujiwhara effect' occurs, we may see changes in the tropical cyclones' movement and speed. This will increase difficulties when their tracks are forecast.

Tracking

https://www.hko.gov.hk/en/education/tropical-cyclone/tracking/00160-what-is-fujiwhara-effect.html

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How to Track and Forecast Tropical Cyclones?

The intensity of a tropical cyclone is determined by the strength of the surface winds near the centre. However, unless there are land stations in the vicinity, or in the rare case of a ship reporting near by, this kind of information is not readily available.

The intensity of a tropical cyclone is determined by the strength of the surface winds near the centre. However, unless there are land stations in the vicinity, or in the rare case of a ship reporting near by, this kind of information is not readily available. The termination of aircraft reconnaissance over the western North Pacific in August 1987 has cut off another important source of information. With the scarcity of direct data, the intensity of a tropical cyclone has to be inferred from analysis of cloud top temperatures as shown on enhanced infra-red satellite pictures.Over the vast expanse of the oceans, satellite analysis plays an important role in locating the centres of tropical cyclones. Satellite pictures are now received at hourly intervals and form a near real-time observational basis for tracking tropical cyclones.The appearance of a well-formed eye is a definite indicator of where the storm is. If an eye is absent, it is still possible to obtain a reasonable estimate of the centre location by tracing the spiral rainbands of the storm or by studying the relative motion of cloud features near its centre.A similar approach can be used to locate the centre of a tropical cyclone if it comes within the effective range of the weather radar of a land station. Different radars have different effective ranges for detecting tropical cyclones depending on the wavelength, power, and the height of the location of the scanner. For Hong Kong, the centre of a tropical cyclone can normally be tracked on the radar if it is located within 500 kilometres of the territory.Despite advances in satellite and radar technology, conventional weather chart analysis remains the basic tool of forecasters. During the approach of a tropical cyclone, surface charts are analyzed on an hourly basis while charts of higher levels in the atmosphere are drawn on a 6-hourly basis.The usefulness of chart analysis is not confined to providing information on the position, directional tendency, and intensity of the approaching tropical cyclone. It also impresses upon forecasters the changing background weather pattern to which the storm is intimately related. If the trends of such changes are correctly diagnosed and anticipated, forecasters will be in a better position to assess the future movement and intensity variation of the tropical cyclone.Left on its own, a tropical cyclone in the Northern Hemisphere has an inertial tendency to drift towards the northwest. Nevertheless, its motion is more often dominated by external forces. The basic concept here is that the environmental current steers the tropical cyclone. An analogy is the movement of a water eddy circulating but moving bodily within the main flow of a stream or river. Over the western North Pacific, most of the tropical cyclones form along the southern or southwestern flank of the subtropical ridge of high pressure where the steering current flows to the northwest. This coincides with the inertial movement tendency and explains the predominant northwestward storm tracks observed.In fact, the prevailing storm tracks reflect the important steering role played by the ridge of high pressure in the Pacific. For example, when the Pacific ridge does not extend beyond Taiwan, tropical cyclones tend to move round the western periphery of the ridge and turn northeastwards, a scenario commonly known as "recurvature". If the ridge extends further west, tropical cyclones will stay under an easterly steering flow and head towards southern China. If the ridge axis shifts to the south, tropical cyclones will travel westwards at lower latitudes.If the ridge is persistently strong and the environmental pattern is slow-changing, then the movement of tropical cyclones will be more straightforward. This is, of course, more often than not, too good to be true. Changes and fluctuations frequently occur during the lifetime of a tropical cyclone and other factors may also become important. As a result, the actual storm track may be highly irregular and eccentric, as in the extreme example of Typhoon Wayne (1986). Examples of factors that need to be considered in track forecasting are given below: - westward extent of the Pacific ridge; - north-south migration of the ridge axis; - interaction with mid-latitude weather systems; - interaction with other tropical cyclone(s); - interaction with terrain; - weak flow region where the steering current is ill-defined. While tropical cyclone track forecasting based on interpretation of past and current weather situations requires a lot of skill and experience on the part of the forecasters, guidance is also available from a host of objective methods.Climatology and statistics are the mainstream objective methods used for forecasting the track of a tropical cyclone. In essence, what is being pursued is to try to learn from the past so as to infer the future. The rationale is that a storm in circumstances (in terms of time of the year, location, and prevailing atmospheric environment) similar to that of previous storms is likely to behave similarly. The end products are normally some kind of average, probability, extrapolation, nomograms, or correlation formulae. Apart from the behaviour of the storm itself, these methods can also provide helpful indications as to the probable local wind and weather conditions.Another major stream consists of various statistical-dynamical methods. Here, the dynamical forces that are responsible for steering the storm are also taken into consideration in projecting future storm movement.A more involved exercise, but one which is gaining popularity all the time, is to simulate the real atmosphere by means of a numerical model. After carefully screening the initial data and meticulously incorporating them into the model, the "mathematical atmosphere" is allowed to run its course according to the imposed physical laws and specified empirical constraints. To deal with the enormous amount of computation, high-speed computers are required to make the exercise operationally viable. Such computers are only available in major meteorological centres around the world. The Hong Kong Observatory receives the numerical products of some of these centres as well as running its own limited-area model. The various models perform very well in the realm of global scale features and systems of higher latitudes. However, the forecasting of tropical and smaller scale systems such as tropical cyclones has only met with limited success and generally falls short of the desired degree of consistency. This remains an area of active research. The continued application of resources to refine these methods is expected to produce improved models and skills as time goes on.The science of tropical cyclone forecasting is by no means perfect. The multitude of contributing factors are so complex that they are as yet not entirely understood, or are only crudely represented (sometimes for practical reasons) in the various forecast schemes. The volatile nature of tropical cyclones, with their inherent fluctuation in both motion and intensity, is also partly to blame.Further uncertainty arises from the problem of identifying the initial position of the tropical cyclone, which many experts regard as a major source of error in forecasting. The question of where the storm is now is not purely for the sake of filling in the numbers in the warning bulletins. It is also critical in predicting where the storm will be. The logic here is quite understandable. If the input data (in this case the present storm position) is imperfectly known, then one should not expect the forecast methods to give perfect output (in this case, future storm position). In fixing the initial position, not every tropical cyclone has the eye-catching feature of an eye for forecasters to pinpoint. Some do not even have spiral bandings to give clues to the possible circulation centre. In poorly organized storms, there can be strong asymmetry and irregularity in the horizontal structure as well as distortion in the vertical alignment.Another problem related to Hong Kong concerns its geographical setting. Most tropical cyclones approach the coast of southern China on a west-northwestward track. With the orientation of the coastline at such an oblique angle to the storm track and Hong Kong being such a small target, a deviation of only 10 degrees in the direction of storm movement is enough to divert the storm to Hainan Island instead of a direct hit on Hong Kong.Faced with an uncertain and ever-changing situation, forecasters need to keep a close watch and be prepared to make timely updates and intelligent decisions. By the same token, the public should be aware of the uncertainties involved and be prepared to respond to the latest development as indicated by the warning bulletins.

Tracking

https://www.hko.gov.hk/en/education/tropical-cyclone/tracking/00156-tracking-and-forecasting-tropical-cyclones.html

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"Best-track"- a 20/20 Hindsight?

After the passage of the storm, the Observatory will collect all the data, including the unconventional ones, for verification and comparison against the real-time data received. The Observatory will then perform a detailed forensic-like analysis to re-construct a complete picture of the storm's movement and intensity change as a reference for future climatological and cases studies.

After the passage of a tropical cyclone, the Observatory will issue a provisional report on the cyclone, including its track and intensity, as well as the local winds, rainfall and water levels brought about by this storm.You might notice that the information or analysis given in the provisional report may differ from the operational one issued when the storm was affecting Hong Kong. That is because some meteorological data may not be available to the Observatory in a timely manner during the passage of the storm. Thus the duty weather forecaster could only exercise his/her professional judgement and issue forecast and warning in a timely manner based on the information available at that time.After the passage of the storm, the Observatory will collect all the data, including the unconventional ones, for verification and comparison against the real-time data received. The Observatory will then perform a detailed forensic-like analysis, known as "best track analysis", to re-construct a complete picture of the storm's movement and intensity change as a reference for future climatological and cases studies. At the time when computers were not yet popular, the best track analysis was drawn manually with the storm's position, intensity and pressure densely plotted on a map, making it hard to read without a magnifying glass!

Tracking

February 2018

https://www.hko.gov.hk/en/education/tropical-cyclone/tracking/00502-besttrack-a-2020-hindsight.html

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Tropical cyclone forecast systems at the Observatory

The article gives an overview on how weather forecasters make use of different types of information and tools to perform analysis and forecasting of tropical cyclone.

