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Satellite Imagery of Super Typhoon Nida

In November 2009, Nida over the western North Pacific developed into a Super Typhoon. It had gone through all the 6 intensity classifications during its life cycle. Satellite pictures capturing images of Nida during each of the intensity classifications are shown below. This series of satellite pictures constitutes a good reference for readers interested in tropical cyclone. From the series of satellite pictures, the increasingly organized banding feature of Nidas cloud as a swirl became progressively more prominent as it intensified from a tropical depression into a severe tropical storm.

The Hong Kong Observatory uses a new tropical cyclone classification scheme with effect from 2009 (see previous article on this).  It consists of 6 categories as follows:In November 2009, Nida over the western North Pacific developed into a Super Typhoon.  It had gone through all the 6 intensity classifications during its life cycle.  Satellite pictures capturing images of Nida during each of the intensity classifications are shown below.  This series of satellite pictures constitutes a good reference for readers interested in tropical cyclone.  From the series of satellite pictures, the increasingly organized banding feature of Nidas cloud as a swirl became progressively more prominent as it intensified from a tropical depression into a severe tropical storm (Figure 1-3).  An eye developed as Nida intensified into a typhoon (Figure 4) and the eye became smaller and yet more well-defined as Nida developed further into a super typhoon (Figure 5-6).

Case Studies

March 2010

https://www.hko.gov.hk/en/education/tropical-cyclone/case-studies/00182-satellite-imagery-of-super-typhoon-nida.html

en

Changes in Typhoon Nuri during Landfall in Hong Kong

Before the landfall of Nuri, the dual Doppler radar winds showed that there were two circulation centres at an altitude of one kilometre, one centre near Tates Cairn and another over the southern part of Hong Kong Island.

Since 1960, about 18 tropical cyclones have made their passage over the territory of Hong Kong.  Amongst them was Super Typhoon Hope in 1979.  Hope made landfall over the eastern part of the New Territories and its eye was clearly discernible on the Hong Kong Observatorys weather radar before landfall (Figure 1).  Another example was Typhoon Nuri in 2008 which passed very close to the Hong Kong Observatory Headquarters.  Nuri made landfall over the eastern part of Hong Kong and weakened into a severe tropical storm on 22 August 2008.  Radar observations indicated that its eye was not well organized then (Figure 2).  Moreover, the circulation of Nuri re-organized itself resulting in complicated changes in its track which could be accurately determined from the network of automatic weather stations operated by the Hong Kong Observatory.The track of Typhoon Nuri over Hong Kong is shown in Figure 3.  Nuri moved northwestwards over the northern part of the South China Sea on 22 August and made landfall near the Hong Kong University of Science and Technology in Sai Kung that afternoon.  As the pressure is low near the centre of a tropical cyclone, the winds spiral inwards towards the centre in an anticlockwise direction (in the southern hemisphere, the rotation is clockwise).  When Nuri was about to make landfall, winds over the northeastern part of the New Territories were east to northeasterlies, with southwesterlies over the southeastern part of Hong Kong and northerlies over other parts of the territory (Figure 4). After making landfall, Nuri was affected by mountains to the northwest of its centre and its circulation re-organized itself. The original centre moved northwestwards and dissipated rapidly.  A new centre soon formed near Tseung Kwan O (Figure 5) and turned to move westwards across the Victoria Harbour.  Winds at Tseung Kwan O changed from northerlies to light winds, and then becoming east to southeasterlies.  As the cyclonic circulation of Nuri passed through Hong Kong, many weather stations recorded the turning of wind direction and a temporary decrease in wind speed.The Observatory operates two Doppler weather radars, one at Tai Mo Shan and another at Tates Cairn.  A Doppler radar is capable of measuring the approach (or departing) speed of raindrops, providing indirectly information on the radial wind speeds.  With both radars operating at the same time, they can provide information on the upper-level winds.  Before the landfall of Nuri, the dual Doppler radar winds showed that there were two circulation centres at an altitude of one kilometre (Figure 6), one centre near Tates Cairn and another over the southern part of Hong Kong Island.  The centre to the north persisted for a while before dissipating, while the centre to the south remained to the southwest or west of the surface centre, indicating that the centre of Nuri was tilting to the west or southwest in the vertical as it passed through Hong Kong.  Thus, Nuri did not possess a distinct and vertically aligned eye as in a well-developed tropical cyclone, such as that shown in the radar image for Super Typhoon Hope in Figure 1.  The complicated track of Nuri over Hong Kong was the result of a weakening tropical cyclone interacting with the terrain.     As a summary, minute-by-minute surface wind and pressure information from automatic weather stations is very useful in determining the tracks of tropical cyclones passing through Hong Kong, enabling the accurate tracking of weaker tropical cyclones without a distinct eye.

Case Studies

December 2009

https://www.hko.gov.hk/en/education/tropical-cyclone/case-studies/00183-changes-in-typhoon-nuri-during-landfall-in-hong-kong.html

en

The Storm Surge Brought by Typhoon Hagupit

During the passage of a tropical cyclone, the associated strong winds pile up the sea water near the coast causing a surge of the sea-level. To a less extent, the low atmospheric pressure of the tropical cyclone also uplift the sea surface on its path. Such phenomenon is called storm surge.

The sea-level rises and falls every day, a daily event called tides, that is of great interest to many people especially mariners and those on fishing sports.  Sometimes, the sea-level can exceed the normal limits and behave extraordinarily.  During the passage of a tropical cyclone, the associated strong winds pile up the sea water near the coast causing a surge of the sea-level (Figure 1).  To a less extent, the low atmospheric pressure of the tropical cyclone also uplift the sea surface on its path (Figure 2).  Such phenomenon is called storm surge.  If storm surge occurs during astronomical high tide, the sea can rise to a high level and flood low-lying areas.On 23 September 2008 evening, Typhoon Hagupit skirted southwest of Hong Kong under a local No. 8 Gale or Storm Signal.  The Observatory issued a warning on storm surge to alert members of the public on the threat of sea flooding over low-lying coastal areas.  The storm surge rode on high tide.  At around 1 a.m. on 24 September, the sea-level at Victoria Harbour reached a maximum of 3.53 metres (Figure 3), the highest level since Typhoon Wanda in 1962. People living or working in low-lying areas should always be alert on the sea-level and pay heed to the warnings issued by the Observatory.  Moving to a safe place on the high ground would be a safety precaution when necessary.

Case Studies

https://www.hko.gov.hk/en/education/tropical-cyclone/case-studies/00184-the-storm-surge-brought-by-typhoon-hagupit.html

en

Occurrence of Multiple Tropical Cyclones

Over the western North Pacific and the South China Sea, the occurrences of multiple tropical cyclones are most common during summer and autumn. From time to time, the Intertropical Convergence Zone (ITCZ) near the Equator becomes active and more than one tropical cyclone may form along the ITCZ. Cases of two tropical cyclones co-existing simultaneously are not rare.

From time to time, you may notice that two or more tropical cyclones are mentioned in the same weather bulletin provided by the Observatory.  How often will this occur in our region?  What will be the behaviour of the tropical cyclones under such a scenario?Over the western North Pacific and the South China Sea, the occurrences of multiple tropical cyclones are most common during summer and autumn.  From time to time, the Intertropical Convergence Zone (ITCZ) near the Equator becomes active and more than one tropical cyclone may form along the ITCZ.  Cases of two tropical cyclones co-existing simultaneously are not rare.  One such case was Severe Tropical Storm Nock-ten and Tropical Storm Muifa over the northern part of the South China Sea and the western North Pacific respectively on 28 July 2011 (Fig.1).   Occurrences of three or more tropical cyclones co-existing at the same time are not quite as often.  Recent examples, based on the Observatorys records, are listed in Table 1. It is known that two tropical cyclones coming within about 1 000 km of each other will undergo the Fujiwhara Effect (Ref. 1) and rotate around each other.  On the basis of similar mechanisms, cyclone tracks will also reflect such interaction in the case of multiple tropical cyclones.  Their behaviour can be illustrated by the example below. On the morning of 9 August 2009, three tropical cyclones, Tropical Depression Goni, Typhoon Morakot and a tropical depression (which later intensified into Tropical Storm Etau) co-existed over the western North Pacific and the South China Sea (Fig.2).  Typhoon Morakot, travelling across the Taiwan Strait, was a dominant feature on the weather map with its large circulation (Fig.2) and an estimated maximum sustained wind of 120 km/h near its centre.  Meanwhile, Goni was located near Hainan Island and was a relatively small feature on the weather map, with an estimated maximum sustained wind of 55 km/h near its centre.  Under the influence of the large circulation of Morakot, the direction of Gonis movement changed abruptly from southwestwards to eastwards (Fig.3).  Satellite images that morning indicated that the circulation of Goni was rather weak and its eastward-drifting circulation centre could only be tracked by the low level cloud lines on the visible satellite image (Fig.4).  The brighter cloud mass of deep convective clouds was sheared off away from the centre of Goni to the west of Hainan Island.  As Goni moved further eastwards that afternoon, it dissipated over the northern part of the South China Sea.  By that evening, its low-level remnant circulation was effectively integrated into the circulation of Morakot (Fig.5).  This illustrates the fact that apart from the erratic tracks, there is also a chance that the weaker tropical cyclone will be captured and absorbed into the circulation of the more intense one as the distance between them decreases. On the other hand, Tropical Storm Etau over the western North Pacific maintained a relatively far away distance from Typhoon Morakot and no interaction between them could be observed.

Case Studies

December 2011

https://www.hko.gov.hk/en/education/tropical-cyclone/case-studies/00181-occurrence-of-multiple-tropical-cyclones.html

en

What is a Weather Radar?

What is a weather radar? What is the working principle of a Doppler weather radar? RADAR stands for RAdio Detection And Ranging. A weather radar detects rain in the atmosphere by emitting pulses of microwave and measuring the reflected signals from the raindrops.

RADAR stands for RAdio Detection And Ranging. Invented just before World War II for military purpose, it has since been applied to many areas, an important one being weather monitoring. Through detecting raindrops in the atmosphere, the weather radar is a very effective tool for monitoring severe weather such as tropical cyclones, thunderstorms and heavy rain in Hong Kong.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.Doppler weather radar has become increasingly popular in recent years. It is capable of measuring the approach (or departing) speed of raindrops. The Doppler principle can be explained by noting the change in pitch of an ambulance siren. The pitch heightens as the ambulance approaches and lowers as it departs. In other words, the faster the ambulance approaches, the higher will be the pitch. For the case of a Doppler radar, the faster the raindrops move towards the radar, the higher will be the frequency (i.e. pitch) of the microwave reflected from raindrops (Fig. 1). The raindrops' approach speed is determined by the frequency shift, and provides a good estimation of the winds, which carry the raindrops.

https://www.hko.gov.hk/en/education/meteorological-instruments/weather-radar/00189-what-is-a-weather-radar.html

en

Weather Radar Observations

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.

IntroductionRADAR stands for RAdio Detection And Ranging. Invented just before World War II for military purpose, it has since been applied to many areas, an important one being weather monitoring. Through detecting raindrops in the atmosphere, the weather radar is a very effective tool for monitoring severe weather such as tropical cyclones, thunderstorms and heavy rain in Hong Kong.Working principle of weather radarA 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.Doppler weather radar has become increasingly popular in recent years. It is capable of measuring the approach (or departing) speed of raindrops. The Doppler principle can be explained by noting the change in pitch of an ambulance siren. The pitch heightens as the ambulance approaches and lowers as it departs. In other words, the faster the ambulance approaches, the higher will be the pitch. For the case of a Doppler radar, the faster the raindrops move towards the radar, the higher will be the frequency (i.e. pitch) of the microwave reflected from raindrops (Fig. 1). The raindrops' approach speed is determined by the frequency shift, and provides a good estimation of the winds, which carry the raindrops.Weather radar in Hong KongThe Hong Kong Observatory's (HKO) first weather radar was installed at Tate's Cairn in 1959. The Decca 41 radar was equipped with a monochrome cathode-ray tube (CRT) for displaying rain areas. A Plessey 43S radar was installed in 1966 adjacent to the first one. Apart from making horizontal and vertical scans of the atmosphere, it was equipped with electronics which allowed the attenuation of echo intensity on the CRT display through different preset gray levels, thus enabling the forecaster to estimate the intensity and vertical extent of rain areas.The first computer-based radar was implemented in 1983 to replace the Decca radar. It displayed radar pictures in different colours according to the rain intensity, resulting in much improved user-friendliness to the forecaster. Apart from making 3-dimensional scans of the atmosphere, it also featured such products as animated display of radar pictures and forecast radar picture based on extrapolation of rain echoes. These features enabled the forecaster to follow the movement and development of rain areas more effectively. (This radar was decommissioned in 2000.)HKO installed in 1994 its first Doppler weather radar (Fig. 2a) to replace the Plessey radar. In addition to measuring the rain intensity, the Doppler radar also provided valuable information on the speed of rain areas. This proved to be very useful in estimating the wind strength of tropical cyclones coming within radar range.In 1998, HKO implemented a Terminal Doppler Weather Radar (TDWR) at Tai Lam Chung, about 12 km northeast of the new airport at Chek Lap Kok. The TDWR (Fig. 2b) is specialized in detecting severe weather near the airport and provides alerts of microburst and windshear associated with convective storms (Fig. 3), thus ensuring safety for landing and departing aircraft under such weather conditions.The implementation of a Doppler weather radar in 1999 at Tai Mo Shan (Fig. 2c) marked another milestone in the history of weather radar in Hong Kong. Scanning the atmosphere from the highest peak in Hong Kong, the radar is equipped with a large antenna (8.5-metre diameter) and a high-stability transmitter. By capturing data in high resolution, it provides a much clearer view of the storm structure, thus facilitating the forecaster's issuance of severe weather warnings.Under severe weather situations, the combined use of both Doppler weather radar at Tai Mo Shan and Tate's Cairn enables a comprehensive radar picture (Fig. 4) to be drawn up, as the coverage of individual radar is limited in some sectors due to possible interference with other radar or blockage by terrain or nearby structures. It also allows the determination of a 3-dimensional wind pattern for Hong Kong (Fig. 5) based on the Doppler wind information obtained with the two radar.Radar information for the public and special usersRadar images are from time to time presented by the Observatory's professionals in their daily weather programmes on the television and in special briefings to the media during severe weather conditions. Radar images are also disseminated to the aviation community for flight planning and operations. For the public, radar information similar to Fig. 6 is updated regularly on the Observatory's Internet website at https://www.hko.gov.hk/en/wxinfo/radars/radar.htm.

https://www.hko.gov.hk/en/education/meteorological-instruments/weather-radar/00192-weather-radar-observations-in-hong-kong.html

en

Application of Phased Array Weather Radar (PAWR)in monitoring inclement weather

Hong Kong’s first phased array weather radar.

The Observatory installed a Phased Array Weather Radar (PAWR), the first of its kind in Hong Kong, at Sha Lo Wan (SLW) in 2021 (Figure 1). The PAWR comprises an array of small antenna units, each can be controlled independently for transmitting and receiving radar signal. By imposing phase shift to the antenna units, a radar beam can be generated for transmission in a designated direction. The merits of PAWR include its small size and simultaneous transmission of multiple radar beams. By utilising both mechanical and electronic scanning methods, a volume scan with a maximum of 68 layers can be completed in 1 minute. This is 5 to 6 times denser than the Tai Mo Shan and Tate’s Cairn weather radars which take around 6 minutes to complete a volume scan with only 12 layers. The PAWR can thus provide high spatial and temporal resolution radar images for monitoring the rapidly changing mesoscale weather systems such as localised rainstorms, hail, tornadoes, etc.Since its trial operation in October 2021, the SLW PAWR performed well in monitoring several inclement weather events. One example was the occurrence of waterspout over the waters to the west of Cheung Chau in the morning of 8 June 2022 which was captured by the Observatory’s network camera (Figure 2a). The SLW PAWR captured clearly the development and dissipation process of the waterspout. In particular, the Doppler velocity overlayed with 3-D radar reflectivity image at around 10:18 a.m. showed the dipole pattern associated with the waterspout, suggesting the presence of a rotating air column. It was estimated that the maximum wind speed associated with this cyclonic vortex at a height of about 2 km exceeded 20 m/s (Figure 2b).As the Observatory collects more PAWR data, weather forecasters will have a better understanding of the characteristics and evolution of mesoscale weather systems which will also pose a positive impact on nowcasting operation.

January 2023

https://www.hko.gov.hk/en/education/meteorological-instruments/weather-radar/00689-application-of-phased-array-weather-radar-in-monitoring-inclement-weather.html

en

How to Interpret Velocity Field from Doppler Weather Radar

Doppler principle is used to detect the approaching (or departing) speed of raindrops. However, this speed is the component of the wind velocity vector along the radar beam direction and is known as radial velocity. If the wind is parallel to the radar beam, the magnitude of the radial velocity is the same as that of the true velocity. If the wind is perpendicular to the radar beam, the radial velocity is then zero. The line with constant zero radial velocity is called the zero isodop.

The article "Weather Radar Observations in Hong Kong" in the educational resources of the Hong Kong Observatory (HKO) website briefly introduces the use of the Doppler principle to detect the approaching (or departing) speed of raindrops. As the raindrops are carried by winds, the speed so detected provides a good estimate of the wind speed. One must however bear in mind that this speed is the component of the wind velocity vector along the radar beam direction and is known as radial velocity (Figure 1). If the wind is parallel to the radar beam, the magnitude of the radial velocity is the same as that of the true velocity. If the wind is perpendicular to the radar beam, the radial velocity is then zero. The line with constant zero radial velocity is called the zero isodop.By using the radial velocity patterns of radar images, interesting phenomena such as the presence of vortices and the wind distribution of a typhoon etc., can be identified. Figure 2 shows the radial velocity patterns for a cyclonic vortex developing at different directions from the radar. The main feature is a dipole pattern, or "wind couplet" , with winds moving away from the radar (positive, warm colour) to right of the zero isodop (grey) and winds moving towards the radar (negative, cold colour) to left of the zero isodop when viewing away from the radar (centre).The Observatory's Terminal Doppler Radar (BP TDWR) at Brothers Point captured the presence of a waterspout that developed near Ma Wan on the morning of 29 August 2018. After formation, the waterspout moved northwards and approached the Ting Kau Bridge before dissipating near Ting Kau (Figure 3). The radial velocity images based on BP TDWR's scans at elevation 17.0° taken at 11:26 a.m. and 11:31 a.m. on that day are shown in Figure 4. The red circles in Figures 4(a) and (b) show a dipole wind structure with northeasterly winds (blue arrows indicating winds blowing towards the radar) and southwesterly winds (red arrows indicating winds blowing away from the radar) forming a "wind couplet" that suggests a cyclonic rotation of air.In Hong Kong, waterspouts usually occur during the rainy season from May to October. However, they are usually rather short-lived and the dimensions are very small. Moreover, being situated at the hill top sites, it is difficult for Tai Mo Shan (around 970 m above mean sea level (msl)) and Tate's Cairn (around 580 m above msl) weather radars to observe such low-level meteorological phenomenon. For example, for the event on 29 August 2018, it was assessed that the waterspout occurred at a level below 600 m based on Figure 3 and the Observatory's cloud base observations at the Hong Kong International Airport at 11:30 a.m. that day. As the BP TDWR is situated at a lower height (less than 100 m above msl) and with higher data resolution, the BP TDWR can capture the waterspout on that day.

January 2019

https://www.hko.gov.hk/en/education/meteorological-instruments/weather-radar/00522-how-to-interpret-velocity-field-from-doppler-weather-radar.html

en

On Terminal Doppler Weather Radar - TDWR

The occurrence of windshear aloft airport areas is one of the causes of aircraft accidents. Since the 1990s, windshear detection radars commonly known as Terminal Doppler Weather Radars (TDWR) had been installed in many modern airports providing windshear information and alerts to landing and departing aircrafts, enhancing aviation safety.

The occurrence of windshear aloft airport areas is one of the causes of aircraft accidents. Since the 1990s, windshear detection radars commonly known as Terminal Doppler Weather Radars (TDWR) had been installed in many modern airports providing windshear information and alerts to landing and departing aircrafts, enhancing aviation safety.The strongest windshear over airport terminal areas is usually generated by microbursts associated with thunderstorm activities. In a mature thunderstorm, a local pool of intense cold air bursts downwards and outspreads near the ground. Abrupt change of wind direction and speeds caused by the advancing air alters the lifting force of aircraft and thus poses a threat to aircraft safety. For details of microbursts and windshear, readers may refer to the Observatory's pamphlet "Windshear and Turbulence"[1] and Thematic Article Series on "Educational Resources" webpages[2].The mission of TDWR is detection of low-level windshear near the airport and is technologically more advanced than traditional weather radars which are used to monitor the development and movement of rain areas. Applying the principle of Doppler effect[3], a TDWR can measure radial winds from the radar. For aircraft landing and departing an airport, changes in the winds along runway direction may affect the lift of aircraft. Hence, a TDWR has to be placed along the direction of the runway in order to effectively monitor windshear near the airport. Locally for the Hong Kong International Airport, west of the two runways is the sea, so a TDWR has to be installed to the east of the runways, near Tai Lam Chung as shown in figure 1. Microbursts are short-lived and usually last for a few minutes only. During thunderstorms, the TDWR has to conduct rapid scans in order to detect the rapid changes of air flow in the affected areas. Calculation must be fast and accurate, and run by the computer automatically so that windshear alerts can be issued to pilots in a timely manner. As such, the quality of radar signal must be very high. This requires a narrow radar beam, sophisticated software to eliminate clutters such as those from birds and terrain, high speed precision motors, and advance design to ensure high availability during thunderstorms.Thunderstorms are common in Hong Kong and windshear caused by microbursts occur from time to time. As the growth of air traffic continues, TDWR will be guardian of the Hong Kong International Airport, ensuring the safety of passengers and aircrafts.

June 2012

https://www.hko.gov.hk/en/education/meteorological-instruments/weather-radar/00188-on-terminal-doppler-weather-radar.html

en

Profiling Rain Areas Using Radar

Dual-polarization Doppler weather radar, unlike traditional single polarization radar, can transmit and receive electromagnetic pulses from both of the horizontal and vertical polarizations. Since the two polarized waves would give rise to echoes of varying characteristics when reflected by water droplets of different sizes or by different ice shapes, these characteristics can be analysed to determine the composition of rain areas as well as the rainfall intensity.

Forms and appearances of clouds in the sky are always changing. The water droplets and ice inside a cloud may change with temperature, humidity and airflows, increasing or decreasing in quantity with water droplets freezing to form ice or ice melting to form water droplets. Moreover, the appearance of ice varies: in the form of crystals or ice pellets of various shapes. In severe convective weather, ice pellets can grow further to become hail. When the water droplets and ice become too heavy, they will fall to the ground as rain, snow or hail (Figure 1 shows ice in various shapes).Weather radar is an effective tool to detect rainfall. It transmits electromagnetic pulses into the atmosphere and measures the signals (echoes) reflected by water droplets or ice in a rain area. Location of the rain area can be determined from the time taken by the echoes returning back to the radar. For rainfall intensity, in general, stronger echoes (reflectivity) indicate heavier rainfall. However, as the reflectivity of ice is higher than that of water droplets, the estimated rainfall intensity may be higher than the actual intensity when there is ice in a rain area. Therefore, if we solely base on the echo intensity to estimate the composition of water and ice in a rain area or the rainfall intensity, there would be some limitations and uncertainties.In 2015, the Observatory commissioned a new dual-polarization Doppler weather radar at Tate's Cairn (Figure 2). Unlike traditional single polarization (electric or magnetic field of electromagnetic wave varies on a single plane) radar, the new radar can transmit and receive electromagnetic pulses from both of the horizontal and vertical polarizations (Figure 3). Since the two polarized waves would give rise to echoes of varying characteristics when reflected by water droplets of different sizes or by different ice shapes, these characteristics can be analysed to determine the composition of rain areas as well as the rainfall intensity. For example, the shape of large water droplets is relatively flat compared to that of small droplets due to gravity. This results in stronger reflectivity for the horizontally polarized wave. Sometimes, insects and birds can also be distinguished from the dual-polarization echo signals. With these additional information, the new radar can more effectively monitor the development of hail, and more accurately estimate the rainfall intensity, opening a new chapter of weather radar’s applications in Hong Kong.Figure 4 is an image captured by the radar at Tate's Cairn on 20 April 2015, showing rain areas with relatively large ice pellets (areas in yellow on the image) and hail (area in red on the image). Under the influence of a trough of low pressure, severe convective activities occurred over the coastal regions of Guangdong that day. Apart from squally thunderstorms and heavy rain, hail was reported in Shantou, which matched the location of hail shown on the radar image.The new weather radar at Tate's Cairn provides weather forecasters with "profiles" of rain areas. The information is useful to accurately monitor the development of rain areas and to estimate the rain intensity, contributing to timely issuance of severe weather warnings for safeguarding the general public.

July 2015

https://www.hko.gov.hk/en/education/meteorological-instruments/weather-radar/00458-profiling-rain-areas-using-radar.html

en

Hail and Hook Echo

Hailstorm is a natural phenomenon which is infrequent in Hong Kong, occurring once every one to two years. Weather radar is the primary tool of the Observatory to monitor hail. The reflectivity and shape of the echoes shown on the radar imagery provide important clues for identifying hail.

Hailstorm is a natural phenomenon which is infrequent in Hong Kong, occurring once every one to two years. Large hails can damage crops, break windows, glass houses and windscreens of cars. In the past two decades (January 2000 - August 2020), there were 11 days with reported hails in Hong Kong. A rather large-scale hailstorm that occurred in the past few years in Hong Kong was on the evening of 30 March 2014. Under the influence of a trough of low pressure, there were widespread torrential rain and severe thunderstorms accompanied by hails that evening, necessitating the Observatory to issue the Black Rainstorm Warning Signal. During the rainstorm, the Observatory received hail reports from many places in Hong Kong, most of which reported hails with size of about 20 to 30 millimetres (Figure 1).Hails are large ice pellets formed in severe thunderstorms. When severe thunderstorms occur, there will be violent vertical motion in the atmosphere. Since temperature generally decreases with height in the troposphere, the moisture in the air will condense when the relatively warm and humid air rises. Due to the violent updraft, water vapour will be carried above the freezing level and keep rolling around there, condensing into ice pellets. During the rolling process, the ice pellets continue to absorb water, growing bigger and bigger like a “snowball”. Finally, when the updraft can no longer support the weight of the ice pellets, these overweight ice pellets will fall to the ground and form hailstorms[1] (Figure 2).Weather radar is the primary tool of the Observatory to monitor hail. The reflectivity and shape of the echoes shown on the radar imagery provide important clues for identifying hail. Since hails are formed in severe thunderstorms, apart from intense echo reflectivity, echo in hook-shaped can sometimes even be observed on the radarscope, which is called "hook echo". For instance, the characteristics of hook echo can clearly be observed on the radar imagery on the evening of 30 March 2014 (Figure 3).Hook echo is an important feature of a supercell thunderstorm, which implies that the development of cumulonimbus associated with the severe thunderstorms has been quite vigorous. During this period, the strong updraft in the cumulonimbus even prevents raindrops from falling to the ground, resulting in a weak echo region at the bottom layer and the formation of hook-shaped echo. In addition to hails and severe thunderstorms, hook echo is sometimes accompanied by tornadoes or waterspouts as well. Generally speaking, hook echo signifies a very destructive feature. If you observe a hook echo on the radar imagery that might have the chance to affect you, you must try to avoid it, and seek shelter immediately in a sturdy building especially if you are outdoors.Hook echo correlates with the occurrence of hails, but only to a certain extent. Not every time when there is a hook echo necessarily implying the occurrence of hails, and vice versa. Forecasters need to assimilate various observations to determine the possibility of hail. For example, the Tate’s Cairn dual-polarization S-band Doppler weather radar (Figure 4) installed by the Observatory in recent years has greatly improved the capability in hail monitoring[2][3].

September 2020

https://www.hko.gov.hk/en/education/weather/weather-phenomena/00550-hail-and-hook-echo.html

en

Use of Building Information Modelling to help maintain Weather Radar

What is Building Information Modelling (BIM)? How can BIM help with the maintenance of weather radar? Building Information Modelling is a technology for managing the digital representation of physical and functional objects through the use of 3-D model and a database.

What is Building Information Modelling? Building Information Modelling (BIM) is a technology for managing the digital representation of physical and functional objects through the use of 3-D model and a database. It is being promoted in the construction industry in Hong Kong as it can bring benefits during various construction stages such as optimising planning and improving coordination [1]. BIM comprises different families of construction components, e.g. door and window, and each component contains certain properties such as material and size etc. The BIM component properties are stored in a database. Using these components, you can then assemble an object virtually. Moreover, the components of one BIM project can be reused in another project. How can BIM help with the maintenance of weather radar? The Hong Kong Observation (HKO) has recently completed a pilot project using BIM to support knowledge transfer. A 3-D model using BIM was developed for the Tate’s Cairn Weather Radar (TCWR) Station (Figure 1). The model allows 3-D visualisation of various radar system components including the embedded ones and those installed inside system enclosures. Animations showing details of radar maintenance procedures have also been developed. For example, one of the critical radar components named Klystron is replaced around once a year (Figure 2). Maintenance staff usually acquires the knowledge and skill for replacing Klystron through on-the-job training. This can only be achieved around once a year as Klystron is embedded inside a container and maintenance staff cannot see the Klystron during normal radar operation. With the use of BIM, simulation can be developed to enable maintenance staff to understand the whole process of replacing Klystron (Figure 3). Making use of the simulations developed from BIM, trainers can share their valuable operational experience to fresh maintenance staff through a virtual environment without the necessity of going to the radar station and interrupting radar operation. This provides a more effective way for knowledge transfer and sharing.

December 2019

https://www.hko.gov.hk/en/education/meteorological-instruments/weather-radar/00538-use-of-building-information-modelling-to-help-maintain-weather-radar.html

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Extraordinary Lightning Protection System for Weather Radar Stations

Weather radar is a primary tool for monitoring severe weather including rainstorms and typhoons and is indispensable for weather forecasting. A weather radar must operate in all weather for safeguarding the life and property of members of the public. During thunderstorms, it is the time when the weather radar is needed most and its normal operation must be guaranteed.

Thunderstorm is a common weather phenomenon during rainy season in Hong Kong.  From April to September each year, thunderstorms may occur frequently.  Figure 1 shows the monthly average numbers of cloud-to-ground lightning strikes over Hong Kong in April to September from 2006 to 2012 and all of them exceeded five thousand.  To protect buildings from lightning strikes, a good lightning protection system is essential.  A brief account of lightning protection theory is described in the article "Don't be a lightning rod" on HKO Educational Resources webpage[1].  In simple terms, a lightning rod will attract nearby electric charges and lead the current to the ground through an earthing device for protecting the building against lightning strikes.  The electrical resistance of the earthing device, called lightning earthing resistance, is a crucial parameter determining effectiveness of the lightning protection system.  The lower the lightning earthing resistance is, the more efficient it will conduct electric current and the lightning protection will perform better.General Specification for Electrical Installation in Government Buildings of the Hong Kong Special Administrative Region[1] published by the Architectural Services Department states that lightning earthing resistance shall not exceed 10 ohms for ordinary government buildings.  This requirement is not applicable to the protection of overhead power lines and other specialized buildings which have their own requirements.  The Observatorys weather radar stations are among such specialized buildings which have to comply with more stringent standard due to the following reasons.Mission criticalWeather radar is a primary tool for monitoring severe weather including rainstorms and typhoons and is indispensable for weather forecasting.  A weather radar must operate in all weather for safeguarding the life and property of members of the public.  During thunderstorms, it is the time when the weather radar is needed most and its normal operation must be guaranteed.Geographical locationWeather radar station usually stands head-and-shoulders above the surroundings to have an unobstructed view of the sky for weather monitoring.  For example, the Tai Mo Shan weather radar station sits on the top of Tai Mo Shan, the highest peak in Hong Kong.  Such locations are particularly susceptible to lightning strikes.  Moreover, weather radars contain sophisticated electronic equipment which are vulnerable to the surge of unstable electrical current induced by lightning.  Thus, a more stringent standard on lightning earthing resistance is required for weather radar stations for safeguarding against lightning strikes.The Observatory adopts a stringent requirement of lightning earthing resistance not exceeding one ohm for weather radar stations in order to reduce the risk of lightning strikes.  Such standard for weather radar stations is also required by the international community, as exemplified in the report "Instruments and Observing Report No. 88 (IOM-88) of the World Meteorological Organization (WMO)"[2] published by Commission for Instruments and Methods of Observations (CIMO) of WMO.  Figure 2 is extracted from IOM-88 which shows a burned radar radome after hit by lightning strikes in radar station overseas.  In the mid-90s of the last century, experts from USA and Australia advising the Observatory on construction of the Tai Lam Chung Terminal Doppler Weather Radar Station also pointed out that lightning earthing resistance of the radar station must not exceed one ohm.A lightning earthing resistance of not more than one ohm can protect a weather radar station from lightning strikes, ensuring normal operation of the radar which is essential to the provision of weather information services to the public in inclement weather.

June 2013

https://www.hko.gov.hk/en/education/meteorological-instruments/weather-radar/00187-extraordinary-lightning-protection-system-for-weather-radar-stations.html

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Application of Virtual Satellite Images

Application of deep learning model to enhance night time weather monitoring for tropical cyclones using satellite.

Satellite images are indispensable for weather monitoring and forecasting. Visible and infrared satellite images are commonly used for such purpose. Image of the former type has higher resolution which enables more detailed analysis of weather systems’ features, but it is generated only during daytime when there is sunlight shining on the Earth. The latter type of image is available round-the-clock despite having a lower resolution.With the advance of artificial intelligence technology, the Observatory has developed a deep learning model for generating virtual nighttime visible satellite images to facilitate weather forecasters in determining the centre of a tropical cyclone during nighttime. It is a conditional generative adversarial networks (CGAN) model comprising a generator and a discriminator (Figure 1). The model has been trained with past satellite images from different channels of the Himawari-8 (H-8) satellite for optimizing the performance of the generator and the discriminator. After training, real-time satellite images could be fed into the CGAN model for producing virtual visible images. Useful results were obtained as exemplified in the case of tropical cyclone Rai in December 2021 (Figure 2).

January 2023

https://www.hko.gov.hk/en/education/meteorological-instruments/meteorological-satellite/00686-Application-of-Virtual-Satellite-Images.html

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How to measure cloud top height from a meteorological satellite?

In Physics, we understand that all object will emit electromagnetic radiation and the amount of which will depend on its own temperature. By measuring the radiation of specific wavelengths emitted from the top of a cloud, we are able to estimate the corresponding temperature according to the Planck's law.

How high a cloud can grow depends on the atmospheric conditions. For example, clouds developed under unstable atmosphere normally grow high up to the upper troposphere. Knowing the cloud top height helps us understand the state of the atmosphere. Meanwhile, knowing the height of the top of a cloud is also valuable to pilots for making decision on whether to fly around or over a convective cloud. Observing the cloud from the above using meteorological satellite allows us to estimate the cloud top height. But how is that done?In Physics, we understand that all object will emit electromagnetic radiation and the amount of which will depend on its own temperature. By measuring the radiation of specific wavelengths emitted from the top of a cloud, we are able to estimate the corresponding temperature according to the Planck's law[1]. For example, infra-red radiation emitted from a cloud can be measured by the infra-red sensor on a geostationary meteorological satellite. Since cloud is opaque to that infra-red frequency, the temperature measured by the sensor is very close to that at the cloud top. After obtaining the cloud top temperature, the cloud top height can be derived from the temperature vertical profile, which in turn can be retrieved from the measurement using the upper air sounding system or estimated from the numerical weather prediction model. Figure 1 shows a schematic diagram indicating how the cloud top height from infra-red measurements is determined.There are also other polar-orbiting satellites equipped with lidar which can directly measure cloud top height. A lidar transmits laser pulses which are scattered backward by the particles in the cloud like water droplets and ice particles. The backscattered pulse is collected by the instrument onboard of the satellite. By measuring the time lapse between transmitting a laser pulse and receiving the pulse backscattered by the cloud top, we can calculate the height of the cloud top. For example, one of the missions of the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) satellite[2] is to measure the vertical structure of clouds using its onboard lidar (Figure 2).

January 2019

https://www.hko.gov.hk/en/education/meteorological-instruments/meteorological-satellite/00519-how-to-measure-cloud-top-height-from-a-meteorological-satellite.html

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What is a Meteorological Satellite?

What is a meteorological satellite? Meteorological satellites carry sensors that are pointing towards the ground, enabling them to have bird eye view of the globe from the space. There are two types of meteorological satellites characterized by their orbits. They are geostationary satellites and polar-orbiting satellites.

Meteorological satellites carry sensors that are pointing towards the ground, enabling them to have bird eye view of the globe from the space. There are two types of meteorological satellites characterized by their orbits. They are geostationary satellites and polar-orbiting satellites (Figure 1).As the name suggests, a geostationary satellite is stationary relative to the earth. That is, it moves above the equator at the same rate as the earth's rotation so that all the time it is above the same geographical area on the earth. In this manner, it is capable of taking cloud images of the same area continuously, 24 hours a day. As it is some 35,800 kilometres from the earth, it is capable of taking cloud pictures covering part of the whole globe.Polar-orbiting satellites are low-flying satellites circling the earth in a nearly north-south orbit, at several hundred kilometers above the earth. Most of them pass over the same place a couple of times a day. As they operate at a distance closer to the earth, they are only capable of taking cloud images of a limited area of the earth each time. Compared with geostationary satellites, polar-orbiting satellites offer fewer and smaller cloud pictures. However, the advantage is that the cloud pictures obtained are of much higher resolution.

https://www.hko.gov.hk/en/education/meteorological-instruments/meteorological-satellite/00196-what-is-a-meteorological-satellite.html

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Meteorological Satellite Reception System

Meteorological satellites carry onboard sensors to capture images of the earth from the space. There are two types of meteorological satellites characterized by their orbits. They are geostationary satellites and polar-orbiting satellites.