Whenever a tropical cyclone may pose a threat to Hong Kong, forecasters at the Observatory will closely monitor and analyse its movement and development round the clock and assess its potential impacts to the territory. The related monitoring and forecasting work involves a large amount of meteorological data and forecasting tools. In 2000, the Observatory developed a “Tropical Cyclone Information Processing System” (TIPS) which makes use of Geographical Information System (GIS) technology to integrate and display such information on a single platform, supporting analysis and decision-making by forecasters. In view of ever-increasing amount and type of weather information, a new-generation TIPS was put into operation in 2022. This article will give an overview on how weather forecasters make use of different types of information and tools to perform analysis and forecasting of storms.Tropical cyclone analysisTo forecast the future movement and development of a tropical cyclone, one must first have a good grasp of its latest location and intensity. As observation data over the ocean are severely limited, forecasters usually determine the location of the tropical cyclone centre by identifying the associated spiral cloud bands or rain bands from satellite or radar imageries. Although satellite and radar information could also help analyse the intensity of a tropical cyclone, such information is not as detailed as observations made on the ground or over the seas. Apart from routinely receiving observation data and ship reports from around the world via the Global Telecommunication System, the Hong Kong Observatory also, under collaboration with the Guangdong Meteorological Service and the State Oceanic Administration, obtains observation data from automatic weather stations in real-time. The latter are particularly helpful in assessing the intensity change and location of tropical cyclones approaching the south China coast.On TIPS, forecasters are able to display the above information on a digital map simultaneously, as well as to overlay the tropical cyclone track, so that a thorough assessment of the latest storm development can be made (Figure 1).Tropical cyclone track forecastsWith the advancement of numerical weather prediction technology, the overall accuracy of tropical cyclone track forecasts has improved in recent years. However, various global computer models differ in their spatial resolution and how atmospheric physical processes are simulated. These, together with inherent uncertainties in the evolution of weather systems, different computer models sometimes predict significantly different tracks for the same tropical cyclone. As with other meteorological centres in the region, the Observatory makes reference to products from multiple computer models when forecasting tropical cyclone tracks.TIPS will automatically collect and process tropical cyclone track forecasts for major computer models from around the world, as well as warning bulletins from meteorological centres in the region. Forecast storm tracks by different models and meteorological centres are then displayed on a map to facilitate easy comparison by forecasters. The system also supports the calculation of an “ensemble track” based on multiple computer models using a weighted-average approach. Forecasters will make reference to this “ensemble track”, other computer model products (such as track probability forecasts) and observation data to prepare the tropical cyclone forecast tracks to be published on the Observatory website and mobile app.Assessment of local weather impactsAfter formulating the forecast track, forecasters then need to assess whether and when high winds associated with the tropical cyclone will affect Hong Kong, which are important in deciding whether and when tropical cyclone warning signals are to be issued. In the past, forecasters have mostly referred to forecasting tools (known as “kidneys” by forecasters) developed based on statistics of past tropical cyclones affecting the territory, yet such tools have their limitations, for example they may not reflect high winds brought by the combined effects of a tropical cyclone and other weather systems. Hence, besides such tools, TIPS also provides overlay of wind speed forecasts from computer models, which serve as additional reference information to forecasters. With the above information, together with actual wind speed data over Hong Kong and neighbouring places, forecasters can more effectively estimate when local winds will pick up.Apart from high winds, forecasters also need to pay attention to the rise in water level caused by storm surges. Another function of TIPS is to provide forecast track information to the Observatory’s storm surge prediction system, which computes the changes in water levels at different locations. Forecasters will then issue alerts to public and relevant government departments according to the system outputs.ConclusionsAnalysing and forecasting tropical cyclones are an enormous challenge. Apart from keeping a close eye on all available observation information, forecasters need to continuously analyse forecasts from computer models and to assess the impacts of the storm to Hong Kong by applying forecasting tools and their experience, so that forecasts and warnings can be issued in a timely manner. The “Tropical Cyclone Information Processing System” (TIPS) is therefore an important tool in tropical cyclone operations at the Observatory. With more and more meteorological observation and forecast data, the development team will continue to enhance TIPS so that it could better support forecasters.

Forecasting

October 2022

https://www.hko.gov.hk/en/education/tropical-cyclone/forecasting/00684-Tropical-cyclone-forecast-systems-at-the-Observatory.html

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Forecast of Tropical Cyclone Tracks with Ensemble Forecast

Movement of tropical cyclones could be predicted with numerical weather models. However, as different models often give different predictions, it is necessary to develop a method to decide on the tropical cyclone forecast tracks scientifically. The Observatory started employing ensemble method for forecasting tropical cyclone tracks back in 2002, and has since accumulated almost ten years of experience.

Movement of tropical cyclones could be predicted with numerical weather models.  However, as different models often give different predictions (as shown in Figure 1), it is necessary to develop a method to decide on the tropical cyclone forecast tracks scientifically.The Observatory started employing ensemble method for forecasting tropical cyclone tracks back in 2002, and has since accumulated almost ten years of experience.  It was observed that ensemble forecast on average performs better than individual models.  The basis of ensemble forecast is to take the average of the latitudes and longitudes of the forecast positions of all model forecasts as the forecast positions.  Members of the ensemble adopted by the Observatory include models from Japan Meteorological Agency (JMA), European Centre for Medium-Range Weather Forecasts (ECMWF), United Kingdom Meteorological Office (UKMO), and National Centers for Environmental Prediction (NCEP).  Members are weighted equally and any member that deviates significantly from others may be taken out before averaging.While the use of ensemble forecast improves the accuracy of tropical cyclone forecast tracks, there are certain limitations in operations:To solve the third problem, the Observatory introduced the use of motion vector consensus in 2009.  This method treats the next 24-hour movements of each model forecast as a motion vector, then takes the average of the motion vectors and applies the resultant vector onto the actual position to obtain the 24-hour forecast position, and so forth for the 48-hour and 72-hour forecast positions.  In addition to eliminating unreasonable movement, it also helps improve the accuracy of forecasts.Forecast positions according to different models could differ significantly, and are sometimes clustered into two or more groups.  Simply averaging the positions may lead to unreasonable forecast tracks.  Currently, forecasters would, based on their professional judgment, subjectively decide whether to remove individual model(s) based on the majority of model outputs and the prevailing synoptic pattern. As the ensemble is based on the average of individual models, it may inherit a large error when most models are giving a wrong prediction concurrently. In operation, one or more ensemble members may not provide all 24-hour, 48-hour and 72-hour forecast positions.  As a result of fewer members for longer range forecast, such as for the 72-hour forecast, the ensemble may give an unreasonable forecast track.

Forecasting

September 2011

https://www.hko.gov.hk/en/education/tropical-cyclone/forecasting/00162-forecast-of-tropical-cyclone-tracks-with-ensemble-forecast.html

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Kidney and Beach Ball (Wind Speed and Direction Forecast)

Observatory colleagues start monitoring the development of a tropical cyclone days before a warning signal is issued. The work involves long hours of close watch and detailed analyses round-the-clock. Over the past few decades a number of tools for such analyses have been developed for the forecaster use.

Forgive me for this rather weird title. The Observatory is under some pressure when it comes to warning people of approaching weather, especially typhoons. Too early a tropical cyclone signal is issued, the public may be baked in brilliant sunshine a couple of days before the effect of the storm is felt. Too late, and Hong Kong will risk the loss of life and properties because of inadequate forewarning.As a matter of fact, Observatory colleagues start monitoring the development of a tropical cyclone days before a warning signal is issued. The work involves long hours of close watch and detailed analyses round-the-clock. Over the past few decades a number of tools for such analyses have been developed for the forecaster use. The device of funny names for these tools is a welcome relief.As a tribute to Observatory colleagues, this blog introduces what are known to them as 'kidney' and 'beach ball'. Some of the tools have been presented to the public, notably in the annual Observatory Open Day and in seminars, as a sharing of scientific knowledge. This has generated considerable interest in Internet forum discussions.What is a 'kidney' and why is it so called? Simply, it represents an area on the map where it is probable that windy conditions will affect a location in Hong Kong once a tropical cyclone comes within that area. Figure 1(a) shows a 50% probability 'kidney' for strong winds (i.e. force 6) at Waglan during the passage of a typhoon (the term 'typhoon', used in this blog covers typhoons, severe typhoons and super typhoons). Typhoon is the strongest amongst all categories of tropical cyclones.The 50% probability means that according to past records of typhoons affecting Hong Kong, half of those located inside the curve had brought strong winds to Waglan, an outlying island about 20 kilometres southeast of Central, Hong Kong. The shape of the curve in Figure 1 gives the tool's namesake. The curve's elongated shape towards the southwest also tells us that Waglan (and Hong Kong as a whole) is more exposed when the storm is located to the southwest of Hong Kong.So how would a forecaster use this tool? What he/she does is to overlay it onto the predicted storm track. This gives an indication of the time when the storm will give strong winds at Waglan, as well as when the winds will start to decrease, as shown in Figure 1(b).Figure 2 presents the 30, 50 and 70% curves. The 70% probability curve means a 70% chance of strong winds at Waglan once a typhoon comes within that region. Naturally it is smaller than the 50% one, which in turn is smaller than the 30% one. How to choose among these curves? The forecaster assesses the extent of the storm itself, and may adopt a smaller probability in the case of an extensive storm and a higher probability for a compact storm.For a different storm category such as tropical depression, tropical storm or severe tropical storm, you need a different set of probability curves. The forecaster may have to grapple with more than one set of curves when the storm shows signs of intensification or weakening. There is also a different set of curves each for other locations such as Victoria Harbour and the airport.Figures 1 and 2 tell us that Waglan is more exposed when the storm is located to our southwest. Well, they don't tell us where the winds are going to come from. To determine the wind direction, you need a 'beach ball'. An example of this is given in Figure 3.Looking at Figure 3, there is probably no need to explain why the tool is called a 'beach ball'. Thus, a storm located to the southwest of Hong Kong, say near Hainan Island, will bring east to southeasterly winds to Waglan.The above tools were possible after years of experience and the accumulation of long periods of observations. The weather station at Waglan, for example, was established nearly 60 years ago, in 1952. The tools are a result of local knowledge.The importance of 'local knowledge' cannot be over-emphasized. Take global weather forecast as an example. There are many inexpensive weather models in operation nowadays, some requiring the power of only a desktop personal computer. Are they accurate? Well, not necessarily. As a matter of fact, 40 degrees Celsius had been predicted for Hong Kong in day-to-day forecasts, whereas the highest temperature ever recorded in Hong Kong was 36 degrees at the Observatory headquarters and 38 degrees elsewhere within the territory.It was for this reason that the Observatory, acting on behalf of the United Nations World Meteorological Organization (WMO) developed the World Weather Information Service (WWIS) in 2000. Day in and day out, the latest weather forecasts for over 1300 cities in the world are collected by a computer at the Observatory and automatically put on a website for free access by everybody. The weather forecasts come from a total of over 120 weather services from around the world and are available in eight languages. These are official forecasts, and are based on local knowledge that even the best weather prediction models in the world may not be able to replicate.This year, a 'Future Version' of WWIS has been developed and is GIS-enabled (GIS stands for Geographic Information System). Now being showcased at the World Expo 2010 in Shanghai, it allows the user to freely pan, scroll, zoom in and out of the virtual globe, and quickly get to the city of interest where the weather forecast, some up to 3 to 7 days ahead, is instantaneously displayed. Next time you go abroad, be sure to use this service: https://worldweather.wmo.int/en/home.html.