HistoryMeteorological satellites carry onboard sensors to capture images of the earth from the space. There are two types of meteorological satellites characterized by their orbits. They are geostationary satellites and polar-orbiting satellites (Figure 1).Geostationary satellites are stationary relative to the earth and as such they capture images of the same geographical area on the earth 24 hours a day. Located some 35,800 kilometres above the equator, they take pictures covering almost half the globe.Polar-orbiting satellites are low-flying satellites circling the earth in a nearly north-south orbit, at several hundred kilometers above the earth. Most of them pass over the same place a couple of times a day. Since they travel at a distance closer to the earth, they are only capable of taking images of a limited area each time. However, the images are of higher resolution than those from the geostationary satellites. In comparison, geostationary satellites offer more satellite images of the same area per day.Reception of Meteorological Satellites in Hong KongSoon after the launch of the Television and Infra-red Observation Satellite (TIROS 8) by the U.S.A. in 1963, the Hong Kong Observatory started using locally designed equipment to receive satellite signals from the Automatic Picture Transmission System (APT) on-board the satellite. The early pictures were displayed on a modified domestic television set.This equipment was used to receive transmissions from the Environmental Survey Satellite (ESSA) and the National Oceanic and Atmospheric Administration (NOAA) series of polar-orbiting satellites until a photographic recorder was purchased in 1968.In 1977, Japan Meteorological Agency (JMA) launched the first of a series of Geostationary Meteorological Satellite (GMS) into an orbit 35,800 km over the equator at longitude 140°E. Images in the visible (VIS) and infra-red (IR) channels were available every three hours in analogue facsimile form. In 1979, the Observatory purchased a reception system to receive the high-resolution pictures (HR-FAX) from the GMS. The GMS was subsequently replaced by its successors GMS-2 in 1981, GMS-3 in 1984, GMS-4 in 1989 and GMS-5 in 1995. The transmission mode of the satellite was also changed from analogue facsimile form to stretched-VISSR (Visible Infra-red Spin Scan Radiometer) digital form, and the frequency of transmission increased to provide satellite pictures every hour. To cope with these changes, the Observatory installed a computer-based reception system in 1988. A more advanced UNIX-based reception system was installed in 1996 to receive and analyze additional satellite images of an additional IR channel and a new water vapour (WV) channel from GMS-5.In late 2001, the Hong Kong Observatory installed a satellite reception system to receive satellite images from the Fengyun-1 series of polar-orbiting meteorological satellites operated by the China Meteorological Administration (CMA) and from the series of polar-orbiting meteorological satellites operated by the U.S. National Oceanic and Atmospheric Administration (NOAA).In mid-2003, the operation of GMS-5 was taken over by the Geostationary Operational Environmental Satellite-9 (GOES-9) of NOAA. The Observatory utilized the satellite reception system for GMS-5 to receive satellite images from GOES-9.In 2004, the Observatory installed another satellite reception system to receive broadcast from MODIS, an acronym for MODerate Resolution Imaging Spectroradiometer, which is a sensor onboard the Earth Observing Systems (EOS) satellites, namely Terra and Aqua, operated by the U.S. National Aeronautics and Space Administration (NASA). MODIS has 36 observational channels, covering a wide frequency spectrum ranging from visible to infra-red radiation, making it valuable data source for meteorology, oceanography as well as environmental monitoring. The high-resolution images from MODIS are useful for monitoring fine features or phenomena such as hill fire and haze.In 2005, the first operational geostationary meteorological satellite of CMA, namely Fengyun-2C (FY-2C), was put into operation. Located over the equator at the longitude of 105°E, FY-2C provides imagery of five channels (VIS, IR1, IR2, WV and IR4) covering most parts of Asia, Australia, Indian Ocean and the Western Pacific. The resolution of the VIS imagery is 1.25 km while that of IR and WV imagery is 5 km. The imagery for the Northern Hemisphere is updated as frequent as 30 minutes during rainy and typhoon seasons.In the same year, the first of the next generation geostationary meteorological satellites of JMA, namely MTSAT-1R, was launched to replace GOES-9. Available at 30 minute intervals, images of MTSAT-1R cover almost the same area as GOES-9. MTSAT-1R satellite provides a new infrared channel (IR4) in addition to the four channels (VIS, IR1, IR2 and WV). The resolution of the VIS imagery is 1 km whereas the IR and WV imagery are at 4-km resolution.Application of Satellite DataThe Observatory receives direct broadcasts from the following satellites round-the-clock to support its weather forecasting and warning services: - FY-1D and FY-2C satellites of CMA. - MTSAT-1R of JMA. - Series of polar-orbiting satellites of NOAA. - EOS-series of satellites of NASA. Meteorological satellite data serve a number of purposes: - Satellite pictures provide a global view of the distribution of cloud cover and weather systems and form an essential source of information particularly over the oceans and other areas where weather observations are sparse. - Clouds of rainstorm develop and ascend to great height and their cloud top temperatures are relatively low. Infra-red satellite images, which depict the cloud top temperatures, are useful in monitoring the development of rainstorms. - The characteristic cloud features of tropical cyclones are easily recognizable on a satellite image and can be of great assistance in tracking their movement and estimating their intensity (Figure 2). - Infra-red satellite images depict the variation in sea surface temperatures, provided the area is not obscured by clouds. The information is useful for forecasting sea fog and monitoring the development of tropical cyclones. - Other than displaying cloud images, some satellite images provide useful information for environmental monitoring such as distribution of haze, location of hill fire, eruption of volcanic ash and location of sandstorm.

https://www.hko.gov.hk/en/education/meteorological-instruments/meteorological-satellite/00201-meteorological-satellite-reception-system-in-hong-kong.html

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A New Era of Meteorological Satellites

How are the new generation of meteorological satellites compared with the previous generation? What are the other technical breakthroughs of Fengyun-4A (FY4A) satellite? Where can we see the high resolution satellite imageries?

"Fengyun-4A (FY-4A)" satellite, the first member of the Fengyun-4 series satellites, was successfully launched at the Xichang Satellite Launch Center of China in the early morning of 11 December 2016. About a month earlier on 2 November, Japan launched the "Himawari-9" meteorological satellite. "GOES-R", an American meteorological satellite, was also put onto the sky on 19 November. The FY-4A, Himawari-9, and GOES-R, together with MTG satellite now under development in Europe all belong to a new generation of geostationary weather satellites. After entering its orbit, the new FY4A satellite will undergo in-orbit tests for several months to a year before being put into operation. When these satellites enter services, a new era of global meteorological satellite services will commence.How are the new generation of meteorological satellites compared with the previous generation?Satellites of the new generation possess more observation channels and are capable of providing more rapid updates which are of higher resolution. For instance, the FY-4A satellite is equipped with 14 observational channels. This is 2.8 times of the 5 observational channels of the previous generation. The speed of observation is also more than doubled where a full-disk scan can be made every 15 minutes and a regional scan of an area of 1000 km by 1000 km can be made every minute. The finest resolution of visible image is also enhanced from 1.25 km to 500 metres per pixel, an improvement by a factor of 1.5. Himawari-9 satellite, a backup of Himawari-8 satellite which has been in operation since mid-2015, is equipped with 16 observational channels where images are updated every 10 minutes. It can undertake regional observation every 2.5 minutes and can be deployed to track the development of typhoons. Figure 1 is a visible image of Typhoon Nida captured by Himwari-8 satellite on 1 August 2016. Fine details of spiral cloud bands around the eye and convective clouds surrounding the eye wall can be observed on the image. With the use of multiple channel data and image analysis techniques, satellites of the new generation can provide images for identification of specific weather or phenomena, such as dust storms, volcanic ash, deep convections, cold and warm air masses, cloud types, etc. In addition, sea surface temperature and aerosol thickness can be derived from the satellite data. Figure 2 highlights volcanic ash erupted from a volcano in Indonesia on 19 July 2015 via processing of Himawari-8 data. The Observatory will from time to time update "Satellite Imagery of Interest" on its satellite webpage. The public can access to the imagery via the Internet link shown in this article.What are the other technical breakthroughs of FY4A satellite?Apart from an Advanced Geostationary Radiation Imager on board for multiple channel observations, FY4A is also equipped with a Geostationary Interferometric Infrared Sounder – the first of its kind installed on board of a geostationary satellite. The Sounder can perform high precision analysis of the atmosphere to obtain profiles of temperature, humidity and stability indices distribution, similar to CT scans in body check. With incorporation of these data into numerical weather prediction models, it is hoped that the development of strong convective weather can be identified a few hours earlier. Furthermore, a Lightning Mapper Imager on board is able to take 500 lightning images every second for detecting the total number of lightning and their strengths. Real-time and continuous observation of lightning together with cloud images and other data, the satellite can help monitor and track the development of strong convections, providing early alerts for thunderstorms and severe weather.Where can we see these high resolution satellite imageries?The Observatory has updated its satellite Internet webpage in phases in March 2016 and in February 2017. With the use of Himawari-8 and other satellite data, the webpage now provides high resolution imageries of southern China and the coastal regions of Guangdong, and global mosaic images (Figure 3), which are useful for a better appreciation of the weather in Hong Kong and its vicinity, and in other parts of the world. Update frequency of satellite image covering eastern Asia was enhanced from once 30 minutes to once 10 minutes. Black and white visible imageries were also upgraded to true colour imageries (Figure 4).URL of Satellite Imagery Webpage is: https://www.hko.gov.hk/en/wxinfo/intersat/satellite/sate.htm

February 2017

https://www.hko.gov.hk/en/education/meteorological-instruments/meteorological-satellite/00488-a-new-era-of-meteorological-satellites.html

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How does a satellite measure the vertical profiles in the atmosphere?

How does a satellite measure the vertical profiles in the atmosphere? How do polar-orbiting meteorological satellites measure the vertical profiles of the atmosphere? How to measure the moisture content of the atmosphere by remote sensing? How to measure temperature using infrared sounders? How to measure the height of water vapour in the atmosphere?

The conventional method for measuring the vertical profiles of the atmosphere is by the using the upper-air sounding system which regularly releases a weather balloon carrying a radiosonde 4 times every day (figure 1). Recently, meteorological centres have started using wind profilers and radiometers (figure 2) to enhance the upper-air observations. Such measurements are usually carried out in a sparse upper-air observation network due to high operating cost. To enhance the spatial coverage, polar orbiting meteorological satellites are now used to measure the vertical profiles of the atmosphere.How do polar-orbiting meteorological satellites measure the vertical profiles of the atmosphere?Some of the polar orbiting meteorological satellites, such as METOP and Feng-Yun 3, are equipped with instruments (such as IASI, AMSU-A and IRAS) for measuring the vertical profiles of the atmosphere. The vertical profiles that they can measure include temperature, moisture content, as well as concentration of trace gases. These instruments, including infrared sounders and microwave sounders, perform such measurements by remote sensing.How to measure the moisture content of the atmosphere by remote sensing?Various gaseous molecules in the atmosphere absorb electromagnetic waves at particular frequencies. Figure 3 shows the absorption spectrum of water vapour. Kirchhoff's law of thermal radiation [1] states that the better an object is in absorbing electromagnetic wave at a particular frequency, the more efficient it is in emitting electromagnetic wave at that frequency thermally. Hence, the atmosphere also emits electromagnetic wave. By measuring the amount of electromagnetic wave emitted by the atmosphere with polar-orbiting meteorological satellites, the vertical profiles can thereby be calculated.How to measure temperature using infrared sounders?The hotter an object is, the more intense electromagnetic wave will be emitted. This is Stefan-Boltzmann law [2]. One example is the incandescent light bulb. The bulb will be brighter when it is hotter.How to measure the height of water vapour in the atmosphere?The vertical profile of the atmosphere can be obtained by taking measurements of the intensity of electromagnetic wave emitted by the atmosphere at multiple frequencies, as different frequencies can indicate the condition of the atmosphere at different heights. For example, if the measurement is taken at a frequency which is strongly absorbed by the atmosphere, the electromagnetic wave measured by the satellite will be mainly emitted from the top of the atmosphere. This is because the electromagnetic wave emitted near to the Earth's surface would be absorbed by the upper part of the atmosphere and ultimately cannot reach the satellite. On the other hand, if the measurement is taken at a frequency which is weakly absorbed by the atmosphere, satellite will then mainly measure the electromagnetic wave emitted near the Earth's surface. This is because the density of air in the lower part of the atmosphere is much higher than that in the upper part of the atmosphere. Electromagnetic wave emitted near the Earth's surface still dominates in intensity even though it is partially absorbed by the upper part of the atmosphere. By measuring various frequencies, the satellite is able to "focus" on different heights of the atmosphere. The vertical profile is then the result of integrating pieces of information obtained at different heights.

July 2015

https://www.hko.gov.hk/en/education/meteorological-instruments/meteorological-satellite/00456-how-does-a-satellite-measure-the-vertical-profiles-in-the-atmosphere.html

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Images of Polar-orbiting Meteorological Satellite

Compared with geostationary weather satellites, polar-orbiting weather satellites provide cloud images with higher resolution. This makes them very good at revealing the fine details of cloud structure, and facilitates the monitoring of the effects of hazardous weather such as tropical cyclones and rainstorms on Hong Kong.

IntroductionCompared with geostationary weather satellites, polar-orbiting weather satellites provide cloud images with higher resolution.  This makes them very good at revealing the fine details of cloud structure, and facilitates the monitoring of the effects of hazardous weather such as tropical cyclones and rainstorms on Hong Kong.  A comparison of the mode of operation of the above two types of satellites is given in Table 1. The Hong Kong Observatory installed a ground reception system at the King's Park meteorological station (Fig. 1) in late 2001 for receiving information from polar-orbiting weather satellites. The high-resolution images and the multiple observational channels on-board these satellites assist the weather forecaster in observing small-scale weather systems and other phenomena on the earth, such as the structure of tropical cyclones, fog, hill fire, sandstorm and dust.  Applications of ImagesTropical CycloneWith resolution of down to about 1 km, polar-orbiting weather satellites enable good depiction of the structure of tropical cyclones and estimation of their location and strength. Figure 2 shows clearly the eye of Typhoon Haiyan as well as its outer circulation and cloud bands.  At the time the image was taken, Haiyan attained a wind strength of 140 km/h near its centre.  FogFog appears near land or sea surfaces.  With the images' high resolution and special observational channels on-board the satellites, fog can be discerned by means of colour enhancement. Figure 3 is a good example of a low visibility event. Widespread fog (depicted in red) affected the coast of Guangdong that morning (17 January 2002), with the visibility in Hong Kong reduced below 500 m.  While the fog dissipated gradually during the day, some fog patches remained over the coastal waters and these were discernible (in pale yellow) on the image in Figure 4.  Hill FireOne of the observational channels on the satellites is particularly sensitive to changes in surface temperatures.  The images obtained with this channel enable observation of hill fires.  The dry weather over southern China on 26 to 27 November 2001 offered a good opportunity for such. With the relative humidity in Hong Kong down to about 50 percent, a number of hill fires were reported on these two days.  The red and dark spots indicated by arrows in Figure 5 show the locations of hill fire, one of which was near Tsuen Wan in the New Territories.  Sandstorm and DustImages from polar-orbiting weather satellites are also useful in discerning sandstorm and dust. Figure 6 presents a satellite image taken in the afternoon of 8 April 2002, showing the extent of dust over the East China Sea to the east of Shanghai. The dust originated a couple of days earlier generally from Xinjian and Nei Mongol. At that time, the East China Sea was covered by an extensive rainband which could effectively wash out dust moving southward. Note: NOAA stands for National Oceanic and Atmospheric Administration of the United States. 

https://www.hko.gov.hk/en/education/meteorological-instruments/meteorological-satellite/00203-polarorbiting-meteorological-satellite-images.html

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Monitoring of volcanic ash

Volcanic ash is mostly glass shards and pulverized rock. It is very abrasive. When an aircraft flies over clouds embedded with dense volcanic ash, the ash will be fused onto the turbine and other parts of the aircraft engines. The glass shards will be melted to form a coating. As a result, the air flow will be affected and the engines would be overheated, and might lead to engine flame-out.

The Grimsvotn volcano in Iceland erupted in late May 2011, flinging large amount of volcanic ash into the air. Reminiscent of the volcanic eruption of Eyjafjallajokull volcano in April to May last year, there was a fear that the volcanic ash might spread widely, leading to complete paralysis of the air traffic in Europe again with most of the flights grounded and a large amount of passengers stranded. Volcanic ash is mostly glass shards and pulverized rock. It is very abrasive. When an aircraft flies over clouds embedded with dense volcanic ash, the ash will be fused onto the turbine and other parts of the aircraft engines. The glass shards will be melted to form a coating. As a result, the air flow will be affected and the engines would be overheated, and might lead to engine flame-out. On 24 June 1982, a British Airways B747 passenger aircraft encountered volcanic ash aloft of Indonesia during its way from Kuala Lumpur, Malaysia to Perth, Australia. All 4 engines lost power one by one and the aircraft descended rapidly from 37000 ft to 12000 ft. Luckily, three of the engines could restart at this point and the aircraft made a successful emergency landing at Jakarta, Indonesia. To safeguard aviation safety, International Civil Aviation Organization (ICAO) has established an International Airways Volcano Watch (IAVW) system. It consists of nine Volcanic Ash Advisory Centres (VAACs). Each centre is responsible for monitoring the occurrence of volcanic ash within its designated region, forecasting the dispersion of the volcanic ash, and disseminating the relevant information. Apart from making use of the information issued by the VAACs, the Hong Kong Observatory also makes use of satellite for the monitoring of volcanic ash. Figure 1 is a true colour image composed from MODIS (Moderate Resolution Imaging Spectroradiometer) data collected on 23 May 2011 on broad of Aqua, a polar-orbiting satellite operated by the National Aeronautics and Space Administration (NASA). Volcanic ash plume is brownish yellow in colour on the image. Clouds are in different shades of white to light grey. The image showed that volcanic ash from Grimsvotn volcano was spreading south from Iceland, drifting towards the northern part of the United Kingdom . To facilitate forecaster in the monitoring of volcanic ash, the Observatory has developed a number of satellite imagery products. Areas with volcanic ash, sand and dust are highlighted in special colours in the products. Figure 2 is a satellite image derived from MODIS infra-red channel data for sand, dust and ash monitoring. Areas with volcanic ash are shown in yellow to red.Since polar-orbiting satellites only pass over the same area twice a day, in order to heighten the monitoring, the Observatory also uses data from geostationary meteorological satellites. Figure 3 is a satellite imagery product for monitoring sand, dust and ash derived from the MTSAT geostationary meteorological satellite data operated by the Japanese Meteorological Agency. The elliptical area encircled in red shows spreading of volcanic ash from Shinmoedake volcano eruption in Kyushu, Japan on 26 January this year. In addition to monitoring volcanic ash, this type of image can also assist forecasters in identifying areas with sand or dust. Those who are interested can find the images on the Observatory's "Sand and Dust Weather Information" web page.When there are indications that the airspace of Hong Kong may be affected by volcanic ash, the Observatory will provide warnings and related information to the Civil Aviation Department and all aviation users according to the ICAO's regulations. This will facilitate airlines and pilots in planning their flight routes to minimize the impact of volcanic ash and to safeguard flight safety.

June 2011

https://www.hko.gov.hk/en/education/meteorological-instruments/meteorological-satellite/00200-monitoring-of-volcanic-ash.html

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The Whereabouts of Radiosondes After Lifting Off

Since 1949, the Hong Kong Observatory has been regularly launching weather balloons equipped with radiosondes every day to collect meteorological data at different altitudes for weather forecasting and for numerical weather prediction models. Sometimes, the radiosondes drift several tens to hundreds of kilometres away from Hong Kong, however, they occasionally land within the Hong Kong territory and are found by citizens.

Since 1949, the Hong Kong Observatory has been regularly launching sounding balloons carrying radiosondes to collect meteorological data at different altitudes in the atmosphere. To this day, the Observatory releases weather balloons equipped with radiosondes at around 08:00 and 20:00 Hong Kong Time every day from King’s Park Upper Air Meteorological Station. The balloon filled with helium gas ascends slowly, so the radiosonde transmits air temperature, humidity and other parameters at various altitudes back to the ground, while wind speed and direction can be derived from the Global Positioning System signals. As the atmosphere becomes thinner at higher altitudes, the balloon gradually expands during ascent and typically bursts at around 30 kilometres above the ground. Then the radiosonde descends with a parachute, landing slowly back onto the ground.During the ascent, the radiosonde flies with the ambient winds and sometimes gets drifted far away from King’s Park, ranging from tens to hundreds of kilometres. However, nothing is absolute, the Observatory occasionally receives reports of found radiosondes from the public. On the morning of 30 July 2023, a citizen found a meteorological observation radiosonde (Figure 1) near the cargo terminal in the airport. It was verified to be the radiosonde released at the King’s Park Meteorological Station that morning. The sounding data for that day (Figure 2) revealed that the sounding balloon initially flew northward under the influence of a low-level southerly flow. Later, as the wind direction changed to northeast with increasing altitude, the balloon gradually drifted southwest. After an approximately 53-minute flight, the sounding balloon eventually burst at around 20 kilometres above Discovery Bay on Lantau Island. The radiosonde descended slowly to the ground with the assistance of a parachute.Upper-air weather observations are crucial for weather forecasting. Once the data is transmitted back to the ground, weather forecasters can analyse the vertical temperature variations and humidity profiles through a tephigram to assess atmospheric stability. This helps them evaluate the cloud cover and the likelihood of severe convective weather for the day. Meteorological agencies worldwide exchange sounding data through the Global Telecommunication System of the World Meteorological Organization, allowing numerical weather prediction models to utilise the sounding data as initial conditions for computation.In addition to supporting weather forecasting, sounding data has a wide range of applications. Paragliding enthusiasts can refer to the thermal index and low-level winds to determine whether it is suitable for paragliding on a given day. Furthermore, the tephigram can reveal the presence of inversion layer and isothermal layer in the lower atmosphere, which greatly assists the photographers in capturing the magnificent moments for “sea of clouds”.When you see an ascending weather balloon, do you ever wonder how far it will fly?

radiosondesr

March 2024

https://www.hko.gov.hk/en/education/meteorological-instruments/automatic-weather-stations/00709-The-whereabouts-of-radiosondes-after-lifting-off.html

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World in Millisecond of Two-Dimensional Video Disdrometer

The disdrometer installed by the Observatory at the Aeronautical Meteorological Station in Chek Lap Kok can derive radar reflectivity from basic physical principles for comparing with weather radar data and can help to identify the characteristics of raindrops under different precipitation mechanisms in different seasons.

Everyone has probably experienced getting caught in the rain without an umbrella and may have noticed the differences between summer and winter rainfall. In general, rainfall in summer comes from cumulonimbus clouds as a result of atmospheric convection so that the raindrops tend to be larger. Rainfall in winter comes mainly from stratiform clouds in the form of light rain or drizzle. Rain gauges and weather radars are two major methods for measuring or estimating rainfall amount but neither of them can provide data on the size distribution of raindrops. Disdrometer can supplement and provide such measurement information.The disdrometer installed by the Observatory at the Aeronautical Meteorological Station in Chek Lap Kok is a Two-Dimensional Video Disdrometer (2DVD; Figure 1). The most important part of the instrument is the measurement area in the center, which is about 10 centimeters long and wide and looks like a “compluvium”, flanked by high-speed scanning cameras on both sides. When a raindrop falls through the measurement area, its sections at different heights are photographed. After processing the observation data by software, the fall speed of the raindrop and even its three-dimensional shape can be reconstructed (Figure 2). During rainfall, the disdrometer can measure each raindrop in milliseconds (thousandths of a second). Taking the heavy rain case on 7 September 2023 as an example, the Hong Kong International Airport at Chek Lap Kok was affected by a rain area with rainfall rate over 30 millimeters per hour (Figure 3). The disdrometer measured more than 140,000 raindrops within the 6-minute interval between 9:54 p.m. and 10:00 p.m. Plotting the fall speed and oblateness data against diameter of these 140,000+ raindrops on two-dimensional histograms (Figure 4), one can observe that the data are not randomly distributed but roughly follow a specific relationship, namely, the larger the raindrop, the higher the fall speed, and the smaller the oblateness and farther away from 1 (that is, the raindrop is more flattened with smaller vertical extent and larger horizontal extent).Upon acquiring the disdrometer data after each rainy event, not only can the dataset be compared with the conventional rainfall rate and rainfall amount recorded by rain gauges, but radar reflectivity can also be derived from basic physical principles for comparing with weather radar data. This helps to identify the characteristics of raindrops under different precipitation mechanisms in different seasons.

Disdrometer

March 2024

https://www.hko.gov.hk/en/education/meteorological-instruments/automatic-weather-stations/00710-World-in-Millisecond-of-Two-Dimensional-Video-Distrometer.html

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What does Heat Stress Monitoring System Measure?

The Observatory’s in-house developed "Heat Stress Monitoring System" consists of dry-bulb thermometer, natural wet-bulb thermometer, and globe thermometer. What is the meaning of Hong Kong Heat Index measured by the system? How is it different from air temperature?

Now it is summer, apart from air temperature, relative humidity and other weather information provided by the Observatory, we may also notice the Hong Kong Heat Index (HKHI). Taking a closer look, you would realize that air temperatures and HKHI have similar trends, but their values are not the same. Using the case on 14 September 2022 as an example, King’s Park recorded a daily maximum temperature of 34.5 degrees, while the HKHI, also measured at King’s Park, was only 27.5. Why would there be such a large difference in values for two indicators of hot weather? Before solving the puzzle, let us first understand the “Heat Stress Monitoring System” measuring HKHI.The Observatory’s in-house developed "Heat Stress Monitoring System" registered a patent in Hong Kong as early as 2009. Initially, the Observatory developed this system to support the Hong Kong Olympic and Paralympic Equestrian Events in 2008. It provided an objective reference indicator of the stress level on horses under high temperatures, allowing concerned parties to take additional measures to prevent the body temperature of the horses from getting too high. This system consists of three thermometers: dry-bulb thermometer, natural wet-bulb thermometer, and globe thermometer.The dry-bulb thermometer is a platinum resistance thermometer housed in a solar radiation shield. The thermometer adopts a platinum wire component with its resistance varying with temperatures. Air temperatures can be known according to the resistance values. The principle of solar radiation shield is the same as that of the Stevenson screen, allowing good ventilation and avoiding thermometer being affected by direct sunshine and rainfall.The natural wet-bulb thermometer also measures temperature by a platinum resistance thermometer. The major difference from the dry-bulb thermometer is that the natural wet-bulb thermometer is covered by a moist muslin and exposed outdoors, allowing the temperature measurements to be influenced by air temperature, humidity, sunlight, and wind. When the water on the moist muslin evaporates, it draws heat from both the muslin and the thermometer, causing the wet-bulb temperature to be lower than the dry-bulb temperature. If the weather is dry, or under blazing sunshine or strong winds, water evaporates more easily. In those days the difference between the two temperatures would be more obvious. Hence the natural wet-bulb temperature can reflect the sweating condition and cooling efficiency of horses or humans in an outdoor environment.The design of the globe thermometer is to enclose a platinum resistance thermometer in a hollow and dull black copper ball. When sunlight shines on the ball surface, most heat radiation would be absorbed and transferred into heat energy. Copper, with a high conductivity of heat, can absorb heat or cool down rapidly. Therefore, the globe thermometer can effectively reflect the effect of solar radiation and winds on temperatures. On a day of abundant sunshine and lighter winds, the globe temperature can be higher than the dry-bulb temperature by more than 10 degrees.When the Heat Stress Monitoring System collects dry-bulb temperature (Ta), natural wet-bulb temperature (Tnw), and globe temperature (Tg), the “Hong Kong Heat Index” can be calculated from their values. The formula is:HKHI = 0.80 Tnw + 0.05 Tg + 0.15 TaHKHI was developed based on a collaborated research study with a local university using meteorological data and local hospitalization data. Generally speaking, the public should take appropriate precautions when the HKHI at King’s Park reaches around 30 or above to avoid adverse health effects brought by hot weather.With the above information, probably you now understand that HKHI is not a temperature reading, it does not have a unit and is an index integrating important effects from temperature, humidity, sunshine and wind speed. In this hot summer, while enjoying outdoor activities, please remember to keep updated of the Observatory’s latest weather information, and take appropriate protective measures against the heat!

Hot and Cold Weather

July 2023

https://www.hko.gov.hk/en/education/meteorological-instruments/automatic-weather-stations/00702-What-does-Heat-Stress-Monitoring-System-measure.html

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Monitoring and forecasting air temperature within urban areas

Urban meteorological monitoring stations can help the public understand the city's microclimate and weather conditions at a higher resolution, thus allow the Observatory to develop better weather information services and enhance the Automatic Regional Weather Forecast.

In Hong Kong, the weather conditions and their changes can vary significantly from district to district. Localised showers and thunderstorms triggered by hot weather are common examples. Apart from rainfall, air temperature also exhibits large spatial difference. A well-recognised phenomenon called the “urban heat island” describes the higher temperature in urban areas than in rural areas at night. However, have you ever felt microclimate difference within an even smaller region in the urban areas? For example at 11 am on 28 July 2022 (Figure 1), the temperatures recorded at the meteorological garden in the Hong Kong Observatory Headquarters and the urban meteorological monitoring station (“urban station” in short) in Mong Kok, both within the Yau Tsim Mong District, showed a difference of up to 4 °C. Moreover, under the continuously sunny and very hot weather in Hong Kong during mid- to late July this year, significant differences in the average temperature daily cycles can be observed at the Hong Kong Observatory and neighbouring urban stations (Figure 2). Let’s have a look into the underlying reasons of the urban-scale temperature variations and the challenges in location-specific weather forecasting.Urban areas in Hong Kong are densely populated and have complex and diverse environments. To have a better understanding of the city’s microclimate for improving weather information services, the Observatory has set up a number of urban meteorological monitoring stations in various districts and the urban-scale weather observations and forecasts were launched in May 2022 for reference by members of the public. While traditional weather stations (e.g. the Meteorological Garden at the Hong Kong Observatory Headquarters) are typically set up on lawns, the urban stations are located very close to urban infrastructures and activities (Figure 1 (right)). The thermometer of an urban station is placed inside a compact solar radiation shield to avoid the effects from direct solar radiation, rain, wind, and man-made emissions, but it is less protected and ventilated than that placed under a thermometer shed or inside a Stevenson Screen. Therefore, temperatures recorded at urban stations are more easily influenced by surrounding environmental factors, including:1. The construction materials and density of buildings2. The human activities, urban vegetation, and air ventilation potentialThrough understanding the causes of temperature differences within urban areas and analysing the data collected from urban stations, development of location-specific weather forecasts at the urban scale can be made. The Observatory utilizes data from the urban station observations and several numerical weather prediction models, the post-processing technique is then applied to verify model forecasts against real-time observations and correct computer model predictions to generate forecasts for the next few hours to days via a fully automatic process. However, since urban stations are more sensitive to the air temperature fluctuations caused by their surrounding environments, it is difficult to produce accurate forecasts for a longer period of time. Currently, hourly observations and automatic forecasts of air temperature and relative humidity at urban stations up to next three days ahead are available in the “Automatic Regional Weather Forecast” web portal. The Observatory will continue to enhance microclimate observations and forecasts, so that the public can gain better access to the detailed weather changes within urban areas for purposes such as the planning of daily activities and assessment of health risks associated with urban high temperature.

October 2022

https://www.hko.gov.hk/en/education/meteorological-instruments/automatic-weather-stations/00683-Monitoring-and-forecasting-air-temperature-within-urban-areas.html

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Portable Upper-air Sounding System

HKO acquired a portable upper-air sounding system (PUPAS) in late 2014. PUPAS comprises a small receiver, a processor, a sonde checker, antennas, etc. The above components can essentially be fitted inside a couple of suitcases. The system is easily transportable by one or two persons to collect meteorological data at different locations, thus enhancing the mobility and the application of upper-air sounding.

The Hong Kong Observatory (HKO) operates an automatic upper-air sounding system (Figure 1) which regularly releases a weather balloon carrying a radiosonde that measures and computes wind speed and direction, temperature, humidity, pressure, etc. at various heights of the atmosphere, providing indispensable data for weather forecasting.  The automatic upper-air sounding system, with a size close to that of a standard container, is located at the King's Park Meteorological Station.To meet operational and development needs, HKO acquired a portable upper-air sounding system (PUPAS) in late 2014.  PUPAS comprises a small receiver, a processor, a sonde checker, antennas, etc.  The above components can essentially be fitted inside a couple of suitcases (Figures 2 and 3).  The system is easily transportable by one or two persons to collect meteorological data (Figure 4)  at different locations or even on board of a ship, thus enhancing the mobility and the application of upper-air sounding.

April 2015

https://www.hko.gov.hk/en/education/meteorological-instruments/automatic-weather-stations/00451-portable-upperair-sounding-system.html

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Let's Talk about Relative Humidity

You may know the term “relative humidity” mentioned in weather reports. But what exactly is relative humidity? Why “relative”? How is it measured? This article introduces what relative humidity is, how it is different from absolute humidity, how it is measured and the recovery of post-war relative humidity records.

“A dry northeast monsoon is affecting the coast of Guangdong. The relative humidity in the territory generally fell to about 40 percent in the afternoon.” If you pay attention to weather reports, the term “relative humidity” should be familiar to you. But what exactly is relative humidity? Why “relative”? How is it measured?Absolute humidity and relative humidityAbsolute humidity is the mass of water vapour per unit volume of the air in which the vapour is contained, usually expressed in the unit of g/m3. It is a measure of the actual amount of water vapour in the air.Relative humidity is the ratio of the actual water vapour pressure in the air to the saturation vapour pressure of water at the same temperature, usually expressed in percent (%).What is saturation vapour pressure then? Imagine a closed container with water at room temperature and assume constant temperature. Initially, some water molecules at the surface with sufficient kinetic energy will evaporate as water vapour and leave the liquid water body. Some of the water vapour in the air will condense again as liquid water. This process will continue until an equilibrium is reached, that is, when the number of returning molecules becomes identical to the number of escaping molecules. At that time, the air in the container cannot hold any more water vapour and is said to be saturated (Figure 1). Under such condition, the pressure exerted by the water vapour on the water surface is called the saturation vapour pressure. Saturation vapour pressure depends on temperature. At a higher temperature, the air can hold more water vapour and the corresponding saturation vapour pressure will increase, vice versa.In layman’s terms, relative humidity can be understood as the ratio of the actual amount of water vapour in the air relative to the maximum amount of vapour that can exist in the air at the same temperature. If we assume the actual amount of water vapour in the air does not change (absolute humidity is constant), when air temperature falls, the maximum amount of vapour that can be held by the colder air will become less (i.e. saturation vapour pressure decreases). This will result in an increase in the relative humidity. Under clear skies with significant night-time radiative cooling, the temperature drop may cause the near-ground air to become saturated with water vapour condensing to form water droplets (i.e. dew). As such, the air temperature to which it must be cooled to become saturated is called dew point temperature.Measuring relative humidityThere are three common methods of measuring relative humidity.Computation using dry- and wet-bulb temperatures: Dry-bulb temperature is simply the air temperature. Wet-bulb temperature is read on a thermometer whose bulb is covered in a water-soaked cloth. Due to evaporative cooling of water on the cloth, wet-bulb temperature is generally lower than dry-bulb temperature. The evaporation rate depends on the amount of water vapour in the air. Under drier air, evaporation is faster and the difference between dry-bulb and wet-bulb temperature is larger. Therefore, the difference between dry-bulb and wet-bulb temperature reflects how far from saturation the air is and can be used to compute the relative humidity. Historically, various methods have been adopted by the Observatory to compute the relative humidity, including the use of hygrometric tables, slide rules (Figure 2) and formulas. Currently, relative humidity is computed using the modified Hooper’s method[1].Resistive humidity probes: measure the variation in resistance of a material whose conductivity changes with humidity.Capacitive humidity probes: measure the variation in capacitance of a dielectric caused by changes in humidity. This type of probes are used at some automatic weather stations of the Observatory.Recovery of post-war historical relative humidity recordsPreviously, the Observatory website only provided relative humidity data starting from 1961. As for the post-war period from 1947 to 1960, relative humidity data are recorded in the weather logbooks kept at the Observatory Headquarters (Figure 3). The Observatory recently quality-checked and digitised these historical relative humidity data, and added them to the Observatory’s database. The daily, monthly and yearly mean relative humidity data recorded at the Observatory Headquarters from 1947 to 1960 were published online in November 2023.Hong Kong has experienced some extremely dry weather events since WW II, including a prolonged drought in 1963 which resulted in an annual mean relative humidity of only 73%, the driest year on record. January of that year was also the driest month on record with a mean relative humidity of only 45%. The mean relative humidity on 16 January 1955 was 21%, the lowest daily value on record. Based on the hourly observations at the Observatory Headquarters, the lowest absolute minimum relative humidity appeared on 16 January 1959, with a relative humidity of only 10% recorded at 3 pm that day (Figure 4).

May 2024

https://www.hko.gov.hk/en/education/meteorological-instruments/automatic-weather-stations/00714-Lets-talk-about-relative-humidity.html

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Evaporation and Potential Evapotranspiration

Evaporation and evapotranspiration are two important elements of hydrological observations. "Evaporation" is the total amount of water entering the atmosphere when water changes from its liquid state to gaseous state. "Potential evapotranspiration" is the amount of evapotranspiration that may occur when the soil often keeps moist.

You may know what "evaporation" is, but have you ever heard of "evapotranspiration"?Evaporation and evapotranspiration are two important elements of hydrological observations. "Evaporation" is the total amount of water entering the atmosphere when water changes from its liquid state to gaseous state. Evaporation is related to sunlight, wind speed, temperature and humidity. The transfer of water from soil and plant surfaces to the atmosphere is called "evapotranspiration". "Potential evapotranspiration" is the amount of evapotranspiration that may occur when the soil often keeps moist. These data are very useful for hydrometeorological and agricultural studies as well as construction of reservoirs and freshwater lakes.At the King's Park Meteorological Station, measurement of water surface evaporation began in 1957, and "potential evapotranspiration" began even earlier in 1951. Both measurements are still in operation nowadays. Traditionally, evaporation is measured using an evaporation pan (Figure 1). The pan is filled with fresh water and equipped with a fixed hookgauge marking the water level before evaporation. Meteorological technician pours water into the pan daily at a certain fixed time and carefully measures its amount until the water surface has just touched the tip of the hookgauge again. This amount of water represents the amount of evaporation in the past 24 hours. If it has rained and the water level is higher than the hookgauge, water needs to be pumped out from the pan and measured. Evaporation of the day equals to the amount of rain minus the amount of water pumped. However, if the pan overflows in torrential rain situation, evaporation cannot be accurately calculated.The "potential evapotranspiration" is measured by using a evapotranspiration measuring device built with bricks and cement. It is covered with soil and planted with short grass on the top (Figure 2). There is an outlet tube extending from the bottom of the device to lead the run-off to another water tank. In the past, meteorological technician regularly sprinkled a certain amount of water on the grass every day to ensure that the soil was saturated with water, causing water to flow out of the outlet tube the next day. Assuming that the water content in the evapotranspiration measuring device remains the same between the two measurement times, the difference between the water sprinkled on the grass surface and the run-off represents "potential evapotranspiration". If there has been rain in the past 24 hours, the rainfall will also be taken into account for the calculation.Since 2016, technical staff of the Observatory have successively automated these two observations, switching to electronic measurement and calculation instead of manual observation. Hookgauges, measuring rulers and water measuring vessels, etc. were no longer used. To measure the change in the height of water level, placing an electronic pressure sensor in the evaporation pan or water tank (Figure 3) is all we need now. The sensor can measure the water level and its changes based on the water pressure, and the "evaporation" and "potential evapotranspiration" can then be calculated. If heavy rain occurs, when the water level reaches a preset height, the device will make a record and then drain the water from the pan or the tank with a pump. This is to prevent overflow which affects the reading. Automated observations not only save manpower, but also eliminate errors arising from observations performed by different people, and increase the data availability on heavy rain days.