Forecasting

September 2010

https://www.hko.gov.hk/en/education/tropical-cyclone/forecasting/00164-kidney-and-beach-ball-wind-speed-and-direction-forecast.html

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Forecasting Yearly Number of Tropical Cyclones Affecting Hong Kong

HKO attempted to use the model data provided by CPC to forecast yearly N500 of Hong Kong. The period of our investigation is 1982-2009. In order to have a fair assessment of the forecast skill of the statistical-dynamical approach, cross-validation was performed. In the cross-validation, a single observation was hidden or deleted from the dataset in turn.

Since 2001, the Hong Kong Observatory has been providing the forecast of annual number of tropical cyclones affecting Hong Kong to the general public via the Internet.  The forecast is generally phrased in a range of consecutive numbers, e.g. 5 to 7 tropical cyclones, and disseminated well before the start of the tropical cyclone season in Hong Kong.  Long range tropical cyclone forecast not only fulfils the interests of the general public and the media, but also provides useful information to decision makers in their planning and decision processes.  For example, with such information, the authorities may have better disaster prevention/reduction planning and preparation ahead of the tropical cyclone season.Formulation of the annual tropical cyclone forecast had been based on a conceptual model using the forecast El Nino/La Nina status of the year as predictor.  After years of implementation and experience, it is found that the skill of this conceptual model approach is more or less on a par with that of the climatology.  In recent years, the Observatory explored the use of dynamical climate model outputs provided by major climate prediction centres to improve the annual tropical cyclone forecast.  These dynamical climate model outputs can be utilized to predict not only regional seasonal temperature and rainfall but also tropical cyclone activity as well.Tropical cyclone activity affecting Hong Kong is currently defined as the number of tropical cyclones coming within 500 km of the city (N500).  This definition is both objective and meaningful because the long term average of yearly N500 is almost the same as the climatological mean of the annual number of tropical cyclones necessitating the issuance of local warning signals.  So a tropical cyclone coming within 500 km of Hong Kong means some impact on the city, whether meteorological or societal.  Fig. 1 shows the monthly average of N500 during the period 1971-2000.  The tropical cyclone season of Hong Kong can be roughly defined as the period June-October in which the average N500 is above 0.5 for each month.  Tropical cyclone activity affecting the territory is most active during July-September.  Normally, the annual tropical cyclone forecast is issued in March every year, i.e. two months before the start of the tropical cyclone season.The Observatory is currently investigating a statistical-dynamical approach to forecast the yearly N500 of Hong Kong.  In many studies, the Poisson distribution is employed to model the distribution of tropical cyclone occurrence in a certain period of time (e.g. summer) in a certain region (e.g. the South China Sea).  The Poisson distribution is solely determined by the average number of occurrence, which is normally unknown and estimated from historical data.  Hence, a simple method of forecasting tropical cyclone occurrence is based on the mean of historical data.  However, such an invariable forecast is bound to be unable to capture the year-to-year variation of the observation.  A better forecast can be achieved by assuming that the average number of occurrence depends on some environmental factors (e.g. winds and sea surface temperature) which vary in time.  This is a rather reasonable assumption as tropical cyclone occurrence depends on where the tropical cyclone genesis takes place and whether the atmospheric circulation is favourable to bring the tropical cyclone to the region of interest.  These environmental factors can be extracted from global dynamical climate model outputs.  Once a statistical relationship is established between the past observations and the corresponding environmental factors, it can be used to forecast new observations.As one of the Global Producing Centres for Long Range Forecasts designated by the World Meteorological Organization (WMO), the Climate Prediction Center (CPC), NOAA, USA, provides operational global long range forecast data as well as re-forecast data generated by dynamical climate model on its website.  HKO attempted to use the model data provided by CPC to forecast yearly N500 of Hong Kong.  The period of our investigation is 1982-2009.  In order to have a fair assessment of the forecast skill of the statistical-dynamical approach, cross-validation was performed.  In the cross-validation, a single observation was hidden or deleted from the dataset in turn.  Predictor selection, forecast model building and prediction for the hidden/deleted observation would be conducted based on the "1-out" dataset.  The predictions would then be verified against the hidden/deleted observations to assess the skill of the method.Fig. 2 shows the time series of actual observations of yearly N500 of Hong Kong and the forecasts given by the new statistical-dynamical approach.  The climatological forecast was used as a reference forecast which produced a root mean squared error (rmse) of 1.87.  The statistical-dynamical approach is able to reduce the forecast error by 30% to 1.28.  The cross-validation result shows that the potential of real life application of the forecast method is high.  Outputs of different climate models will be tried out and further study on the extraction of environment factors will be conducted with a view to improving the forecast skill.

Forecasting

June 2011

https://www.hko.gov.hk/en/education/tropical-cyclone/forecasting/00163-forecasting-yearly-number-of-tropical-cyclones-affecting-hong-kong.html

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Typhoon 5-6 (Average Number of Tropical Cyclones)

The tropical cyclone records in Hong Kong reveal that there were very large inter-annual and inter-decadal fluctuations in the annual number of tropical cyclones affecting Hong Kong. The lowest record is two in a year (in 1997 and 2007) while the highest record is 11 (in 1974).

This blog title may sound a bit peculiar to many people. "5-6" is certainly not a new name of tropical cyclone in the western North Pacific. In fact, 5 and 6 are the two most frequently occurred numbers of tropical cyclones affecting Hong Kong[Note] each year, based on the statistics in the past five decades (See Figure 1). The tropical cyclone records in Hong Kong reveal that there were very large inter-annual and inter-decadal fluctuations in the annual number of tropical cyclones affecting Hong Kong. The lowest record is two in a year (in 1997 and 2007) while the highest record is 11 (in 1974). The average numbers of tropical cyclones for various periods are given in Table 1. While all the averages shown fall well within the range between 5 and 7, the 30-year average decreases from the 6 to 7 for the 30 year periods of 1961-1990 and 1971-2000 to between 5 and 6 for 1981-2010. The long term 50-year average from 1961 to 2010 is about 6. The decrease in the 30-year averages in the last few decades is an interesting subject to look into. The inter-annual and inter-decadal variations in tropical cyclone frequency in the western North Pacific are likely related to a number of factors which affect the sea surface temperature and atmospheric circulations in the region. These factors include the El Niño and La Niña as well as the Pacific Decadal Oscillation[1-2] which may have effects for periods of several years to decades. Moreover, some recent research suggested that the long term sea surface temperature rise due to human-induced climate change[3] could also affect the tropical cyclone frequency, intensity and movement. Since the causes of the variations in the tropical cyclone frequency in the western North Pacific and the South China Sea are rather complex and may comprise both natural and human-induced influences, more studies are still required to further understand the relative contributions of natural variations (e.g. El Niño and La Niña) and human-induced factors to the past and future changes in tropical cyclone activity in the region.In view of the presence of significant inter-decadal or even multi-decadal fluctuations in tropical cyclone activity in our region, the 30-year average of tropical cyclone counts may exhibit noticeable changes from one period to the other. Therefore, in depicting the norm for the annual number of tropical cyclone affecting Hong Kong, it may be more appropriate to refer to the long term average of 1961-2010 which is about 6 in a year.Note: Refer to tropical cyclones crossing within 500km of Hong Kong (22.3o N, 114.17o E). The use of 500 km range as a proxy indicator of tropical cyclones affecting Hong Kong is because the average number of tropical cyclones entering this range (about 6) is roughly the same as the long-term average of the annual number necessitating the issuance of local tropical cyclone warning signals in Hong Kong. 

Forecasting

April 2012

https://www.hko.gov.hk/en/education/tropical-cyclone/forecasting/00161-typhoon-56-average-number-of-tropical-cyclones.html

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Beware of Multi-Hazard Combined Effect

The concurrence of multiple hazards can lead to superposition of their effects, resulting in exacerbated disasters that may exceed the resilience limits of the urban infrastructure.

When natural hazard such as high winds or heavy rain occurs individually, well-designed urban infrastructure can normally withstand the associated disasters with tolerable impacts being expected on the society. However, the concurrence of multiple hazards can lead to superposition of their effects, resulting in exacerbated disasters that may exceed the resilience limits of the urban infrastructure. The longer the combined effect lasts, the higher the risks and the greater the societal impacts potentially. Without multi-hazard combined effect, flooding may occur only at some low-lying areas or spots with poor drainage in Hong Kong even if a rain episode is intense enough to warrant the Amber Rainstorm Warning signal. In contrast, another rain episode with similar rainfall intensity can pose rather different flood risk in the presence of a tropical cyclone (TC). The more persistent the rain is, the higher the cumulative flood risk. Imagine that the Strong Wind Signal, No.3 has been in force with strong winds generally blowing over Hong Kong (even occasional gales over some areas) for some time. A lot of dead leaves, branches, gravels or debris may have flurried to the ground and be washed down to different locations, resulting in potential blockage to the entrances of drainage systems at such locations. Moreover, as the storm approaches, the sea level may rise significantly above the normal position due to storm surge and/or pounding waves, resulting in potential seawater intrusion and flooding over relatively low-lying areas along the coast (Figure 1). This example illustrates how the impact on flooding due to the combined effect of TC and prolonged rainstorm can be way more severe than the totality of impacts from the two hazards occurring individually. Figure 1 Similar rainstorm situations with (right) and without TC (left). Effects from TC-induced high winds, storm surge, pounding waves, etc., can add up and degrade drainage. Whenever inclement weather events are expected to superimpose, you may need to start preparations earlier and adopt multiple precautionary measures as appropriate. Pay attention to outdoor conditions, in particular whether there is traffic disruption or not, and reserve sufficient time plus safety margin for your trip before you start. To allow early protective measures by members of the public, the Observatory will promulgate on any significant wind-rain combined effect, be it actual or forecast. Related Articles Previous Next Without multi-hazard combined effect, flooding may occur only at some low-lying areas or spots with poor drainage in Hong Kong even if a rain episode is intense enough to warrant the Amber Rainstorm Warning signal. In contrast, another rain episode with similar rainfall intensity can pose rather different flood risk in the presence of a tropical cyclone (TC). The more persistent the rain is, the higher the cumulative flood risk. Imagine that the Strong Wind Signal, No.3 has been in force with strong winds generally blowing over Hong Kong (even occasional gales over some areas) for some time. A lot of dead leaves, branches, gravels or debris may have flurried to the ground and be washed down to different locations, resulting in potential blockage to the entrances of drainage systems at such locations. Moreover, as the storm approaches, the sea level may rise significantly above the normal position due to storm surge and/or pounding waves, resulting in potential seawater intrusion and flooding over relatively low-lying areas along the coast (Figure 1). This example illustrates how the impact on flooding due to the combined effect of TC and prolonged rainstorm can be way more severe than the totality of impacts from the two hazards occurring individually. Figure 1 Similar rainstorm situations with (right) and without TC (left). Effects from TC-induced high winds, storm surge, pounding waves, etc., can add up and degrade drainage. Whenever inclement weather events are expected to superimpose, you may need to start preparations earlier and adopt multiple precautionary measures as appropriate. Pay attention to outdoor conditions, in particular whether there is traffic disruption or not, and reserve sufficient time plus safety margin for your trip before you start. To allow early protective measures by members of the public, the Observatory will promulgate on any significant wind-rain combined effect, be it actual or forecast. Related Articles Previous Next Whenever inclement weather events are expected to superimpose, you may need to start preparations earlier and adopt multiple precautionary measures as appropriate. Pay attention to outdoor conditions, in particular whether there is traffic disruption or not, and reserve sufficient time plus safety margin for your trip before you start. To allow early protective measures by members of the public, the Observatory will promulgate on any significant wind-rain combined effect, be it actual or forecast.

Weather Effects and Impact

May 2022

https://www.hko.gov.hk/en/education/tropical-cyclone/weather-effects-and-impact/00674-Beware-of-Multi-Hazard-Combined-Effect.html

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Why Windows Shattered by Typhoons Fall Outside the Building?

Whenever a typhoon hits Hong Kong, there may be news reports about shattered glass windows of buildings from time to time. If you examine them more closely, you may notice that the broken glass windows were not necessarily on the windward side of the buildings. Sometimes you will see glass debris or even air conditioners falling outside of the building. What are the reasons behind?

Whenever a typhoon hits Hong Kong, there may be news reports about shattered glass windows of buildings from time to time. If you examine them more closely, you may notice that the broken glass windows were not necessarily on the windward side of the buildings. Sometimes you will see glass debris or even air conditioners falling outside of the building (Figure 1). What are the reasons behind?In fact, the breaking of glass windows during typhoons is mainly due to the following three reasons: (1) impact on the windows by hard windborne debris; (2) the pressure exerted by winds (i.e. wind loading [1]) directly on the windward side of the building exceeds what the glass window can withstand; (3) high winds blow across buildings leading to a difference in air pressure between indoor and outdoor and the glass window cannot withstand the induced force. We may explain the last point with the Bernoulli's principle. The Bernoulli's principle is derived from the law of conservation of energy (i.e. the sum of kinetic energy, potential energy and internal energy must be kept constant). The simple interpretation is: when a fluid (e.g. air) increases in speed of motion and its height remains the same, its pressure will decrease.There are many applications of the Bernoulli's principle. For example, an aircraft wing is specifically designed in a way that the air flowing over the top surface of the wing moves faster than that below its bottom. According to the Bernoulli's principle, the pressure on the upper surface of the wing will be lower than that from below. This pressure difference results in an upward lifting force (Figure 2), which counteracts the aircraft’s own weight, enabling it to float in the air. In the city, the distance between the outer walls of some adjacent buildings is relatively small. When air passes through the narrow corridor between them, it will increase in speed due to the wind tunnel effect. According to the Bernoulli's principle, the air pressure in the corridor will thereby fall and the buildings on both sides of the corridor will experience a suction force. This will pull the objects on the surface of the buildings (such as glass or air conditioner) away from the buildings (Figure 3). Even for a single-block building, when high winds blow across it, a positive pressure area will be formed on the windward side of the building due to reduced speed of the airflow. If an opening exists on the windward side of the building (e.g. gaps between windows), positive pressure will also be built up inside the building. Coupled with the pressure difference due to the Bernoulli's principle, an outward force will be exerted to the sides and leeward surface of the building (Please see Figure 4), pulling the objects on the surface of the building away from it.Therefore whenever a typhoon strikes, you should be cautious about the glass windows of the building even if they are not facing headwinds!

Weather Effects and Impact

March 2020

https://www.hko.gov.hk/en/education/tropical-cyclone/weather-effects-and-impact/00540-why-windows-shattered-by-typhoons-fall-outside-the-building.html

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Can Aircraft Take-off and Land at the Hong Kong International Airport under the Influence of a Tropical Cyclone?

The stronger the crosswind, the greater is the potential risk for aircraft taking-off and landing. Moreover, the strength of crosswinds over the Hong Kong International Airport is closely related to the position of the tropical cyclone relative to Hong Kong. Do you remember the two tools mentioned in the Observatory’s blog "Kidney and beach ball"? The "Beach ball" can help us predict whether a tropical cyclone will bring crosswinds to the airport.