March 2020

https://www.hko.gov.hk/en/education/meteorological-instruments/automatic-weather-stations/00541-Evaporation-and-Potential-Evapotranspiration.html

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Conventional Instruments installed at an Automatic Weather Station in Hong Kong

AWS network collects vital meteorological data essential for the provision of weather forecasting and warning services in Hong Kong. A typical AWS usually comprises several conventional instruments to measure air temperature, relative humidity, wind speed, wind direction, rainfall and atmospheric pressure.

The Hong Kong Observatory (HKO) established its first two Automatic Weather Stations (AWS) in 1984, one at its Headquarters in Tsim Sha Tsui and another one at Sha Tin.  By the end of 2014, the number of AWS operated by HKO has increased to over 80, and is believed to be one of the densest AWS networks in the world.  This AWS network collects vital meteorological data essential for the provision of weather forecasting and warning services in Hong Kong.  A typical AWS usually comprises several conventional instruments to measure air temperature, relative humidity, wind speed, wind direction, rainfall and atmospheric pressure.Although it appears to be simple to measure the above-mentioned meteorological elements, proper measuring instruments and site selection are required to meet certain standards and specifications.  In the HKO’s AWS network, a platinum resistance thermometer (PRT) is used to measure the air temperature (dry-bulb temperature).  Another PRT wrapped around by wet cloth is used to measure the wet-bulb temperature (Figure 1).  The good linear correlation between the resistance of platinum and air temperatures makes PRT an ideal tool for measuring air temperature.  Relative humidity can then be computed from the dry-bulb and wet-bulb temperatures.  These two PRTs are placed inside a Stevenson screen box about 1.2 meters above the ground.  The best site for temperature measurements is over level ground with grass surface, freely exposed to sunshine and wind and not shielded by obstructions[1]. Most HKO AWS use a cup anemometer to measure wind speed and wind direction (Figure 2).  The wind speed sensor consists of a three-cup assembly mounted on a vertical spindle rotating in ball bearings[2].  The wind cups rotate as the wind blows and the rate of rotation reflects the wind strength.  The wind vane on the other hand measures the wind direction.  The sharp pointer indicates the direction from which the winds are blowing.  The anemometer is generally required to be installed in open terrain at a height of around 10 metres above the ground.Rainfall is usually measured by a tipping-bucket raingauge.  A tipping-bucket raingauge consists of a light bucket which is divided into two equal compartments pivoted at its centre like a see-saw (Figure 3).  Rain is collected at one compartment and when a predetermined amount of water is received, the bucket overbalances and tilts, discharging the collected water and allowing the other compartment to begin filling.  The number of tilts during a certain period is used to calculate the total amount of rainfall recorded during the period.  The raingauge should also be installed at a certain distance from obstacles.Atmospheric pressure is usually measured by capacitive pressure transducer.  The pressure sensor contains two closely spaced, parallel and electrically-isolated metallic surfaces.  One of which is essentially a diaphragm that can be bent slightly in response to the variation of air pressure.  Its small mechanical movement, caused by the change of air pressure, alters the gap between the two metallic surfaces and thus creates effectively a variable capacitor.  The resulting change in capacitance is then used to determine the atmospheric pressure.  The pressure sensor is generally installed indoors with relatively stable environmental conditions.

April 2015

https://www.hko.gov.hk/en/education/meteorological-instruments/automatic-weather-stations/00454-conventional-instruments-installed-at-an-automatic-weather-station-in-hong-kong.html

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Thermometers Under the Shed

An official name for the shed is "Thermometer Shed". Thermometers respectively for measuring dry- and wet-bulb temperatures are placed under the shed. Thermometer shed is a special shelter used in tropical regions to house the thermometers. A thermometer shed has been used at the Observatory Headquarters in the late 1880s. The shed is covered by mattress and palm leaves which are required to be repaired or re-surfaced once every 5 years or so.

During the Observatory's Open Day in March every year, lots of visitors are attracted by the historical 1883 Building and take photos right in front of it. Meanwhile, a shed on the Observatory's lawn just next to the building also draws the visitors' attention. An official name for the shed is "Thermometer Shed". Thermometers respectively for measuring dry- and wet-bulb temperatures are placed under the shed.The air temperature is directly measured from the dry-bulb thermometer while the relative humidity is calculated from the difference between temperatures measured by the dry- and wet-bulb thermometers. To obtain the actual temperatures, the thermometers are placed under a shed to avoid the measured temperatures being affected by direct sunlight and adverse weather conditions, and to allow good ventilation at the same time.Thermometer shed is a special shelter used in tropical regions to house the thermometers. A thermometer shed has been used at the Observatory Headquarters in the late 1880s. The shed is covered by mattress and palm leaves which are required to be repaired or re-surfaced once every 5 years or so. Due to its bulky size, the Observatory has only installed a thermometer shed at its Headquarters. The conventional Stevenson Screen is used at the automatic weather stations over Hong Kong for housing thermometers.

May 2019

https://www.hko.gov.hk/en/education/meteorological-instruments/automatic-weather-stations/00526-thermometers-under-the-shed.html

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Enhancing Aviation Weather Services Through Familiarization Flights

The first-hand experience of encountering significant weather in flight as well as direct exchanges with pilots are useful for enhancement of the Observatory's aviation weather services.

Since 2005, the Hong Kong Observatory has been sending staff members to participate in actual flying in the cockpit as arranged by airlines, which allows colleagues involved in aviation weather services to interact and exchange ideas with pilots and crew members. Due to the pandemic, these familiarization flight arrangements were suspended for several years. With the support of the Hong Kong Cathay Pacific Airways Limited, a new round of familiarization flights resumed in October 2023, and lasted till January 2024.Through interviews with pilots, colleagues from the Observatory gained insights into how they utilize weather information during flights, including the practical operation of the "MyFlightWx" electronic flight bag application developed by the Observatory for flights using the Hong Kong International Airport. Colleagues can collected feedback on the Observatory's existing aviation weather services and products, and engage in discussions regarding future services and product developments (Figure 1).During the familiarization flights, colleagues from the Observatory have the opportunity to witness firsthand how pilots acquire the latest weather information and respond to different weather conditions in the cockpit. This leaves a profound impression on the participating colleagues, and highlighted the importance of aviation weather services to the aviation industry. High-quality aviation weather information allows pilots and crew members to be well-prepared when dealing with adverse weather situations, thereby enhancing flight safety and ensuring the regularity and efficiency of air traffic operations. The first-hand experience of encountering significant weather in flight as well as direct exchanges with pilots are useful for enhancement of the Observatory's aviation weather services.Apart from adverse weather, various atmospheric conditions are encountered along flight routes. Different kinds of atmospheric phenomena may therefore be observed en-route. For example, when an aircraft flies over the clouds made up of liquid water droplets, with the right alignment of sunlight, one may observe the aircraft’s shadow surrounded by alternating reddish and bluish rings. This phenomenon, known as a “glory”, is one of the atmospheric optical phenomena often be observed during flight (Figure 2). If encountered at high altitudes below the freezing point, it may even serve as a sign of potential icing conditions that needs attention. These various observations during flight help deepen the participating colleagues’ understanding of aviation meteorology.

March 2024

https://www.hko.gov.hk/en/education/aviation-and-marine/aviation/00713-Enhancing-aviation-weather-services-through-familiarization-flights.html

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Services provided by Aeronautical Meteorological Adviser in Integrated Airport Centre

The article describes the services provided by the Aeronautical Meteorological Adviser of the Hong Kong Observatory at the Integrated Airport Centre in the Hong Kong International Airport.

Beginning from the end of June 2022, the Observatory dispatches Aeronautical Meteorological Advisers to the Integrated Airport Centre (IAC) of the Hong Kong International Airport (HKIA) to provide services to the aviation community. Customers include the Airport Authority Hong Kong, Civil Aviation Department, airlines, ground handling agents, cargo terminal operators and ramp operators, etc.The services of Aeronautical Meteorological Adviser include, among others, delivering regular daily briefings on local weather and inclement weather with high impact in nearby regions, distribution of concise comprehensive weather charts, and provision of probabilistic forecasts of weather elements with high impact on HKIA’s operations.By delivering briefings through video conferencing, customers can better appreciate the latest weather conditions of HKIA and the Hong Kong Flight Information Region, which would facilitate more effective airport operation arrangements. Aeronautical Meteorological Adviser will inform the customers upon the assessment that any airport in the adjacent regions will be affected by inclement weather. For example, if a tropical cyclone approaching Japan will affect airports with frequent flights to and from Hong Kong, the flights departing from Hong Kong may need to be suspended or rescheduled. Relevant stakeholders can inform passengers in advance through different channels and request them not to proceed to the airport at the moment to avoid creating chaos due to the crowding of passengers waiting at the airport. Therefore, appreciating the weather situation at nearby airports by the customers is also highly related to the operation of HKIA. Should the customers have any enquiries at Aeronautical Meteorological Adviser’s briefing, they can raise their questions immediately for further discussion. This highly enhances the communication between the Observatory and the aviation customers. The content of the briefing will be summarized in a concise weather chart and uploaded to a dedicated webpage for easy reference by the customers.Aeronautical Meteorological Adviser will also provide customers with probabilistic forecast of weather elements that have a high impact on airport operations, including runway high winds, crosswind and extreme high temperature, etc.. This kind of forecasts can facilitate decision-making of the aviation customers in their daily operations, or their activation of contingency measures of different levels.Aeronautical Meteorological Adviser on duty at the IAC can respond face-to-face to various enquiries from customers. During rapidly changing inclement weather situations such as tropical cyclones or significant convection affecting the airport operations, Aeronautical Meteorological Adviser can provide appropriate information and opinions more effectively. During the case of some uncertain weather conditions, face-to-face consultation can help explain various factors that may affect the airport operations, and facilitate customers to assess weather changes and make appropriate decisions. This kind of real time and up-to-date consultation service can facilitate flight planning and operational decision-making of the customers, thereby significantly improving the safety and efficiency of the aviation services.

January 2023

https://www.hko.gov.hk/en/education/aviation-and-marine/aviation/00688-Services-provided-by-Aeronautical-Meteorological-Adviser-in-Integrated-Airport-Centre.html

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Tailor-made Meteorological Services in Support of Airport Runway Re-designation

Meteorological services for Hong Kong International Airport Runway Re-designation.

The pavement of the Third Runway of the Hong Kong International Airport has been completed and the runway will become the new “North Runway” in future. The former North Runway was re-designated as “Center Runway” on 2 Dec 2021. The name of the runway was changed from 07L and 25R to 07C and 25C respectively (Figure 1). Additional designator “C” is used for Centre Runway in order to identify the relative position of the parallel runways.The Hong Kong Observatory started to prepare for runway re-designation more than a year ago. Relevant meteorological systems and work procedures were modified so that aviation users such as air traffic controllers, pilots and other aviation users can accurately and easily grasp meteorological data compiled for individual runways (Figure 2). In addition to working closely with Civil Aviation Department on system transition, the Hong Kong Observatory also provided weather services to support Airport Authority to conduct outdoor works required for runway re-designation.The Hong Kong Observatory is enhancing the meteorological systems and equipment to provide the necessary weather services to support the future operation of Three-Runway System.

January 2022

https://www.hko.gov.hk/en/education/aviation-and-marine/aviation/00667-Airport-Runway-Re-designation.html

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Collaboration of Aviation Weather Services in issuance of SIGMET

What is SIGMET? Why coordination is needed for SIGMET issuance? Through the SIGMET Coordination web platform, seamless and consistent SIMGET messages can be issued easily.

Provision of timely information on adverse weather affecting en-route flights within the Hong Kong Flight Information Region (HKFIR) (Figure 1) to ensure flight safety is one of the roles and responsibilities of the Airport Meteorological Office (AMO) of the Hong Kong Observatory (HKO). AMO will issue various meteorological advisories whenever hazardous weather is affecting or is expected to affect the region. These advisories, called Significant Meteorological Information (SIGMET), need to be issued to pilots timely in accordance with the regulations set by the International Civil Aviation Organization (ICAO) to enable them to take early appropriate actions. For example, when thunderstorm is affecting or is forecast to affect a certain FIR, pilots may take necessary actions such as flight deviation or hovering in holding area upon receipt of the SIGMET message so as to avoid potential hazards to flight safety due to significant convection or even hail. The latest SIGMETs issued by the AMO can be found on the HKO's website. There are three types of SIGMET dedicated for different types of hazardous weather, namely, the WC SIGMET (tropical cyclones) [1], WV SIGMET (volcanic ash), and WS SIGMET (other meteorological phenomena such as thunderstorms, turbulence, icing and etc.). Each SIGMET message contains the description of the weather phenomenon, location, height, speed and direction of movement, intensity change and forecast position [2]. To facilitate exchange, SIGMET is encoded. Automatic translation of SIGMET message issued by AMO in the form of plain language is available at the above-mentioned website. Why coordination is needed for SIGMET issuance? Since forecasters are responsible for issuing SIGMET within their own FIR, when a weather hazard, e.g. thunderstorm cloud, extends beyond one FIR, pilots may receive SIGMET messages that are not fully consistent with each other across FIR boundary/boundaries due to different operational practices or assessments for the same weather system by forecasters responsible for different FIRs. Figure 2 illustrates schematically one of the cases. An airplane is approaching a thunderstorm cloud but the SIGMET issued by forecasters from "FIR A" predicted the cloud to be moving to the east, forecasters from "FIR B" predicted it to be stationary, while forecasters from "FIR C" didn't even issue any SIGMET. Under this situation, pilots may find the SIGMET information ambiguous and confusing. To further improve the SIGMET services, in recent years, ICAO promotes cross-border cooperation and coordination so as to harmonise meteorological related products [3]. To provide and promote SIGMET coordination among neighbouring FIRs, the Hong Kong Observatory has developed a web platform, named the Regional SIGMET Coordination Platform [4]. Using the chat-room function (Figure 3), forecasters could exchange views and agree on the final warning information. Based on the agreed information, the platform will then automatically generate the SIGMET message for respective FIRs. This web platform has been put into operations since 2017 and has been provided to a number of MWOs within the region for their daily operations or trial operations [5]. Through the SIGMET Coordination web platform, seamless and consistent SIMGET messages can be issued easily. The improved quality of the coordinated SIGMET within the region is well received by the pilots.

December 2019

https://www.hko.gov.hk/en/education/aviation-and-marine/aviation/00537-collaboration-of-aviation-weather-services-in-issuance-of-sigmet.html

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ICAO Meteorological Information Exchange Model 101

IWXXM is made up of two components, namely Unified Modelling Language(UML)conceptual model and eXtensible Markup Language(XML). In day-to-day operations, IWXXM can be considered as the conversion of Traditional Alphanumeric Code (TAC) into XML format for all meteorological products as stipulated in Annex 3 to the Convention on International Civil Aviation.

'ABC of meteorological data encoding' in HKO Educational Resources points out that the traditional decoding method cannot effectively handle the exponentially increasing data exchange between meteorological centres. Higher fidelity meteorological data simply adds a further burden to the situation. The same thing also happens to Aeronautical Meteorology. In this regard, the World Meteorological Organization (WMO) set up a Task Team on Aviation XML in 2011 to join force with the International Civil Aviation Organization (ICAO) in formulating the next-generation standard on exchanging information for aeronautical meteorology –– this is the origin of the ICAO Meteorological Information Exchange Model (IWXXM).What is IWXXM?IWXXM is made up of two components, namely Unified Modelling Language(UML)conceptual model and eXtensible Markup Language(XML). In day-to-day operations, IWXXM can be considered as the conversion of Traditional Alphanumeric Code (TAC) into XML format for all meteorological products as stipulated in Annex 3 to the Convention on International Civil Aviation. Among the meteorological products, aviation weather reports (METAR), aerodrome forecast (TAF) and significant weather information (SIGMET) are some examples. Benefits of implementing IWXXMThe XML format facilitates information exchange among computer systems and conversion to other formats to fit for various purposes. XML is easily extensible to handle new information or data. The structure and metadata of XML also make data validation more efficient and effective. IWXXM will allow better exchange of aeronautical meteorological data, in line with the Aviation System Block Upgrade (ASBU) under the ICAO Global Air Navigation Plan, to support more advanced aeronautical meteorological services that require more accurate data in future.IWXXM version 1.0 was first released in September 2013. The latest version 3.0 will be approved by WMO in mid-2019. IWXXM is expected to become mandatory from 2020 when Amendment 79 to ICAO Annex 3 becomes effective. Let's get ready for a data-booming era!

January 2019

https://www.hko.gov.hk/en/education/aviation-and-marine/aviation/00520-icao-meteorological-information-exchange-model-101.html

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Planes: Special Air Reports

Introduce special air report made by aircraft as a source of aviation weather observation.

According to Chapter 5 of ICAO Annex 3 - Meteorological Service for International Air Navigation, aircraft are required to make special observations if they encounter certain hazardous weather phenomena including moderate/severe turbulence or icing. These special observations, called special air reports (ARS), could be used as one of the many valuable sources of information for aviation forecasters to issue accurate forecasts or warnings. The ARS also serve as alerts to other flights along the same flight path.Special air reports are first provided by pilots to air traffic services (ATS) units through voice communications. Upon the receipt of such reports, the ATS units would relay them without delay to their associated meteorological watch offices (MWOs). In Hong Kong, the information is passed from the air traffic control officers of the Hong Kong Civil Aviation Department to the Airport Meteorological Office (AMO) of Hong Kong Observatory (HKO).Aviation forecasters at AMO will then encode the information into the ICAO-specified format, and disseminate the ARS as soon as possible via Air Traffic Services Message Handling System (AMHS) to relevant parties around the globe including ATS units, World Area Forecast Centres (WAFCs), MWOs, airlines, etc. The encoded information will also be distributed via VOLMET broadcast/D-VOLMET, which are periodic broadcasts of meteorological information. Aviation forecasters will also assess the intensity and persistence of the phenomenon, then issue Significant Weather Information (SIGMET) when necessary (Figure 1).HKO also receives reports of windshear and turbulence from departing or arriving flights through the aerodrome control tower. Aviation forecasters will make use of such information and consider if there is a need to issue windshear warnings or SIGMETs. Such windshear and turbulence information will also be included in the Aviation Weather Report (METAR/SPECI).In addition to real time operational usage, ARS are also used for development of aviation hazardous weather forecast products. The collected data help developers understand the climatology of hazardous weather, evaluate the performance of forecast products and improve the forecast accuracy. An example of ARS on March 2016 is illustrated in Figure 2.

January 2023

https://www.hko.gov.hk/en/education/aviation-and-marine/aviation/00687-Planes-Special-Air-Reports.html

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Aviation Weather Observation and Reports

The aviation weather observation and reports are made round-the-clock by the weather observer at the Airport Meteorological Office located inside the Control Tower of the Hong Kong International Airport. Since April 2000, the station of synoptic weather observation for international exchange has been relocated from the Hong Kong Observatory Headquarters in Tsim Sha Tsui to the Hong Kong International Airport at Chek Lap Kok.