You may have come across the following situations: While flights were still able to depart and land with the No. 8 Gale or Storm Signal in force, but a significant number of flights were delayed or even cancelled when the No. 3 Strong Wind Signal had been issued. But why is this the case?Wind direction and wind speed over the airport depends on the position of tropical cyclonesThe stronger the crosswind, the greater is the potential risk for aircraft taking-off and landing (see "Crosswind and Aviation Safety"). Moreover, the strength of crosswinds over the Hong Kong International Airport is closely related to the position of the tropical cyclone relative to Hong Kong. Do you remember the two tools mentioned in the Observatory’s blog "Kidney and beach ball"? The "Beach ball" can help us predict whether a tropical cyclone will bring crosswinds to the airport. For instance, if a tropical cyclone adopts a track (see Track A in Figure 1) which passes to the north of Hong Kong, when it approaches Hong Kong from the east, the airport will first experience crosswinds from the northwest which would affect both the take-off and landing of the aircraft. When the tropical cyclone departs Hong Kong to the northwest later on, winds over the airport will become southwesterly, nearly parallel to runways. However, if a tropical cyclone passes to the south of Hong Kong (see Track B in Figure 1), the situation is just the opposite: The airport area will first be affected by the northeasterly winds nearly parallel to runways, and then the departing tropical cyclone to the southwest will bring southeasterly crosswinds to the runways which could have a greater impact on aircraft operations.The track which brings about sustained crosswindsIf you take a closer look at Figure 1, you may notice that the area in which tropical cyclones may bring southeasterly winds to Hong Kong is greater than the one that will bring northwesterly winds. In other words, even when a tropical cyclone is located away at Hainan Island, significant crosswinds could still occur at the Hong Kong International Airport. Generally speaking, taking-off and landing of aircraft will be affected when crosswind reaches 20 knots or above. To facilitate aviation forecasters to predict the occurrence of crosswinds brought by tropical cyclones, the Observatory has produced a set of "Crosswind Kidneys" specifically for the Hong Kong International Airport. They are actually probability curves of crosswinds at runways exceeding 20 knots when a tropical cyclone is in the vicinity of Hong Kong (see Figure 2). The "asymmetric butterfly-shaped" curves show that the airport is more likely to be affected by significant crosswinds when a tropical cyclone is passing to the south of Hong Kong (Track B). In this case, there is a higher chance of encountering crosswinds though the storm is departing and the overall threat to Hong Kong is diminishing.A tale of two tropical cyclones - "Usagi" and "Kalmaegi"Super Typhoon Usagi in September 2013 and Typhoon Kalmaegi in September 2014 both took west-northwesterly tracks. Usagi passed to the north of Hong Kong while Kalmaegi passed to the south of the territory (see Figure 3).As shown in the time series of the Figure 4, northwesterly crosswinds of 20 knots had started affecting the airport runways about 10 hours before Usagi came closest to Hong Kong. When Usagi departed Hong Kong, with local winds backing to southwesterly, crosswinds over the runways weakened and their impact on aircraft operations also reduced.Contrary to the case of Usagi, before Kalmaegi's closest approach to Hong Kong, east to northeasterly winds prevailed over Hong Kong and crosswinds at the runways were not significant. However, when Kalmaegi moved away from Hong Kong, crosswinds picked up instead. Crosswinds reaching 20 knots affected the runways for more than ten hours after its closest approach, bringing a longer lasting impact on aircraft operations. In short, tropical cyclones adopting different tracks across the coast of Guangdong may result in vastly different impacts on the aircraft operations in Hong Kong. Can aircraft take-off or land when a tropical cyclone warning signal is in force?As shown in Figure 4, the taking-off and landing of aircraft do not have a straight forward relationship with the tropical cyclone warning signals; whether or not an aircraft can take-off or land is mainly determined by the crosswinds as well as windshear and turbulence conditions (see "Impact of Strong Wind Associated with Tropical Cyclones on Aviation Operation"). Even when the No. 8 signal is in force, if the winds were blowing in nearly the same direction of the runways, they generally have a minor impact on aircraft operations. On the other hand, if the winds were blowing across the runways, particularly the southeasterly winds through the mountain gaps of Lantau Island, even though the No. 3 signal has replaced the No. 8 signal for the departing storm, the crosswinds may still exceed the operating limit of aircraft, and flights would be delayed or cancelled. Apart from meteorological factors, airlines may also change their flight schedules due to other factors. Therefore, please be advised to check the latest flight information before departing for the airport.

Weather Effects and Impact

July 2015

https://www.hko.gov.hk/en/education/tropical-cyclone/weather-effects-and-impact/00459-can-aircraft-takeoff-and-land-at-the-hong-kong-international-airport-under-the-influence-of-a-tropical-cyclone.html

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Very Hot Weather and Storm Surges Triggered by Tropical Cyclones

Occasionally, the weather in Hong Kong could be very hot together with intense sunshine and light wind conditions when a tropical cyclone is in the vicinity of Taiwan or Luzon strait. This is caused by the subsiding air at the outer circulation of the tropical cyclone.

The Hong Kong Observatory (HKO) has conducted meteorological monitoring for over 130 years starting with the measurement of basic meteorological elements like air pressure, temperature, humidity, wind speed and direction, rainfall etc., and then followed by making upper air and tidal measurements etc. Continuous and regular acquisitions of meteorological data are essential for establishing a climatological database for Hong Kong. Such database enables us to obtain the climatological average values of different meteorological elements with respect to various seasons and provides the foundation for weather forecasting and climate change monitoring. In this article, we have extracted some tropical-cyclone-related records in recent years from the database, and will briefly discuss the causes of these records.Up to 2020, the top two maximum temperatures recorded at the HKO Headquarters were 36.6°C on 22 August 2017 and 36.3°C on 8 August 2015 (Table 1) since records began in 1884. Both of these records were associated with the subsiding air at the outer circulation of tropical cyclones (Hato in 2017 and Soudelor in 2015).Occasionally, the weather in Hong Kong could be very hot together with intense sunshine and light wind conditions when a tropical cyclone is in the vicinity of Taiwan or Luzon strait. This is caused by the subsiding air at the outer circulation of the tropical cyclone. When the intense rising air near the centre of a tropical cyclone reaches the top of the troposphere, it can no longer rise but spread sideway due to the generally stable atmosphere aloft (stratosphere). It will eventually subside at the outer edge of the tropical cyclone (Figure 1). The subsiding air will raise the air temperature and suppress the formation of clouds. As a result, the weather in the affected regions is often sunny, hot and stuffy with haze.The weather normally deteriorates with squalls and heavy rain, and may be even storm surge as the tropical cyclone edges closer to Hong Kong. The low-pressure centre of the tropical cyclone sucks up sea water, while the high winds can push the water towards the shore (Figure 2). The combined effect causes the sea levels near the centre of the tropical cyclone to rise and trigger storm surge.Storm surge is calculated by subtracting the astronomical tide (created by gravitational attractions of the Moon and the Sun) from the sea level measured (Figure 3). The maximum storm surge recorded at Quarry Bay/North Point tide gauge was 2.35 m on 16 September 2018 (Table 2) [1], associated with Super Typhoon Mangkhut [2].The top two highest sea levels recorded at Quarry Bay/North Point were 3.96 m on 1 September 1962 and 3.88 m on 16 September 2018 (Table 3) [3], which were associated with tropical cyclones Wanda and Mangkhut respectively. The day Mangkhut hit Hong Kong fell on the seventh day of the eighth month of the lunar calendar, which was close to a neap tide day. Should Mangkhut have hit Hong Kong during the spring tide, the damages must be even more devastating. The sea level would also be affected by other factors such as the monsoon, shape of the coastlines, etc. Various factors could superimpose to affect the sea levels at different locations to different extents.Apart from squalls and heavy rain, very hot weather and storm surges associated with tropical cyclones should not be overlooked. With a warming globe and rising sea level, more frequent occurrences of extreme weather would be expected and the impact of which would even more pronounced. Let us contribute through continuously and actively adopting greener living style, saving energy and reducing carbon emissions.

Weather Effects and Impact

April 2021

https://www.hko.gov.hk/en/education/tropical-cyclone/weather-effects-and-impact/00562-Very-Hot-Weather-and-Storm-Surges-Triggered-by-Tropical-Cyclones.html

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Know More about Storm Surge

There are several factors that determine the storm surge generated by a tropical cyclone at a given location, including cyclone size and intensity, the direction of cyclone motion relative to the coast, and the shape of the coastline. A storm surge model is often used to simulate the effects of these factors.