As the designated meteorological authority in Hong Kong, the Hong Kong Observatory provides the weather services for international air navigation in accordance with the standards and requirements of the International Civil Aviation Organization (ICAO) and the World Meteorological Organization (WMO).The aviation weather observation and reports are made round-the-clock by the weather observer at the Airport Meteorological Office located inside the Control Tower of the Hong Kong International Airport.  Since April 2000, the station of synoptic weather observation for international exchange has been relocated from the Hong Kong Observatory Headquarters in Tsim Sha Tsui to the Hong Kong International Airport at Chek Lap Kok.  The weather observer at AMO is now responsible for issuing weather reports to the aviation community as well as to the meteorological community.What is the difference between meteorological observation and report?Meteorological observation is the evaluation of one or more meteorological elements, while a meteorological report is a statement of observed meteorological conditions related to a specified time and location.What is SYNOP?Synoptic weather observation is a surface meteorological observation made at internationally agreed standard times, e.g. 00, 06, 12, 18 UTC (HKT = UTC + 8 hours).  The meteorological elements observed at a particular location and time are encoded in agreed formats of a meteorological report called SYNOP for the exchange among countries of the world at periodic times such as 3-hourly and 6-hourly.  The data are used for weather forecasting or climatological studies.What is METAR?METAR is an aviation routine weather report issued at hourly or half-hourly intervals. It is a description of the meteorological elements observed at an airport at a specific time.  The elements include surface wind, visibility, runway visual range, present weather, clouds of operational significance, air temperature, dew-point temperature and atmospheric pressure. The aviation weather report also includes a section containing the trend forecast, which indicates the expected change in meteorological conditions in the next two hours.What is SPECI?SPECI is aviation special weather report issued when there is significant deterioration or improvement in airport weather conditions, such as significant changes of surface winds, visibility, cloud base height and occurrence of severe weather. The format of the SPECI report is similar to that of the METAR and the elements used have the same meaning. The identifier METAR or SPECI at the beginning of the weather report differentiates them.How are METAR/SPECI and SYNOP reports exchanged internationally?METAR/SPECI are exchanged internationally through the Aeronautical Fixed Telecommunication Network (AFTN).  This aging system is gradually being replaced by the Aeronautical Message Handling System (AMHS) which supports higher bandwidth and the transmission of binary data.  As for the data format, the aviation community is also moving from alpha-numeric to XML which is more friendly to automatic systems and support future data-centric operation of aviation users.SYNOP reports are exchanged internationally via the WMOs Global Telecommunication System (GTS).  The times of observations, the format of data and messages, the schedule of exchange and the responsibility of the reporting station are coordinated in the system.  To enhance regional and global connectivity and information management, the WMO Information System (WIS) is being built upon the GTS and will be the core information system in the future.  It makes use of dedicated telecommunication means such as data networks and satellite-based telecommunications to ensure service quality, as well as the Internet to enable flexible data delivery service.  As for the data format, table-driven code form will still be used for the exchange of SYNOP reports.

June 2012

https://www.hko.gov.hk/en/education/aviation-and-marine/aviation/00207-aviation-weather-observation-and-reports.html

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Crosswind and Aviation Safety

Strong crosswinds during landing might blow an aircraft off the runway centreline. When performing crosswind landing, pilots will either use "crabbing" (by pointing the aircraft towards the wind so that the aircraft heading forms an angle with the runway alignment) or use "wing-low" (adding a small rolling angle such that the two wings are not on the same levels) to counter balance the effect of crosswinds.

Crosswinds are winds that blow from the side.  As an example, for departing or landing aircraft, winds blowing across the runway are crosswinds( Figure 1).  Besides aircraft, ships and cars are also affected by crosswinds.  Crosswinds can be useful in propelling the yacht.  On an athlete's track, the magnitude of crosswinds can affect the runner's performance as well!Strong crosswinds during landing might blow an aircraft off the runway centreline.  When performing crosswind landing, pilots will either use "crabbing" (by pointing the aircraft towards the wind so that the aircraft heading forms an angle with the runway alignment) or use "wing-low" (adding a small rolling angle such that the two wings are not on the same levels) to counter balance the effect of crosswinds.  As the first technique could potentially damage the landing gear, whereas the second technique may cause wing tip or engine to hit the ground when landing, the stronger the crosswinds, the larger the potential hazard to safe landing.  In general, when the aircraft is larger, its ability to overcome crosswinds would be better.  Crosswinds over 25 knots are deemed as significant.The two runways of the Hong Kong International Airport (HKIA) are aligned approximately along the east-west direction.  High winds from the north or south will therefore produce high crosswinds.  Winter monsoon and tropical cyclone are the two major weather systems which favour the occurrence of high crosswinds.  Tropical cyclones usually pose larger impact to aircraft operation.  For aircraft approaching or departing HKIA, southerly crosswinds normally have bigger impact than northerly crosswinds.  The main reason is that as southerly winds blow across the hilly ranges over Lantau, streaks of strong and weak winds could appear downwind.  These streaks cross the approach and departure paths of the airport to bring significant crosswinds.  Apart from that, they can also produce intermittent severe windshear and turbulence which may further affect the flight safety.

June 2013

https://www.hko.gov.hk/en/education/aviation-and-marine/aviation/00208-crosswind-and-aviation-safety.html

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Unnoticed cross-mountain climber - Mountain Wave

This article explores Mountain Waves and its hidden dangers to aviation safety.

Disturbance behind TranquillityAir in the atmosphere is constantly in motion and causes different phenomena. For example, clashes of two different air masses may trigger heavy downpour; winds over the ocean may roll up layers of waves. When a layer of stable air flows across a mountain range, the upwind side may be smooth, the downwind side on the other hand may be quite choppy. As air flows over the mountain, it will be deflected, forming a fluctuating airflow that rises and sinks called mountain wave. Mountain wave can propagate beyond the height of the mountain, and may sometimes reach the stratosphere. If a jet stream happens to occur near the mountain ranges, more intense mountain waves may result.Master of hidingMountain wave is a master of hiding its whereabout. Although invisible, the wave can be felt when an aircraft flies over the mountain. Depending on the shape and height of the mountain as well as how wind changes with height, mountain waves can take different form. On the upwind side, smooth rising airflow can help an aircraft to gain height. However, on the downwind side, due to the variations in vertical wind speed at different locations of the wave, the aircraft could be lifted and then sunk for hundreds of metres. If the vertical wind change is significant, it might form eddies causing severe turbulence to an aircraft. In serious cases, they might even damage the wings or engine of the plane, bringing danger to the flight.Under certain weather conditions, mountain waves will drop some clues and can be visible to pilots. Lenticular clouds or a rotor clouds can appear downwind in the sky meaning that downdraft and turbulence are strong. Pilots should avoid flying into these clouds which are signs of mountain waves in the area.Forecasting Mountain WaveForecasting small scale mountain wave is not easy. Forecasters should have a good knowledge of the topography and climate of the region as well as a good appreciation of the current weather conditions, the necessary forecasting tools and pilot reports to identify potential areas of severe mountain wave. When necessary forecaster would issue advisory of Significant Meteorological Information (SIGMET) to alert pilots from flying through the concerning area.

January 2022

https://www.hko.gov.hk/en/education/aviation-and-marine/aviation/00666-Mountain-Wave.html

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Terrain-induced Windshear & Turbulence over the Hong Kong International Airport

According to the Observatory's statistics, about 1 in 500 flights landing at or departing from the Hong Kong International Airport (HKIA) reported encountering significant windshear, whereas about 1 in 2,500 flights reported encountering significant turbulence. The majority of significant windshear events at HKIA could be associated with terrain disruption of airflow, which occurs predominantly in the late Spring season.

According to the Observatory's statistics, about 1 in 500 flights landing at or departing from the Hong Kong International Airport (HKIA) reported encountering significant windshear, whereas about 1 in 2,500 flights reported encountering significant turbulence. The majority of significant windshear events at HKIA could be associated with terrain disruption of airflow (Figure 1), which occurs predominantly in the late Spring season. As HKIA is located just north of the Lantau Island, winds blowing across the Lantau mountains from the east or southeast would become disturbed by terrain and might bring about significant windshear or turbulence downstream. A significant terrain-induced windshear episode recently occurred on 5 March 2015 (Figure 2). The arrival of an intense northeast monsoon resulted in 64 reports of significant windshear by landing/departing aircraft at HKIA, the highest number received by the Observatory within a day in recent years under non-tropical cyclone situations. As revealed by the Observatory's LIDAR scans in the afternoon of 5 March, the fresh-to-strong easterly winds near the surface gradually veered to southeasterly with height, leading to highly-disrupted airflow immediately to the west of HKIA downstream of the mountains (Figure 3). An aircraft landing from the west (which was the direction in use on the day) would experience sharp changes in wind speed/direction before touch-down and hence could be expected to encounter significant windshear.While all 64 encounters were successfully covered by the Observatory's windshear and turbulence alerting service (Figure 4), a number of affected aircraft had to initiate go-around or even diversion to neighbouring airports. The episode highlighted the potential impact of low-level windshear and turbulence to aviation operations and the importance of their timely detection and alerting using advanced remote-sensing equipment.

July 2015

https://www.hko.gov.hk/en/education/aviation-and-marine/aviation/00457-terraininduced-windshear-turbulence-over-the-hong-kong-international-airport.html

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Roadblocks in the Sky Convective Weather Impact on Aircraft

To avoid severe convective weather enroute, every pilot will study the flight plan before taking off. Apart from weather information such as the temperature, wind direction, and wind speed, a flight plan also includes forecast locations of thunderstorms. Pilots would pay special attention to forecast thunderstorms on the flight route or at the destination. They might even consult the meteorological office of the destination airport regarding thunderstorm forecasts if deemed necessary.

Located in the subtropical region, members of the public should find severe convective weather such as thunderstorms a commonplace in Hong Kong. However, not everybody may know that severe convective weather can pose a big impact to air traffics.A thunderstorm will bring not only lightning and heavy showers but can also bring squalls, hailstones and turbulence. Lightning strikes and hailstones can cause damage to cockpit windows and the skin of the aircraft. Radio communication between pilots and air traffic controllers could be severely interrupted while an aircraft is flying within a thunderstorm. Furthermore, turbulence and microburst (small scale but high speed downdraft beneath the cloud base) caused by intense thunderstorms can pose serious threat to an aircraft during landing and takeoff.To avoid severe convective weather enroute, every pilot will study the flight plan before taking off. Apart from weather information such as the temperature, wind direction, and wind speed, a flight plan also includes forecast locations of thunderstorms. Pilots would pay special attention to forecast thunderstorms on the flight route or at the destination. They might even consult the meteorological office of the destination airport regarding thunderstorm forecasts if deemed necessary. Extra fuel will be carried to cater for the situations that the aircraft has to amend its course to steer away from thunderstorms enroute or carry out holding to wait for weather improving at the destination airport.Thunderstorms are usually associated with a particular type of cloud called cumulonimbus with the abbreviation 'CB'. It can be developed from a single cell or multi-cells. The clouds can extend from near the ground up to 10 kilometres or higher (Figure 1). Thunderstorm clouds associated with synoptic scale weather system such as Mei Yu trough can form in clusters or lines with horizontal distance up to one hundred kilometres or more. The top of CB can far exceed the altitude where commercial aircrafts normally fly. They therefore look like huge towers or great walls in the sky. To avoid these "roadblocks" in the sky, the flight path has to be re-routed resulting in use of extra fuel.The writer had a chance of sitting at the cockpit observing the pilots controlling the aircraft during a return flight to Hong Kong. On its way, there was a huge CB at the port side of the aircraft (Figure 2).  At that time, the cruising level was 30,000 feet. An anvil-shaped cloud was seen at the cloud top (Figure 3), indicating that it had already reached the tropopause, the upper boundary of the troposphere. Fortunately, the CB did not sit in the course of the flight but moved further to the port side (i.e. moving East) away from the flight route.On 9 September 2010, there was a typical example of flights blocked by huge thunderstorm clouds. At around midnight, an aircraft entered the Hong Kong Flight Information Region and approached the northern runway. The first landing attempt failed because of the microbursts brought about by thunderstorms right before touch down. The pilots determined to perform a go-around for a second attempt. However, when it came to about 20 nautical miles to the southwest of the airport, there was a line of thunderstorms in front of it (Figure 4). The line of thunderstorms was about 60 nautical miles long oriented north to south blocking the way like a huge wall. As there was no gap for the aircraft to pass through, the pilots finally decided to divert to the Macau airport.

June 2013

https://www.hko.gov.hk/en/education/aviation-and-marine/aviation/00209-roadblocks-in-the-sky-convective-weather-impact-on-aircraft.html

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St. Elmo's Fire as Seen from Aircraft

The St. Elmo's Fire is not a real fire. It is a weather phenomenon that occurs in strong electrical condition in the atmosphere, usually in the presence of thunderstorms, or volcanic eruption nearby. The charges in the cloud induces charges on surface objects and the ground, resulting in a high voltage between the ground surface and the cloud base.

When people started to use ships for sailing, they occasionally observed mysterious glowing balls of blue or violet light on the ships at sea, as if the ship was caught by fire. This glowing light was believed to be revelation of spirits and would bring good luck. It was referred to as "St. Elmo's Fire", named after Saint Elmo who was the patron saint of sailors in the 3rd Century. The sailors believed that when the St. Elmo's Fire appeared, the patron saint was present to protect them. In Chinese culture, the same phenomenon is called "Fire of Mazu", the goddess who guards the seas. Actually, the St. Elmo's Fire is not a real fire. It is a weather phenomenon that occurs in strong electrical condition in the atmosphere, usually in the presence of thunderstorms, or volcanic eruption nearby. The charges in the cloud induces charges on surface objects and the ground, resulting in a high voltage between the ground surface and the cloud base. The established voltage is particularly intense when the surface object has a large curvature like a ship mast. This is sometimes called the sharp point effect (see "Don't be a lightning rod"). When the voltage is high enough, the air molecules surrounding the object are ionized to form a region of plasma. As the plasma discharges, the energy is released in the form of glowing light. The colour of the glow is usually violet or blue due to abundance of nitrogen and oxygen in the air. The appearance of the glow resembles a flame. Apart from ships, wing tips and nose cones of in-flight aircrafts are also common platforms for the St. Elmo's Fire to occur. The image in Figure 1 shows the St. Elmo's Fire as seen from the cockpit of an aircraft flying in close proximity to thunderstorms. Although St. Elmo's Fire might look a bit scary, it is generally harmless, and without scorching heat. The main hazards to aircrafts are brought by atmospheric conditions such as thunderstorms or volcanic ashes which initiate the St. Elmo’s Fire (see "Roadblocks in the Sky Convective Weather Impact on Aircraft" and "Monitoring of Volcanic Ash"). Nonetheless, if the magnetic field generated by the electrical current in the plasma is strong enough, temporary interruption to on-board instruments is possible, probably resulting in unreliable measurements or failure in radio transmissions. To ensure aircraft safety, the Airport Meteorological Office of the Hong Kong Observatory issues Significant Weather Information (SIGMET) and significant weather charts to warn flights within the Hong Kong Flight Information Region from getting close to hazardous regions such as thunderstorms and volcanic ash.

December 2019

https://www.hko.gov.hk/en/education/aviation-and-marine/aviation/00534-st-elmos-fire-as-seen-from-aircraft.html

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Caution! Slippery Road!

Monitoring and reporting of accumulated water depth on runway during heavy rain.

The International Civil Aviation Organization (ICAO) has mandated the requirement for worldwide airports to provide real-time Runway Condition Report (RCR) to pilots from November 2021 onwards, so as to alert pilots of possible dangerous runway conditions when aircraft are landing at high speeds. For example, whether the runway is wet and slippery, having standing water or icing conditions. Pilots may, therefore, get ready to take necessary actions, such as applying the reverse thrust to stop the aircraft safely. Air Traffic Controllers from the Civil Aviation Department (CAD) may also apply a farther separation between aircraft in the air.At the invitation of the Airport Authority Hong Kong (AAHK), the Hong Kong Observatory (HKO) has designed an automatic system to monitor the rainfall condition over the Hong Kong International Airport and deduce the standing water depths on runway surfaces under heavy rain round the clock. With the use of nine visibility meters installed at both ends and at the middle of each of the three runways (Figure 1), real-time rainfall rates are measured and converted to standing water depths on runways, as well as the percentage of runway surface covered by water. RCRs are then provided to AAHK on whether the runway is “Dry”, “Wet” or having a “Standing Water” depth of more than 3 mm.Abrupt heavy rain often occurs in the summer of Hong Kong. If the cumulonimbus is small in spatial scale, the associated heavy rain is often localised and only lasted for a very short time (as seen in Figure 2). If the RCR is issued upon instantaneous rainfall figures, the report may easily jump from one category to another in a few minutes, causing nuisance to Air Traffic Controllers and pilots in grasping the actual situation. The RCR system incorporates the SWIRLS nowcasting system of HKO in determining whether the heavy rain will persist based on the predicted rainfall amount at the airport in the coming 15-20 minutes. Upon confirmation by AAHK personnel, the information will be passed to the Air Traffic Controllers of CAD for broadcast to pilots.With international travel restrictions easing progressively, the airport is becoming increasingly busy. When you bump into a heavy rain at the airport next time, you may remember that the Observatory and airport personnel are working hard to uphold aviation safety by closely monitoring and responding to the slippery runways.

January 2023

https://www.hko.gov.hk/en/education/aviation-and-marine/aviation/00690-caution-slippery-road.html

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Severe thunderstorms bringing hazard to aviation

Severe thunderstorms may cause significant disruption to airport operation and hazard to aircraft safety. The abrupt change of wind direction and speed caused significant windshear at the airport.

Severe thunderstorms may cause significant disruption to airport operation and hazard to aircraft safety.  On 16 April 2009, a band of intense rain and thunderstorms over inland Guangdong advanced southward and affected the coastal region in the small hours, necessitating the issuance of Thunderstorm Warning and Amber Rainstorm Warning by the Hong Kong Observatory.  The intense rain and thunderstorms affected the Hong Kong International Airport (HKIA) between 1:30 a.m. and 2:00 a.m.  During the passage of the rainband, winds at the HKIA strengthened rapidly from about 5 m/s from the south to 25 m/s from the north with gusts reaching about 38 m/s during the period.  The abrupt change of wind direction and speed caused significant windshear at the airport.  The Windshear and Turbulence Warning System (WTWS) indicated significant windshear along the runways.Significant windshear may cause an aircraft to deviate seriously from its intended flight path during landing or departure.  Thunderstorms and strong winds may also affect aerodrome facilities and the safety of airport staff working in open areas.  To ensure aviation safety, the Airport Meteorological Office (AMO) issues windshear warning for possible low-level windshear and turbulence within 3 nautical miles of the runways.  Aerodrome warnings of thunderstorms, strong surface wind and gusts will also be issued when situation warrants.Details of the aviation weather services provided by the Hong Kong Observatory can be found on the Observatorys webpage for "Aviation Weather Service". 

June 2011

https://www.hko.gov.hk/en/education/aviation-and-marine/aviation/00206-severe-thunderstorms-bringing-hazard-to-aviation.html

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Nocturnal Jet

Nocturnal jet is a fast-moving air current in the lower atmosphere during nighttime when the skies are clear. As air temperatures near the ground drop after sunset, an inversion layer is formed in the lower atmosphere during the night. Within the inversion layer, air temperature rises as height increases. This tends to enhance atmospheric stability and suppress atmospheric vertical motion.