What is storm surge?Storm surge is a rise of sea level due to the combined effects of low atmospheric pressure and high winds associated with tropical cyclones. Water is primarily being pushed towards the shore and piled up against the coast by the force of high winds associated with tropical cyclones (Figure 1).Low pressure at the centre of tropical cyclones also has the effect of enhancing the magnitude of a storm surge. As pressure is relatively higher around the periphery of a tropical cyclone, it tends to push down on the sea water outside and sucks up water near the cyclone centre (Figure 2).Storm surge is the sea level rise generated by a tropical cyclone over and above the astronomical tide and does not have a reference level. Storm tide is the sea level during a tropical cyclone due to the combination of storm surge and astronomical tide (Figure 3). Like mean sea level, storm tide requires a reference level. In Hong Kong, the reference level is called "Chart Datum" (please see "Terminology" below), determined by the Survey and Mapping Office of Lands Department.When storm surge occurs during astronomical high tide, the storm tide can get very high, and low-lying areas may be flooded. In general, if the storm tide in the Victoria Harbour is 3 metres above Chart Datum or higher, flooding may occur in the low-lying areas of Hong Kong.Major factors affecting storm surgeThere are several factors that determine the storm surge generated by a tropical cyclone at a given location, including cyclone size and intensity, the direction of cyclone motion relative to the coast, and the shape of the coastline. A storm surge model is often used to simulate the effects of these factors.Cyclone sizeWinds in a larger tropical cyclone (usually expressed in terms of radius of maximum winds) are pushing on a more extensive area of the ocean. Strong winds in a larger tropical cyclone also tend to affect an area for a longer period of time. For tropical cyclones passing to the south of Hong Kong (Figure 4(a)), storm surge at the Victoria Harbour generated by a tropical cyclone of minimum central pressure of 980 hPa with a radius of maximum winds of 100 km would be higher than that generated by a tropical cyclone of the same minimum central pressure but with a smaller radius of maximum winds of only 50 km (Figure 4(b)) as shown in the model simulation results.Cyclone intensityLower central pressure (stronger winds) will produce a larger storm surge. Along the track shown in Figure 4 (a), model-simulated storm surges at the Victoria Harbour generated by a tropical cyclone with a radius of maximum winds of 50 km and a minimum central pressure of 920 hPa would be higher than that generated by a tropical cyclone of the same size but with a higher minimum central pressure of 980 hPa (Figure 5).Direction of approach in cyclone motionAn approaching tropical cyclone bringing on-shore winds to the coast will generate a larger storm surge than a tropical cyclone with off-shore winds (Figure 6(a) & 6(b)). With winds swirling anti-clockwise around a cyclone in the Northern Hemisphere, cyclones passing to the south or southwest of Hong Kong (Track 1) will bring local winds from the east or southeast and water will pile up against the coast. For cyclones passing to the east or north of Hong Kong (Track 2), local winds will generally be from the north or northwest, pushing water away from the shore. As such, the storm surge effect will be significantly less even though the passage of the cyclone is much closer to Hong Kong. Shape of coastlineStorm surge is higher at bag-shaped bays than in channels. The sea water pushed towards the coast by on-shore winds will be trapped more easily by bag-shaped bays and the resultant sea level will be higher. This can be illustrated by storm surges at Tai Po Kau with a bag-shaped coastline in comparison with storm surges at the Victoria Harbour (Figure 7).TerminologyAstronomical tideAstronomical tide is the rise and fall of sea levels caused by the combined effects of gravitational forces exerted by the Moon, the Sun and the rotation of the Earth.Storm surgeStorm surge is a rise of sea level due to the combined effects of low atmospheric pressure and high winds associated with tropical cyclones.Storm tideStorm tide is the sea level during a tropical cyclone due to the combination of storm surge and astronomical tide.Hong Kong Principal Datum (HKPD)In Hong Kong, all heights and levels on land refer to the Principal Datum, formerly known as the Ordnance Datum. HKPD was previously determined by Dr. William Doberck, the first Director of the Hong Kong Observatory, as 1.125 m below the Mean Sea Level for the years 1887-1888. This figure was superseded by the updated Mean Sea Level derived using 19 years (1965-1983) of observation records from the automatic tide gauge at North Point, Victoria Harbour operated by the Hong Kong Observatory (Mean Sea Level (1965-1983)). HKPD was then determined as 1.230 m below Mean Sea Level (1965-1983).Chart Datum (CD)The Chart Datum, formerly known as the Admiralty Datum, is approximately the level of Lowest Astronomical Tide and is adopted as the zero point for Tide Tables since 1917. At present, CD is 1.376 m below Mean Sea Level (1965-1983), or 0.146 m below HKPD.

Weather Effects and Impact

September 2014

https://www.hko.gov.hk/en/education/tropical-cyclone/weather-effects-and-impact/00444-know-more-about-storm-surge.html

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Impact of Strong Wind Associated with Tropical Cyclones on Aviation Operation

Among the various weather hazards associated with tropical cyclones, strong wind often has impact on airport operation for a prolonged period of time. Strong winds over HKIA also adversely affect personnel working outdoors on aircraft parked on the ground for unloading or loading and at other airport facilities.

Among the various weather hazards associated with tropical cyclones, strong wind often has impact on airport operation for a prolonged period of time.On the morning of 29 September 2011 when Typhoon Nesat was centred around 380 km south of Hong Kong, strong to gale force easterly winds prevailed over and around Hong Kong International Airport (HKIA) (Figure 1), bringing high headwind (wind blowing towards aircraft) for aircraft landing or taking off in the runway direction towards the east-northeast.  Pilots normally prefer to land and take off in headwind as it increases the lift.  However, easterly airstreams of such high wind speeds, after passing through the mountain gaps, usually induce significant windshear and turbulence causing difficulty in the control of aircraft (Figure 2).  The Hong Kong Observatory therefore issued windshear and turbulence alerts and warnings to aircraft landing at and departing from HKIA.As Nesat moved further away from Hong Kong and made landfall over Hainan Island in the afternoon, wind speeds at HKIA gradually subsided to fresh to strong force (Figure 3).  However, with Nesat southwest of HKIA at that time, winds turned southeasterly.  This resulted in high crosswind (wind blowing from either sides of the aircraft) and controlling the aircraft for landing or taking off in either runway directions of HKIA became even more difficult.  Aircraft may not be able to land when the crosswinds exceed certain limit which depends on aircraft types, loading and other factors.  Airlines and pilots would usually make reference to weather forecasts of destination airports to plan for extra fuel required for landing or diverting to other places.  During the passage of Nesat, over 40 flights were cancelled, around 490 flights were affected and 44 aircraft were diverted due to adverse weather.Strong winds over HKIA also adversely affect personnel working outdoors on aircraft parked on the ground for unloading or loading and at other airport facilities.  The Observatory issues aerodrome warning of strong wind to aviation communities in HKIA for their necessary actions to protect outdoor workers at the airport.Details of the aviation weather services provided by the Hong Kong Observatory can be found on the Observatorys webpage for "Aviation Weather Service".  

Weather Effects and Impact

December 2011

https://www.hko.gov.hk/en/education/aviation-and-marine/aviation/00175-impact-of-strong-wind-associated-with-tropical-cyclones-on-aviation-operation.html

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Influence of Tropical Cyclones on Visibility in Hong Kong

Reduction in visibility is mainly caused by the absorption and scattering of visible light by suspended particulates. In summer, although the visibility in Hong Kong is usually better than in other seasons, reduction in visibility often occurs when a tropical cyclone is situated in the vicinity of Taiwan.

Reduction in visibility is mainly caused by the absorption and scattering of visible light by suspended particulates.  In summer, although the visibility in Hong Kong is usually better than in other seasons, reduction in visibility often occurs when a tropical cyclone is situated in the vicinity of Taiwan.  According to a study by the Observatory, the causes of reduced visibility are associated with the change in the source of air mass, the convergence of horizontal and vertical airstreams as well as the low wind speed (see Figure 1).Take Typhoon Haitang in 2005 as an example, when Haitang was moving in the vicinity of Taiwan on 19 and 20 July, the visibility in Hong Kong dropped significantly.  Figure 2 shows that on 14 July, when Haitang was still quite far away from Hong Kong, the air mass in Hong Kong was mainly from the relatively clean ocean.  However, with Haitang gradually approaching on 19 and 20 July, the outer circulation of the typhoon brought along air to Hong Kong from the mainland where human activities were high.  During the period when Haitang traversed Taiwan, the surface isobars near Hong Kong were slackened and hence the wind speed was low.  The outer circulation of Haitang also caused the convergence of the continental (west to northwesterly) airstream and the maritime (west to southwesterly) airflow near Hong Kong.  The air ascended near the centre of Haitang.  However, in places relatively far away from Haitang, such as Hong Kong, the air current was generally descending. Low wind speed and the convergence of horizontal and vertical airstreams hindered the 3-dimensional dispersal of suspended particulates and thus causing a drop in visibility.

Weather Effects and Impact

https://www.hko.gov.hk/en/education/weather/visibility/00089-influence-of-tropical-cyclones-on-visibility-in-hong-kong.html

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Very Hot Weather Ahead of an Approaching Tropical Cyclone

Do you notice that sunny and very hot weather usually precedes the approach of tropical cyclones in Hong Kong? More often than not, the severe thunderstorms that follow will bring down temperatures towards the evening.

Do you notice that sunny and very hot weather usually precedes the approach of tropical cyclones in Hong Kong?  More often than not, the severe thunderstorms that follow will bring down temperatures towards the evening.The weather on 27 and 28 July 2008 is a good example. The temperature at the Hong Kong Observatory on 28 July rose to 34.6 degrees, the highest of the month.  The severe thunderstorms that developed over inland Guangdong moved south to affect the territory in the afternoon of 27 July as well as the evening of 28 July. Hails were reported on 27 July during the passage of thunderstorms.At the time, Typhoon Fung-Wong over the western Pacific was moving towards Taiwan (Figure 1).  There was severe convection inside the circulation of the typhoon, resulting in intense rising air and hence formation of rain bands.  However, when the rising air reached the top of the troposphere, they could no longer rise but spread sideways and subside in the typhoons neighbouring areas.  The subsiding air brought fine and light wind conditions to areas beneath.  On that day (27 July), the subsiding air ahead of Fung-Wong brought fine weather to southern China (including Hong Kong, Figure 2), and abundant sunshine caused temperatures over inland areas to rise sharply.As typhoons in the Northern Hemisphere are swirling in an anti-clockwise sense, Fung-Wong near Taiwan brought light northerly winds to Hong Kong.  These winds carried the very hot air over inland areas to the territory.  Added to this was the subsiding air, which brought temperatures further up (Note: contrast this with rising air, which tends to cool with height).  Together with intense sunshine and light wind conditions, the local weather was very hot and uncomfortable.The very hot air near the surface also set off convection.  However, the ascending air was capped by the subsiding air from Fung-Wong and did not go very far up.  As the day proceeded surface temperatures continued to rise, and the convection became severe enough to overcome the subsiding air and reached a greater height, setting off thunderstorms (Figure 3) and even hails over the inland areas.  They followed the northerly winds to affect downwind Hong Kong (Figures 4 and 5).  Hails were reported at Sha Tau Kok, Fanling and Yuen Long during the passage of the thunderstorms that day.  The thunderstorms also brought with them cooler air from above to the ground surface and lowered the temperature.Typhoon Sinlaku which affected Taiwan around Mid-Autumn Festival (Figure 6) is yet another example of tropical cyclone bringing hot and uncomfortable weather to Hong Kong. The temperature at the Observatory reached a high of 34.2 degrees on 13 September.  The high of 33.8 degrees on 15 September is the highest recorded on the ensuing day of Mid-Autumn Festival since records began.  The thunderstorms brought about by Sinlaku were not as severe nor widespread as Fung-Wong.  However, the subsiding air of Sinlaku and the light wind condition rendered the dispersion of suspended particulates in the air rather difficult.  As a result, haze lingered over Hong Kong during the period and the visibility at Chek Lap Kok once dropped to 3600 metres on 15 September.