Nocturnal jet is a fast-moving air current in the lower atmosphere during nighttime when the skies are clear.  As air temperatures near the ground drop after sunset, an inversion layer is formed in the lower atmosphere during the night.  Within the inversion layer, air temperature rises as height increases.  This tends to enhance atmospheric stability and suppress atmospheric vertical motion.  Air then tends to flow horizontally and becomes less inhibited by turbulence and convection which tend to dominate during daytime when the ground surface heats up as a result of insolation.  Many literatures have reported on the occurrences of nocturnal jets in Europe, the Great Plains in USA and Africa, etc.  In Hong Kong, weather conditions in autumn are often favourable for the formation of nocturnal jets, as illustrated by the example below. In late October 2010, a dry northeast monsoon brought fine weather to Hong Kong.  The Observatory's wind profiler at Cheung Chau captured the development and dissipation of nocturnal jets on 30-31 October (Figure 1).  Solar heating during daytime caused air temperatures near the ground to rise above 20oC, triggering turbulent mixing that tend to reduce the wind speeds in the lower atmosphere.  The vertical profile of winds on the afternoon of 30 October 2010 (blue line) showed that wind speeds below 1,800 m were less than 10 m/s and the vertical variations of wind speed were generally small.  After sunset, a jet with maximum wind speed exceeding 20 m/s appeared at an altitude between 1,600 m and 1,800 m (purple line).  After midnight, the jet core descended to an altitude between 800 m and 1,000 m (green line).  The formation of the jet also meant an increase in vertical wind shear, i.e. abrupt changes of wind speeds with height.  After daybreak the next morning, the jet weakened as wind speeds at the jet core dropped below 15 m/s (pink line).  In the afternoon, the vertical profile of wind speeds resumed a pattern similar to the day before (orange line).The Observatory's Tai Mo Shan wind station at a height of over 950 m above mean sea level also recorded the occurrence of nocturnal jet during the 2-day period (Figure 2).  The time series of wind speeds showed that wind speeds started to increase significantly by late afternoon on 30 October.  Winds exceeded 12 m/s and the jet was largely maintained up to around 8 a.m. the next morning.  Wind speeds then decreased sharply during the day before another rise appearing in the evening.Nocturnal jet and its associated vertical wind shear can pose a threat to aviation safety.  Camping on high grounds during the cool season may be affected by the occurrence of strong winds that can bring down tents or blow away loose items.

March 2012

https://www.hko.gov.hk/en/education/aviation-and-marine/aviation/00210-nocturnal-jet.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".  

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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What is "Clear-air Turbulence"?

High in the sky, the 'fasten seat belt' sign is on inside the plane. When you look out from the window, the nearest cloud is kilometres away. The weather is brilliant. You may wonder, what could you possibly run into at this height? Something out there, something called clear-air turbulence - an invisible trouble-maker for aircraft.

High in the sky, the 'fasten seat belt' sign is on inside the plane.  When you look out from the window, the nearest cloud is kilometres away.  The weather is brilliant.  You may wonder, what could you possibly run into at this height?  Something out there, something called clear-air turbulence - an invisible trouble-maker for aircraft. What is turbulence?Turbulence is caused by irregular motion of air.  It brings about rapid bumps or jolts to an aircraft.  In severe cases, the aircraft might go momentarily out of control.  Turbulence usually occurs in areas where air masses with different speed, direction or temperature meet each other. What causes turbulence? and clear-air turbulence?Turbulence is sometimes associated with thunderstorms and cold and warm frontswhere clouds and weather provide visible clues to the existence of turbulence.  However, turbulence can also occur in places where clouds are not present.  This kind of turbulence is called clear-air turbulence (CAT).  CAT usually occurs at relatively high altitudes of 20,000 feet (around 6 kilometres) or above.  It typically occurs near jet streams (i.e. narrow bands of strong winds) and other regions ofsignificant wind changes in the vertical direction.  It can also occur when strong winds blow across mountain ranges. How often is CAT encountered near Hong Kong? When does it occur most frequently in the year?During the past couple of years, on average CAT was reported in about 15 days per year in the vicinity of Hong Kong. Out of these 15 days, severe CAT was reported in one day. A majority of the CAT events were reported in the winter months of December, January and February. How does the Hong Kong Observatory (HKO) alert aircraft to CAT?Weather forecasters at the HKO's Airport Meteorological Office constantly monitor the weather conditions in the vicinity of Hong Kong.  They are on the lookout for likely signs of CAT on satellite imageries, on winds aloft obtained with balloons, and from results of computer modelling of the air.To alert the pilot to possible CAT along the way, the forecasters issue the following products: (a) significant weather information for aircraft in flight; and (b) significant weather charts for flight planning by airline operators and pilots. What should aircraft passengers do to avoid injury?A majority of turbulence-related injuries worldwide are related to passengers not having fastened their seat belts.Passengers should buckle up at all times.  Make sure hand baggage are safely stowed away.  When the aircraft encounters turbulence, stay calm, listen to the aircrew and follow their instructions.Remark: Here in this article, a turbulence report was classified as a Clear Air Turbulence when there was no thunderstorm nor layer cloud except cirrus within 1 degree latitude and longitude from the aircraft position.

https://www.hko.gov.hk/en/education/aviation-and-marine/aviation/00221-what-is-clearair-turbulence.html

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Safeguard flight operations - A rare occurrence of volcanic ash near Hong Kong

An introduction to aviation safety hazards induced by volcanic ash and a rare occurrence of volcanic ash near Hong Kong.

Threat to aviation by volcanic ash has become better known after two volcanic ash incidents reported by Boeing 747 airplanes flying over Indonesia and Alaska in 1982 and 1989 respectively. Fortunately, both airplanes recovered from power loss and landed safely despite the heavy damages in the jet engines.Volcanic eruption ejects a large amount of mixture of pulverized rocks, minerals, crystals and glass particles, collectively called volcanic ash, to the atmosphere. If an airplane flies into the volcanic ash clouds, the ash particles would melt and fuse on the turbine blades. This could interrupt the airflow, thereby reduce the engine performance. In severe cases, the build-up of molten ash particles could lead to engine flame-out. The pitot-static system, which determines the airspeed of an airplane, is also prone to malfunction as volcanic ash blocks the sensors. Pilots are therefore trained to avoid observed or predicted volcanic ash plumes.On 13 August 2021, the eruption of Fukutoku-Oka-no-Ba, an underwater volcano in the Ogasawara Islands of Japan, pushed a significant amount of steam and volcanic ash to the upper air of height exceeding 50,000 feet. The volcanic ash plumes were then driven westward by the easterly winds aloft. As Fukutoku-Oka-no-Ba is situated more than 2,700 kilometres from Hong Kong, the eruption did not pose any immediate risk to our vicinity. However, the ash plumes subsequently spread further downwind towards the northern part of South China Sea.The Observatory’s Airport Meteorological Office (AMO) actively monitored the dispersion of the volcanic ash plumes using satellite data as well as closely communicated with neighbouring Meteorological Watch Offices (MWOs). The plumes reached Hong Kong Flight Information Region (HKFIR) on 14 August, posing threats to flights within the area. Before the ash plumes reached HKFIR, AMO forecasters issued Significant Meteorological Information (SIGMET) messages detailing the expected location and altitude of ash clouds to warn flight operations according to the International Civil Aviation Organization (ICAO) regulations. This volcanic ash event within the HKFIR was the first in the past thirty years since the eruption of Mount Pinatubo in the Philippines back in 1991 which warranted the issuance of SIGMET message by HKO.

January 2022

https://www.hko.gov.hk/en/education/aviation-and-marine/aviation/00668-A-rare-occurrence-of-volcanic-ash-near-Hong-Kong.html

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Impact of sand and dust storms on aviation safety

Sand and dust weather caused by rolling up of sand and dust obscures visibility and affects takeoffs and landings, causing flight delay, cancellation and diversion to other airports. Besides, static charge built up by sand particles in the air may interfere with aircraft instruments and cause communication failure. Sucking sand and dust into the turbine engine will cause erosion of the engine and performance degradation which may lead to flight accident.

There are some famous deserts in western and northern China and Mongolia. In the spring time from March to May, when there are strong winds coupled with unstable atmosphere, for example the passage of strong cold fronts, sandstorms may occur due to dry weather and sparse vegetation on the ground. In some cases, the raised sand can reach several kilometers in height and can be transported by upper airstream to thousands of kilometers away. The process may last up to a week until the wind subsides. On 10 to 11 April 2020, sandstorm weather over Tarim Basin, Xinjiang, China is an example of sandstorm triggered by cold air advection activities.Sand and dust weather caused by rolling up of sand and dust obscures visibility and affects takeoffs and landings, causing flight delay, cancellation and diversion to other airports. Besides, static charge built up by sand particles in the air may interfere with aircraft instruments and cause communication failure. Sucking sand and dust into the turbine engine will cause erosion of the engine and performance degradation which may lead to flight accident.In order to ensure flight safety, all aviation meteorological offices monitor sand and dust weather. Sand and dust weather can be classified into four levels, namely ‘dust or sand’, ‘blowing dust or sand’, 'duststorm or sandstorm’, ‘heavy duststorm or sandstorm’. In case of heavy duststorms or sandstorms, aviation meteorological offices will issue Significant Weather Information (SIGMETs) and/or aerodrome warnings to ensure flight safety. The Hong Kong Observatory makes use of weather observations, satellite images and air trajectories to monitor sand and dust weather. Please refer to the “Sand and Dust Weather Information” webpage for details.

January 2021

https://www.hko.gov.hk/en/education/aviation-and-marine/aviation/00556-impact-of-sand-and-dust-storms-on-aviation-safety.html

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Assessing Airport Runway Visual Range to Support Aircraft Landing and Take-off

Hong Kong Observatory employed both transmissometer and forward scatter for assessing RVR. The real-time RVR information is relayed to air traffic controllers who would further relayed the RVR to pilots in flight.

During period of low visibility, pilots and air traffic controllers must have access to real-time information on runway visibility conditions to determine whether it is suitable for aircraft landing and take-off. Runway visual range (RVR) is one of the most critical information.International Civil Aviation Organization (ICAO) defines RVR as the range over which the pilot of an aircraft on the centre line of a runway can see the runway surface markings or the lights delineating the runway or identifying its centre line. In practice, RVR cannot be measured directly. It is assessed based on the runway visibility, background luminance and runway light intensity and is intended to represent what a pilot would see on the runway as far as possible.ICAO recommends that RVR be assessed using either transmissometers (Figure 1(A)) or forward scatter meters (Figure 2(A)) as visibility sensor equipment. The operating principle of these two types of equipment is given in Figure 1(B) and Figure 2(B).In Hong Kong International Airport, the Hong Kong Observatory employed both transmissometer and forward scatter for assessing RVR. They are located next to each runway that provide a measurement of the visibility at key points along a runway, including touchdown, midpoint, and runway end. The location should be as close to the runway as possible but the adverse impact to flight operation should be avoided. Since the runway lights are near ground level and the average eye level of pilot is about 5 m above the runway, the sensor for measuring visibility is therefore installed at a height of about 2.5m above level of runway centre line as far as practicable so that the assessment for RVR can be a good representative of a pilot’s viewing level. Apart from visibility sensor, the equipment also measures the background luminance and collects the runway light intensity information for assessing RVR. The real-time RVR information is relayed to air traffic controllers who would further relay the RVR to pilots in flight.

March 2024

https://www.hko.gov.hk/en/education/aviation-and-marine/aviation/00711-Assessing-airport-runway-visual-range-to-support-aircraft-landing-and-take-off.html

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The Impact of Low Visibility on Aviation

Low visibility in fog will have serious impact on air traffic, and may even lead to aviation accident.

In many horror movies, scenes of ghost ships sailing in fog often set up a haunting atmosphere to create a sense of fear among the audiences.  In real life, low visibility in fog will have serious impact on air traffic, and may even lead to aviation accident."Instrument Landing System" installed at modern airports can help flights landing safely under poor visibility condition.  However, in general in the final phase of the approach, pilots need visual reference from the runway to maneuver the airplane to touch down.  The runway visual range (RVR) is the range over which the pilot of an aircraft on the centre line of a runway can see the runway surface markings or the lights delineating the runway or identifying its centre line.  The minimum RVR requirement for landing depends, amongst others, on the airport facilities, aircraft equipment, pilots' training and airlines' policy.  RVR is deduced from the visibility measurement of a transmissometer or a forward scatterer along the runway.  The acceptable minima for landing operations under different categories of "Instrument Landing System" can be found in Table 1.  If the visibility deteriorates just before the plane touches down, that would pose a great challenge to the pilot.  Flights would hold in the air and wait for the visibility to improve.  If low visibility condition persists, a flight may have to divert to another airport due to fuel consideration.After landing, an airplane in fog would move slowly to avoid collision with other aircraft, vehicles or equipment on the apron.  As airport operation has to slow down, it may cause delays to passengers disembarking and retrieval of luggage.On 28 February 2011 morning, fog affected airports in the Pearl River Delta. The lowest visibility recorded at the Hong Kong International Airport was about 200 metres while the visibility at Macao and Shenzhen were as low as 100 metres. Some flights had to divert.  After sunrise, fog began to dissipate and airport operation gradually returned to normal.

June 2012

https://www.hko.gov.hk/en/education/weather/visibility/00083-effects-of-low-visibility-to-aviation.html

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Advantages of Short Range LIDAR in Windshear Alerting

The Observatory currently uses a suite of meteorological instruments, including the long-range Light Detection And Ranging (LIDAR) systems, for windshear alerting at the Hong Kong International Airport (HKIA). The instruments are useful in detecting windshear associated with terrain and thunderstorms. However, for windshear/turbulence which have even smaller spatial scales, such as those as

The Observatory currently uses a suite of meteorological instruments, including the long-range Light Detection And Ranging (LIDAR) systems, for windshear alerting at the Hong Kong International Airport (HKIA). The instruments are useful in detecting windshear associated with terrain and thunderstorms. However, for windshear/turbulence which have even smaller spatial scales, such as those associated with buildings/man-made structures, it would be beneficial to employ meteorological instruments with even higher resolutions. For this purpose, in 2009-2012, the Observatory has arranged the field study of a short-range LIDAR (SRL) on the rooftop of AsiaWorld-Expo to test the possibility of enhancing windshear alerting over the eastern arrival runway corridor of the north runway (i.e. 25RA runway corridor). Compared to the existing long-range LIDARs, SRL has a spatial resolution improved by about 29% (105 m of long-range LIDAR vs. 75 m of SRL) and temporal resolution improved by 83% (120 seconds of long-range LIDAR vs. 20 seconds of SRL).During the field study, the Observatory had tried different algorithms to alert windshear using the SRL. Based on the study results of the past 4 summers in 2009-2012, the use of SRL could improve the hit of windshear by about 11% based on pilot reports (hitting 214 reports out of a total of 240 reports with the use of SRL, compared to the hitting of 193 reports by the Observatory's Windshear and Turbulence Warning System (WTWS) alone). At the same time, the  total alert duration is increased only by a comparable amount of 13%. Moreover, SRL has demonstrated the capability of capturing some windshear features that could not be seen by the existing instruments of WTWS. An example of windshear event captured by SRL but missed by WTWS is shown in the figure. The Observatory is considering the cost-benefit of permanent deployment of a SRL at HKIA.

January 2013

https://www.hko.gov.hk/en/education/aviation-and-marine/aviation/00212-advantages-of-short-range-lidar-in-windshear-alerting.html

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The Impact of Global Warming on Aviation

In principle, for an aircraft to take off, the lift has to be greater than its total weight. An important factor determining the lift is air density. The higher is the air temperature, the lower is the air density. Hence, warmer air will produce less lift for an aircraft.

Summer in Hong Kong can be very hot. Due to the vast land surface with little vegetation, air temperature over the Hong Kong International Airport (HKIA) can easily reach 33 °C or higher in summer. Apart from bringing discomfort to outdoor airport staff and passengers who are required to embark/disembark aircrafts at remote stands, the high temperature also affects the loading capacity of departing aircrafts.In principle, for an aircraft to take off, the lift has to be greater than its total weight. An important factor determining the lift is air density. The higher is the air temperature, the lower is the air density. Hence, warmer air will produce less lift for an aircraft. This implies that when air temperature rises, an aircraft will require a longer distance to accelerate to sufficient speed for taking-off. Sometimes it may even have to off load some of its cargo to reduce its weight if needed. The World Meteorological Organization has ranked 2014 the hottest year on record as part of a continuing global warming trend. Hong Kong has just experienced record-breaking summer months in 2014. The average temperature between June and September in 2014 was the highest since records began in 1884. Statistics over HKIA also indicates that the number of very hot days (daily maximum temperature ≥33.0°C ) at the airport is showing an increasing trend - from an annual average of around 60 days between 2000 and 2007 to around 70 between 2008 and 2014 (Figure 1). As the number of very hot days is forecast to increase further under global warming, there will be a higher chance for cargo loading being affected in the future.Furthermore, the latest, Fifth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC AR5) predicts that global mean sea level would rise under global warming. Research conducted by the Hong Kong Observatory suggested that under the high greenhouse gas concentration scenario, the annual mean sea level in Hong Kong and its adjacent waters would rise by 0.63 to 1.07 m by the end of this century (2081-2100), when compared to the average sea level in 1986-2005 (HKO, 2015). Moreover, in the context of global warming, climate models mostly projected that the intensity of tropical cyclones in the western North Pacific basin would increase in the 21st century (Ying et al., 2012). The height of the runways of HKIA is about 6.7 m (22 feet) above the mean sea level. Under global warming, there may be an increased risk of the runway being flooded by higher storm surges brought by intense tropical cyclones with time.

July 2015

https://www.hko.gov.hk/en/education/aviation-and-marine/aviation/00462-the-impact-of-global-warming-on-aviation.html

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Why do Wing Shapes Vary Across Different Types of Aircraft?

When an aircraft generates lift and flies, especially around the wingtips, small vortices are formed. These vortices are actually thieves to steal the aircraft’s drive. Different shapes of the aircraft wings harvest the pressure difference in Bernoulli's principle to create lift in helping the aircraft to take off.

How lift is generated on a wing?Figure 1 illustrates the airflow around a typical aircraft wing as it is dashing on a runway, with the blue lines representing the paths of air particles as they whoosh over and under the wing during flight. This clever design of the aircraft wing lies in the curvature on the top surface, which narrows the cross-section of the wing’s upper part (line segment CD) compared to the cross-sections at the inlet and outlet of the wing (line segments AB and EF). The air passing over the top surface of the wing, due to its narrower cross-section, must flow faster than the air passing through the wider sections at inlet and outlet to maintain mass conversation. This is similar to the principle of squeezing a water hose, where water sprays out faster when the hose is constricted. The fast-moving air over the top surface of the wing creates a region of lower pressure. According to the Bernoulli's principle, the pressure difference creates lift, helping the aircraft to take off.Tapered versus rectangular wingsWhen an aircraft generates lift and flies, especially around the wingtips, small vortices are formed (see The “Tail” of an Aircraft). These vortices are actually thieves to steal the aircraft’s drive. Therefore, wing designs with gradually narrowing tapered tips (Figure 2) can reduce the wing’s surface area, resulting in fewer vortices being formed. Overall, the drag due to vortices will be reduced, making the flight more efficient. However, the production costs for tampered wings can be more expensive, which is one of the reasons why training aircraft still use the traditional rectangular wings (Figure 3).Swept-back wingsThe airflow passing over the aircraft wing, due to its specific design, can achieve a speed greater than the aircraft’s own speed. As a result, before the aircraft reaches the speed of sound, the airflow surpasses the speed of sound and generates shock waves. The immense drag caused by these shock waves decelerates the aircraft.Swept-back wings deflect part of the airflow over the wings to delay the formation of shock waves. From Figure 4, it can be observed that the airflow (arrow A) impacting the wing is separated into two components. Part of it crosses the wing (arrow B), while the other part flows sideways along the wing span (arrow C). This separation effectively reduces the speed of the airflow crossing the wing, delaying the formation of shock waves. Furthermore, swept-back wings help maintain the aircraft's straight flight.Let's discuss other considerations in designing aircraft wings later.