Weather Effects and Impact

https://www.hko.gov.hk/en/education/tropical-cyclone/weather-effects-and-impact/00173-very-hot-weather-ahead-of-an-approaching-tropical-cyclone.html

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Combined Effect of Tropical Cyclone and Northeast Monsoon

A tropical cyclone can give rise to, among other things, strong winds. Likewise, strong wind is one of the effects of northeast monsoon. The combined effect of a tropical cyclone and the northeast monsoon to a place means the effect experienced by this place when it is affected by both the tropical cyclone and the northeast monsoon simultaneously.

Have you come across the phase "combined effect of a tropical cyclone and the northeast monsoon"? This phase is sometimes used in weather bulletins or tropical cyclone warning bulletins issued by the Hong Kong Observatory. To help you better understand the phase, its meaning is explained here.  A tropical cyclone can give rise to, among other things, strong winds. Likewise, strong wind is one of the effects of northeast monsoon. The combined effect of a tropical cyclone and the northeast monsoon to a place means the effect experienced by this place when it is affected by both the tropical cyclone and the northeast monsoon simultaneously. One of the combined effects is stronger winds.In this article, the concept of isobar is used to explain the combined effect. The meaning of "isobar" is explained in a previous article. Suffice to recap that one can deduce wind speed from isobars in weather charts. Everything else being equal, winds tend to blow harder (gentler) where isobars are closer together (further apart). Technically, the distance apart between two isobars depicts the pressure gradient of the areas around. The pressure gradient is tight (slack) when the isobars are closer together (further apart). Tighter pressure gradient results in stronger winds.When a tropical cyclone affects a place, the weather map shows that isobars around the place are packed together with very tight pressure gradient (Figure 1). Similarly, a northeast monsoon will also bring tight pressure gradient to a place (Figure 2). When the place is being affected by both a tropical cyclone and the northeast monsoon, the pressure gradient around the place as revealed by the small distance between isobars are indeed tight (Figure 3). The daily weather chart of 12 October 2009 (Figure 4) shows a good example of the combined effect of Tropical Storm Parma and the northeast monsoon to Hong Kong and its vicinity. Under the combined effect of the northeast monsoon and Parma, Hong Kong was windy from 11 to 14 October.  The Strong Monsoon Signal was in force for 37 hours 20 minutes from 0250H, 11 October to 1610H, 12 October.

Weather Effects and Impact

March 2010

https://www.hko.gov.hk/en/education/weather/monsoons/00076-combined-effect-of-tropical-cyclone-and-northeast-monsoon.html

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To the East, To the West? (Effects of Landfall Position)

Apart from the intensity of the tropical cyclone, winds will become more violent and rain will be more persistent when the centre of the storm or the associated spiral rainbands draw closer to Hong Kong. However, owing to the structure of the tropical cyclone and orographic effect, the weather affecting Hong Kong is also determined by the landfall location of the storm.

Usually starting from May every year, Hong Kong will be prone to the impacts of tropical cyclones of different strength. The frequency peaks between July and September. In general, apart from the intensity of the tropical cyclone, winds will become more violent and rain will be more persistent when the centre of the storm or the associated spiral rainbands draw closer to Hong Kong. However, owing to the structure of the tropical cyclone and orographic effect, the weather affecting Hong Kong is also determined by the landfall location of the storm. Generally speaking, tropical cyclones making landfall to the west of Hong Kong often bring more severe weather to our region than those land to our east. The reasons are elaborated below.Semicircle EffectIn the Northern Hemisphere, winds surrounding a tropical cyclone blow in an anticlockwise direction. Imagining if a tropical cyclone is divided into two semicircles along its direction of movement, winds on the right semicircle will be in the same direction of the storm's translation motion while winds over the left semicircle will be in the opposite direction. As a result of the superposition of the cyclonic winds and the translational motion of the storm, winds on the right semicircle are usually stronger than those on the left semicircle, which are thus termed the dangerous semicircle and navigable semicircle respectively[1].Furthermore, in the western North Pacific and the South China Sea, subtropical ridge usually stays on the northeastern side of a tropical cyclone, resulting in a tighter pressure gradient and thus stronger winds in between them. As tropical cyclones over the region usually track northwestwards, the dangerous semicircle will overlap with the tight gradient over the northeastern flank of the storm such that winds over the right semicircle will strengthen further (as shown in Figure 1).Orographic EffectSituated along the coast of southern China, Hong Kong is sheltered from the north by east-west oriented mountain ranges inland but faces the open sea to the south. In addition to the frictional effect over land, northerly winds over Hong Kong are usually weaker than southerlies if other factors are held the same. As winds of a tropical cyclone rotate in an anticlockwise direction in the Northern Hemisphere, prevailing winds are usually southeasterlies and thus stronger when a tropical cyclone lands to the west of Hong Kong. In contrast, relatively weaker northwesterly winds will prevail when a tropical cyclone makes landfall to the east of Hong Kong.Storm SurgeAn offshore rise of water can be induced by the winds of a tropical cyclone and the low pressure near its centre. This effect is called a storm surge. (For details, please refer to the past article "What is a storm surge?" at the Observatory's Blog.)[2] When a tropical cyclone lands to the west of Hong Kong, prevailing southeasterly winds will push on the sea surface and pile up sea water over the coast. Coupled with the low pressure centre of the storm, serious storm surge can be resulted. On the contrary, when a tropical cyclone makes landfall to the east of Hong Kong, prevailing northwesterly winds will push offshore water back to the sea, thus counteracts the effect of the low pressure. Storm surge is usually insignificant under this circumstance. Due to the reasons mentioned above, tropical cyclones which make landfall to the west of Hong Kong usually bring more severe weather to our region than those land to our east. Table 1 compares the impacts to Hong Kong brought by two tropical cyclones with similar intensity but different landing locations.Nonetheless, each tropical cyclone is unique. It is not appropriate to conclude that storms landing to our east will bring lesser threat to us. Apart from the location of landfall, the impact of a storm to Hong Kong includes many factors, like the structure and intensity of the storm, the distance of the storm centre from Hong Kong, the distribution of its spiral rainbands etc. Take an example, in July of 2001, although Typhoon Utor made landfall to the east of Hong Kong (near Shanwei), the No. 8 NE, NW and SW Gale or Storm Signals were issued successively and winds over the territory generally reached strong to gale force. A number of trees and scaffolding were toppled. The rainbands of Utor also brought more than 150 mm of rainfall to most parts of the territory. All in all, whenever a storm is approaching Hong Kong, even though it is forecast to land to our east, we should maintain vigilant and should not underestimate the associated severe weather.

Weather Effects and Impact

June 2011

https://www.hko.gov.hk/en/education/tropical-cyclone/weather-effects-and-impact/00166-to-the-east-to-the-west-effects-of-landfall-position.html

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Swells from distant typhoons

Hong Kong is facing the South China Sea and is exposed to the threat of possible calamity inflicted by the sea during the approach of tropical cyclones. While most people realize such dangers during the close approach of typhoons, it may not be so clear to them that a distant tropical cyclone several hundred kilometers away can bring about swells to Hong Kong. These swells may drag people near the shore line out to the sea.

Hong Kong is facing the South China Sea and is exposed to the threat of possible calamity inflicted by the sea during the approach of tropical cyclones.  While most people realize such dangers during the close approach of typhoons, it may not be so clear to them that a distant tropical cyclone several hundred kilometers away can bring about swells to Hong Kong.  These swells may drag people near the shore line out to the sea.The force of winds causes motion of sea surface.  Waves are raised by winds blowing locally, the stronger the winds, the higher the waves.  Swells are caused by winds far away.  The winds far away generate waves in that distant area.  When these waves propagate to reach one's location, they are called swells.Huge waves are usually whipped up by high winds associated with tropical cyclones. For a distant tropical cyclone, its waves may travel towards Hong Kong to become swells (Figure 1).  These swells travel at speeds much faster than the movement of tropical cyclones.  When a tropical cyclone is a few hundred kilometers away, local weather may be deceptively fine with light winds.  However, severe swells generated by the tropical cyclone could have already reached the coastal areas, often catching people by surprise.  When the swells enter shallow waters, their heights would increase.  The Chinese saying that "three feet wave in no wind" really has its scientific backing.  These swells can pose hazards to people staying close to the shoreline or engaging in fishing or water sports.  There have been occasions in the past that these swells cause casualties in Hong Kong.  One recent example was a university student being dragged to the sea in Tai Long Sai Wan when Typhoon Ketsana traversed the South China Sea at more than 700 kilometers away from Hong Kong (Figure 2).In future, even if a tropical cyclone is located several hundred kilometers away, beware of the possible swells that it can bring to Hong Kong and be extra careful if you really need to engage in activities close to the shore line.

Weather Effects and Impact

December 2009

https://www.hko.gov.hk/en/education/aviation-and-marine/marine/00171-swells-from-distant-typhoons.html

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Significant Weather Information of Tropical Cyclone (TC SIGMET)

Whenever a tropical cyclone (TC) of tropical storm intensity or above is affecting the HKFIR or is forecast to enter the HKFIR within the following 12 hours, the Aviation Forecaster at the AMO will issue a TC SIGMET for the tropical cyclone.

Under the stipulation of the International Civil Aviation Organization (ICAO), a Meteorological Watch Office (MWO) is responsible for issuing significant weather information (SIGMET) concerning the occurrence or expected occurrence of specified en-route weather phenomena which may affect the safety of aircraft operations.  The MWO is required to follow the Asia/Pacific Regional SIGMET Guide in the issuance of SIGMET.In Hong Kong, the Airport Meteorological Office (AMO) of the Hong Kong Observatory is a MWO responsible for issuing SIGMET on hazardous weather including tropical cyclone, thunderstorm, turbulence, icing and volcanic ash cloud when it is affecting or is expected to affect the Hong Kong Flight Information Region (HKFIR) over the northern part of the South China Sea.  As SIGMET contains warning information, it is of the highest priority among other types of meteorological information in the dissemination through dedicated channels so as to ensure timely transmission to the pilots.Whenever a tropical cyclone (TC) of tropical storm intensity or above is affecting the HKFIR or is forecast to enter the HKFIR within the following 12 hours, the Aviation Forecaster at the AMO will issue a TC SIGMET for the tropical cyclone. The TC SIGMET contains information on the observed position and/or the forecast position of the TC, the vertical and horizontal extent of its associated cumulonimbus clouds, the direction and speed of movement as well as the forecast TC position for the next 6 hours.  Examples of TC SIGMET in text and graphical format for tropical cyclone Nock-ten in July 2011 are given in Figure 1 and 2 respectively.

Tropical Cyclone Warning

September 2011

https://www.hko.gov.hk/en/education/tropical-cyclone/associated-weather-warnings/00176-significant-weather-information-of-tropical-cyclone-tc-sigmet.html

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Tropical Cyclone Season in 2011 (When would the Observatory issue the No.8 Signal)

Whenever a tropical cyclone enters or develops over the South China Sea, family and friends used to ask if a No. 8 Signal would be issued. This is understandable since most members of the public will get an extra day or half-day off and do not have to go to work or school when a No. 8 Signal is issued. Notwithstanding this, the primary consideration

Stepping into December, successive cold surges have already affected southern China, signifying nearly the end of the tropical cyclone season this year. Up till now, there were 6 tropical cyclones coming within 500 kilometres of Hong Kong this year. This is in line with the forecast of 6 to 9 tropical cyclones made earlier in March and also matches the revised forecast of 5 to 7 made in September. Among the 6 tropical cyclones, 5 necessitated the issuance of tropical cyclone signals.It is worth mentioning Typhoon Nesat that affected Hong Kong in end-September. It brought the first and was very likely the only No. 8 Signal (i.e. Gale or Storm Signal) of the year. It was also the first No. 8 Signal since Typhoon Koppu in September 2009.Whenever a tropical cyclone enters or develops over the South China Sea, family and friends used to ask if a No. 8 Signal would be issued. This is understandable since most members of the public will get an extra day or half-day off and do not have to go to work or school when a No. 8 Signal is issued. Notwithstanding this, the primary consideration of the Observatory in issuing a No. 8 Signal is public safety. The Tropical Cyclone Warning System operates on the basis of objective criteria and scientific approach. The actual and forecast winds in Hong Kong are the bases for issuing tropical cyclone signals. So, how to determine if a No. 8 Signal is necessary? Here below are our considerations.When the Observatory issues a No. 8 Signal, it means gale or storm force wind is expected or blowing generally in Hong Kong near sea level. Factors affecting the wind strength in Hong Kong include: a) Intensity of tropical cyclone; b) Distance of tropical cyclone from Hong Kong; and c) Wind structure of tropical cyclone. Intensity of Tropical CycloneTropical cyclone is classified based on the maximum sustained wind speed near its centre. The classification adopted in Hong Kong[1] is as follows:From this table, if a tropical cyclone just reaches the intensity of a Tropical Depression, then no matter how close the Tropical Depression is from Hong Kong or even if it passes overhead Hong Kong, it will not bring sustained gale force wind to the territory. As such, there is no need to issue a No. 8 Signal in this case. However, when a tropical cyclone reaches the intensity of a Tropical Storm or above, we then need to analyze and predict its distance from Hong Kong and its wind structure before determining the need of a No. 8 Signal.Distance of Tropical Cyclone from Hong KongIn general, apart from the weaker winds at the centre of a storm (usually called an eye[3]), the winds are the strongest near the centre and are weaker at greater distance from the centre. So, when a storm comes closer to Hong Kong, the chance of high winds affecting us is greater, whereas the chance of high winds is lower when the storm is farther away.While many friends would like to know how close a storm will bring a No. 8 Signal, there is actually no fixed distance within which a No. 8 Signal is necessary. Of course, in general if the tropical cyclone is at quite some distance from the territory, the chance of No. 8 Signal is lower. In fact, the farthest tropical cyclone since 1957 that brought No. 8 Signal to Hong Kong is Typhoon Joe in September 1980. Its closest distance to Hong Kong was 350 km. So, when a tropical cyclone's closest distance to Hong Kong is still some 400 km or farther, the need to issue No. 8 Signal is generally rather low.Wind Structure of Tropical CycloneThe distribution of wind strength around a storm can be very asymmetrical as a result of its structure and its interaction with the surrounding atmospheric circulation. Asymmetric wind distribution is also quite common[4]. Figure 1(b) is the wind distribution around Typhoon Nesat to the southwest of Hong Kong on 29 September this year. It shows that the winds on the eastern and northern quadrants of Nesat were generally stronger than those on the southern and western quadrants. This was related to the combined effect of Nesat with the northeast monsoon over southern China at that time. Thus, winds can differ considerably at different places even if they are at the same distance from the storm's centre. We cannot solely rely on the distance of a storm from Hong Kong to determine the need of a No. 8 Signal, we also need to analyze and estimate the storm's wind distribution and its changes.Take Figure 2 as an example, Typhoon Yutu skirted to the south of Hong Kong in September 2001. Its closest distance from the territory was 180 km. Gale force winds associated with Yutu affected Hong Kong, mainly over the southern districts. The No. 8 Signal was issued. In comparison with Yutu, Nesat was farther away from Hong Kong. Based on the forecast track at 8 p.m., 28 September 2011 (dashed line in Figure 2), Nesat would pass about 400 km from Hong Kong. As mentioned above, no No. 8 Signal was ever issued for this distance. Moreover, the area of gale winds of Nesat was not extensive at that time (roughly the yellow region in Figure 1(a)) and was mostly concentrated over the northern part of the South China Sea. Based on the then forecast track, the chance of gales generally affecting Hong Kong was rather low. However, Nesat took a more northwesterly track overnight (Figure 2), making it closer to Hong Kong. Its closest distance to Hong Kong was 350 km, about 50 km closer than that previously forecast. Meanwhile, the region of gales of Nesat showed signs of considerable extension (Figure 1(b) indicates that the gale region of Nesat covering Hong Kong in the morning of 29 September). In considering that Nesat became closer to Hong Kong than previously forecast and that local winds were strengthening, the Observatory issued the No. 8 Signal at 4:40 a.m., 29 September.In conclusion, all the above three factors, i.e. intensity of tropical cyclone, its distance from Hong Kong and its wind structure, will vary with time. Local winds[5] will also change as a result. The Observatory will closely monitor the variation of winds in Hong Kong and neighbouring areas as well as the future development and movement of tropical cyclone, in order to provide timely and effective information for the public. Since the state-of-the-art technology still cannot entirely grasp the future evolution of a storm accurately[6], forecast errors and changes are unavoidable. Through more channels in the future, we will encourage and facilitate members of the public to pay more attention to changing weather, to raise their awareness to inclement weather against casualties and losses.

Tropical Cyclone Warning

December 2011

https://www.hko.gov.hk/en/education/tropical-cyclone/associated-weather-warnings/00179-tropical-cyclone-season-in-2011-when-would-the-observatory-issue-the-no8-signal.html

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