March 2024

https://www.hko.gov.hk/en/education/aviation-and-marine/aviation/00708-Why-do-wing-shapes-vary-across-different-types-of-aircraft.html

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The "Tail" of an Aircraft

Wake flow from an aircraft mainly comprises a pair of counter-rotating vortices (Figure 2). Since these vortices may contain severe turbulence and subsidence, a trailing aircraft along the same flight path must maintain a suitable distance (or "aircraft separation") from the preceding aircraft in order to prevent encountering its wake flow, which might affect flight safety.

It has been mentioned in an earlier article (“In the wake of a duck”, June 2013) that, whenever a duck swims in the water (or any object propels itself using the reaction force from a fluid) it would naturally leave behind a series of wave patterns (Figure 1).  The same also applies to an aircraft in flight.When an aircraft is flying, the air pressure below its wings is higher than that above (thus providing lift). This pressure difference leads to the generation of vortices. The superposition of these vortices in turn generates a wake pattern, usually known as "aircraft wake turbulence" or "aircraft wake vortices" in the aviation community.Wake flow from an aircraft mainly comprises a pair of counter-rotating vortices (Figure 2). Since these vortices may contain severe turbulence and subsidence, a trailing aircraft along the same flight path must maintain a suitable distance (or "aircraft separation") from the preceding aircraft in order to prevent encountering its wake flow, which might affect flight safety. It is worth mentioning that aircraft wake flow is generally invisible to the naked eye under clear air conditions, in contrast to the phenomenon of "contrail" (the latter is mainly composed of ice crystals resulting from the condensation of water vapour within aircraft engine exhaust, hence more easily observable).Vortices within aircraft wake flow are of relatively small size1, and often have a lifetime of only 1 to 2 minutes (or less) in the near-surface atmosphere. To observe these invisible airflow, remote-sensing instruments with high spatial and temporal resolution, such as the short-range LIDAR (light detection and ranging), would be required (Figure 3).Within the display of wind velocities as measured by the short-range LIDAR, the counter-rotating vortex pair stands out from the background wind field with its unique signature. If you take a look at the planar wind speed distribution in Figure 4, doesn't it resemble two long "tails" left behind by the landing aircraft?Footnote: [1] The width of wake vortices is generally comparable to an aircraft's wingspan. For example, the vortices left behind by a Boeing 747-400 (with wingspan of about 65 metres) would have a diameter of about 30 to 40 metres on each side when close to the aircraft.

July 2015

https://www.hko.gov.hk/en/education/aviation-and-marine/aviation/00463-the-tail-of-an-aircraft.html

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Observation of aircraft wake vortices at Hong Kong International Airport

Detection and research of wake turbulence.

The article "The ‘Tail’ of an Aircraft" in the Educational Resources of the Hong Kong Observatory introduces the principles and observation of aircraft wake vortices. As an aircraft flies, airflow rolls up behind and at the tips of the wings to form a pair of counter-rotating vortices, i.e. aircraft wake vortices (Figure 1). Aircraft wake vortices are generally as wide as the wingspan of the aircraft and they tend to descend and gradually dissipate in the atmosphere, lasting for about one to three minutes. Under light wind and stable atmospheric conditions, the vortices may last longer.Aircraft wake vortices cause disturbances to the atmosphere, commonly known as wake turbulence. When an aircraft encounters the wake turbulence generated by another aircraft, it may experience rapid bumps or jolts; and in severe cases, it might even go momentarily out of control. It would be even more dangerous if the aircraft is taking off or landing at the time of the encounter, as there might not be sufficient altitude or time for the pilot to react. Aircraft must therefore comply with the minimum separation requirements, i.e. keeping distance from other aircraft in flight, to minimise the chance of wake turbulence encounter and its impact on flight safety. On the other hand, such minimum separation requirements also limit the landing and take-off frequencies as well as capacity of runways. Hence, the detection and research of wake turbulence can facilitate evaluation of the minimum separation requirements to ensure flight safety and more efficient runway operations.As early as in 2014, during a field campaign, the Observatory conducted a trial of using short-range LIDAR (“SRL”) to scan vertically across a departure/approach corridor of the Hong Kong International Airport (Figure 2) and successfully detected the two-dimensional wind structure of aircraft wake vortices (see Reference for details). For further detection and research of wake turbulence, the Observatory installed a unit of SRL at each end of the airport’s new North Runway in mid-2022. Following initial tuning of the scanning configuration of the SRLs, aircraft wake vortices, which cause wake turbulence, can be observed and tracked from the radial wind velocity measured during vertical scans performed in high temporal and spatial resolutions. Figures 3 to 5 show the vertical sections of radial wind velocity measured by an SRL at the new North Runway on a certain day before and after a landing aircraft passed through. In the figures, warm colours such as yellow and red (and cool colours such as green and blue) represent winds blowing away from (and towards) the SRL. Figure 3 presents the situation just before the passage of the aircraft. One can tell from its greyish and yellowish colours that winds were blowing away from the SRL. After the aircraft passed through, quadruple colour patches of alternating and opposite radial wind directions appeared in the scan image (Figure 4), depicting a pair of counter-rotating vortices in the air (i.e. aircraft wake vortices). The evolution of the aircraft wake vortices can be observed from subsequent scan images (Figure 5): the vortices descended to the ground, at the same time drifted away from the SRL with the background winds and gradually dissipated.Compared with the field campaign in 2014, the radial resolution of the SRL at that time was 30 metres and the vertical resolution was about 7 metres; whereas the SRLs installed at the new North Runway can achieve a radial resolution of 1.5 metres with new signal processing techniques. The radial and vertical resolutions adopted in the presented case (Figures 3 to 5) are 1.5 metres and about 2.7 metres respectively. Due to the higher radial and vertical resolutions, the new SRLs can visualise the features of aircraft wake vortices more clearly, capture smaller vortices and provide finer data, all of which will benefit the detection and research of wake turbulence by the Observatory.

January 2023

https://www.hko.gov.hk/en/education/aviation-and-marine/aviation/00691-observation-of-aircraft-wake-vortices-at-hong-kong-international-airport.html

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Magnetism of Airport Runways

At the Hong Kong International Airport (HKIA), the runway number corresponds to the direction of the runway in degrees from magnetic north. For example, 07L refers to the runway on the left side when an aircraft is heading in the direction of about 70 degrees from magnetic north, while 07R is the runway on the right side.

The naming convention of Airport Runways In general, the name of a runway is a number based on the points on a compass, from 01 to 36, reflecting the magnetic compass reading to the nearest 10 degrees and dropping the last digit. Large airports have additional designators for multiple runways, such as L or R respectively for the left or right runway in a parallel pair. At the Hong Kong International Airport (HKIA), the runway number corresponds to the direction of the runway in degrees from magnetic north. For example, 07L refers to the runway on the left side when an aircraft is heading in the direction of about 70 degrees from magnetic north, while 07R is the runway on the right side. Compass is the most convenient way of measuring directions. For convenience of application, aeronautical charts and instruments are based on magnetic bearings and runways are normally designated by their magnetic headings. What is Geographic North and Magnetic North? The Earth rotates around an axis which points to the geographic North Pole and South Pole (aka "True North" and "True South"). True North is located in the middle of the Arctic Ocean whereas the South Pole lies in the Antarctica which can be considered stationary in general applications. Magnetic north refers to the North Magnetic Pole. The Earth's magnetic poles exist because of its magnetic field. The magnetic field is produced by electric currents in the liquid part of its core which is constantly moving. Complex fluid motion in the outer core of the Earth (the molten metallic region that lies from about 2,800 to 5,000 km below the Earth's surface) causes the magnetic field to change slowly with time. In general, a compass needle aligns itself with the Earth's magnetic field and is pointing to the Magnetic North Pole. The horizontal angular difference between true north and magnetic north is called MAGNETIC VARIATION or DECLINATION. It will change with time and geographic location. Problems brought about by using the magnetic north convention Magnetic anomaly caused by nature As Earth's magnetic field lines are slowly drifting on the surface, the magnetic direction is continuously changing. This magnetic North Pole could shift by as much as 65 kilometres a year, and is steadily moving from somewhere over Canada towards Russia. When pre-planning a flight course, small aircraft pilots would be using true north on an aviation sectional map to plot their route. True north bearings would be converted to magnetic north for in-plane navigation using compass. These bearings are then adjusted on a pre-flight plan by adding or subtracting local variations of magnetism. Effect of man-made structure in the surroundings According to a case reviewed by the U.K. Air Accidents Investigation Branch (AAIB) on 31 October 2006, a Raytheon Hawker 800XP departing from London City Airport (LCY) for Brussels observed a significant heading difference between the two primary flight displays (PFD 1 and PFD 2), also between the PFD 1 and the standby instrument. The problem persisted with unreliable readings even after switching to the AHRS (attitude and heading reference system) according to their emergency procedure. Eventually, the pilots requested for air traffic control assistance for their return to LCY. AAIB looked into the history of LCY. The airport once was a shipping dock in 1987, with railway lines running around the warehouses area. There were also large cast iron bollards along the dock walls for tying up ships. It was found that the railway lines were not totally removed when building the airport, where only sections of the bollards above the dock wall were removed. Furthermore, a steel-encased concrete pile which was some out-of-service oil pipeline was found underneath an aircraft holding area. The AAIB inspector observed a needle deviation as much as +/- 60 degrees on a hand-held magnetic compass by walking around this Runway 28 holding area. The investigation concluded that the compass large deviations were caused by several ferrous magnetic signature anomalies, mainly from the piled-beam structures (bollards) underneath, steel-reinforced concrete and the railway lines below the holding area. In view of the problem above, why we still use the magnetic north naming convention for airport runways? We will discuss this in the next article of airport geomagnetism, stay tuned.

December 2019

https://www.hko.gov.hk/en/education/aviation-and-marine/aviation/00535-magnetism-of-airport-runways.html

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Magnetism of Airport Runways (Sequel)

Why are we still using the Magnetic North naming convention for Airport runways? What are Magnetic Variation Effects over Hong Kong?

Why are we still using the Magnetic North naming convention for Airport runways? In the previous article, we have talked about the problems that may arise in using the Magnetic North convention, namely "Magnetic anomaly caused by the nature" and "Effect of man-made structures in the surroundings". In this article, let us take a closer look to the consequences arisen and the reasons for still using it. 1. Consequences of magnetic variations Change of Airport Runway name Since runway names indicate the headings in degrees magnetic, if the latter changes too much, the runway name may disagree with the prevailing magnetic heading. Some airports in the far north do not use magnetic headings for their runways. At Resolute Bay Airport (IAEA designated code "YRB") in Canada, the runway name is 17T/35T using True North bearings convention, not magnetic ones. The variation was once recorded as much as 30-degrees west over that area. Make over, signs, manual, documentations If the name of a runway is changed, it is required to repaint those runway numbers at the ends of each runway and replace all related signage. All reference manuals, documentation and approach plates have to be updated for pilots and air traffic controllers to study and use. Maps Readings Since the poles are moving constantly, printed maps require reprinting, digital maps used on smartphones and mapping software used by military and government agencies have to be updated. For both printed and digital mapping products updates, this is where the World Magnetic Model (WMM) comes into play. WMM is the standard model jointly developed by mapping agencies of the United States and the United Kingdom. It is used by governments and organisations for navigation, attitude and heading referencing systems using the geomagnetic field. It is also widely used in the civilian navigation areas. The latest version is WMM 2020. It predicts the pole movements and the magnetic field changes over that period. 2. Why still use the Magnetic North convention? Magnetic declination plays an important role in air navigation. Most simple aircraft navigation instruments are still using heading information of the magnetic North via a compass based magnetic device. This ensures that aircraft equipped with even only a basic magnetic compass can still navigate. Also, Very High Frequency (VHF) Omni-Directional Range (VOR), a type of short-range radio navigation system for aircraft using radio beacons to transmit signals for use in navigation, broadcast their "radial" information in magnetic degrees so that one can magnetic heading to fly to or from a VOR. Although GPS system is commonly used nowadays, compass device is still used for redundancy. On the contrary, if True North were used for the whole navigation scheme, the fall-back procedure for reverting to the compass and VOR in case of GPS failure would be very complex. For long over-water flights, night flights or flights in unmapped regions are examples that flying by compass are still necessary. 3. The Magnetic Variation Effects over Hong Kong The Magnetic variation effects over Hong Kong are small. HKO would commission a contractor normally every 5 years to conduct magnetic measurement at HKIA to fulfil requirements of the International Civil Aviation Organization (ICAO). The last survey at HKIA conducted in 2015 revealed that the Magnetic Declination measured at HKIA's Compass Calibrated Pad (CCP) was 2°36'51.4" west of True North. The next survey will be conducted in 2020.

June 2020

https://www.hko.gov.hk/en/education/aviation-and-marine/aviation/00543-Magnetism-of-Airport-Runways-Sequel.html

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Storm Surge and Sea Waves

Winds blow over sea surface and generate waves that propagate along the wind direction. Storm surge is a rise in sea level on top of the normal tide due to the combined effects of low atmospheric pressure and high winds associated with tropical cyclones.

Winds blow over sea surface and generate waves that propagate along the wind direction. Waves grow with the strength of the winds. Huge waves generated by typhoons can rise above 10 metres in the open sea. Storm surge is a rise in sea level on top of the normal tide due to the combined effects of low atmospheric pressure and high winds associated with tropical cyclones. Primarily, the water is pushed towards the shore and piled up against the coast by the force of high winds associated with tropical cyclones to generate a storm surge (Figure 1).Super Typhoon Mangkhut in 2018 brought huge waves to Hong Kong apart from severe storm surge (Figure 2). These waves approaching shores might even overtop seawalls to become overtopping waves. Overtopping is a sophisticated process which occurs intermittently at times of attack of individual high waves. Severe inundation could be resulted along the coastal areas due to overtopping waves in combination with storm surge under the effects of typhoons (Figure 3). How high the waves will get to and hence the degree of inundation depends on a number of non-weather related factors, such as shape of coastline, orientation, depth of seabed and structure of seawall, etc. Therefore, the wave heights experienced at different locations in Hong Kong can vary significantly as in the case of Mangkhut.Currently, there are 12 tide gauge stations in Hong Kong operated by the Hong Kong Observatory, Marine Department, Drainage Services Department and the Airport Authority Hong Kong to monitor real-time tidal variation at various locations in Hong Kong. The Civil Engineering and Development Department (CEDD) has also been operating two bed-mounted wave recorders near Kau Yi Chau and West Lamma Channel for long-term wave monitoring (Figure 4).During the passage of Mangkhut in 2018, the sea level in Quarry Bay rose to a maximum of 3.88 metres above the Chart Datum and a peak storm surge of 2.35 metres, while the maximum wave height recorded at Kau Yi Chau and West Lamma Channel was about 6.8 metres.

May 2019

https://www.hko.gov.hk/en/education/aviation-and-marine/marine/00524-storm-surge-and-sea-waves.html

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On sea level rise and abnormal sea level in Hong Kong

The impact of global warming includes a rise in temperature, melting of ice and snow, rise in mean sea level, etc. The rise in mean sea level is caused by the thermal expansion of sea water as well as the land-based ice and snow melting and flowing into the oceans.

The impact of global warming includes a rise in temperature, melting of ice and snow, rise in mean sea level, etc. The rise in mean sea level is caused by the thermal expansion of sea water as well as the land-based ice and snow melting and flowing into the oceans. Globally, the rate of sea level rise since the mid-19th century has noticeably accelerated. In Hong Kong, based on the records of the tide gauges installed at North Point/Quarry Bay, the sea level in the Victoria Harbour has been rising at a rate of about 3 cm per decade from 1954 to 2017 (Figure 1). The rate of sea level rise is similar in other regions in Hong Kong, such as Tai Po Kau in the Tolo Harbour.Based on the Fifth Assessment Report of the United Nations Intergovernmental Panel on Climate Change, the Observatory has made an assessment of the potential impacts to Hong Kong in terms of rising temperature and sea level in the 21st century. Under the high greenhouse gas emission scenario, the sea level in the vicinity of Hong Kong is expected to rise by 0.63 to 1.07 m by the end of this century (2081-2100) when compared to the average sea level in 1986-2005[1].With the elevated sea level, the risk of typhoon-generated storm surges affecting Hong Kong will be enhanced. Storm surge is the rise of sea level due to the combined effects of low pressure and high winds associated with a tropical cyclone. When storm surge occurs during astronomical high tides (near New Moon or Full Moon), the resultant sea level can be quite extreme, leading to flooding in low-lying areas. Since the beginning of instrumental records in 1954, the maximum sea level in the Victoria Harbour associated with storm surges was 3.96 m above Chart Datum (mCD) during the passage of Super Typhoon Wanda in 1962. In comparison, the sea level in the Victoria Harbour rose to 3.57 mCD during the passage of Super Typhoon Hato in 2017, second only to the record set by Wanda (Table 1).Apart from storm surges induced by tropical cyclones, there exists one other situation which also gives rise to abnormal sea level in Hong Kong. Such events normally occur during autumn or winter around the days of astronomical high tides when the winter monsoon brings strong winds to the south China coastal waters, resulting in appreciable sea level rise. One recent example occurred in Tai Po Kau on the night of 5 November 2017 when the sea level rose to a maximum of 2.99 mCD (Figure 2), about half a metre higher than the normal astronomical tide and causing minor flooding along the banks of Shing Mun River in Sha Tin[2]. One point worthy of note is that when the passage of a tropical cyclone coincides with the winter monsoon during astronomical high tides, the storm surge impact could still be quite significant even though the tropical cyclone may be skirting past at a relatively large distance. Typical cases are Bess, Nalgae and Elaine as shown in Table 1, all in the month of October when tropical cyclones still roam the South China Sea and the winter monsoon is also becoming increasingly active.

April 2018

https://www.hko.gov.hk/en/education/aviation-and-marine/marine/00505-on-sea-level-rise-and-abnormal-sea-level-in-hong-kong.html

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Why are waves always parallel to the shore on approaching the seashore?

We know that waves are mainly driven by the wind. However, the wind does not always blow straight in towards the shore. Out in the ocean, it may be blowing from every direction. The waves we see at the shore are those that are travelling more or less in our direction. Otherwise, we would never see them. So, the waves that we see do not normally come straight in, i.e. they approach at an angle to the shoreline. The question then is: how does a wave know when it is approaching the shore, change its direction, and become parallel to the shore?

On approaching the seashore, why are waves always parallel to the shore (no matter what the orientation of the shoreline is)?We know that waves are mainly driven by the wind. However, the wind does not always blow straight in towards the shore. Out in the ocean, it may be blowing from every direction. The waves we see at the shore are those that are travelling more or less in our direction. Otherwise, we would never see them.So, the waves that we see do not normally come straight in, i.e. they approach at an angle to the shoreline. The question then is: how does a wave know when it is approaching the shore, change its direction, and become parallel to the shore?Let's consider a wave that is coming at an angle, with the shoreline to its left. The part of the wave to hit shallow water and scrape bottom will be its left side. This side will be slowed down because of friction, while the middle and right side will continue marching at the original speed. This results in the wave turning to the left, i.e. towards the shore. When the middle and right side hit shallow water, they too will slow down because of friction. Thus, the whole wave gradually turns to the left - until it becomes parallel to the shore.On approaching the shore, waves break because of the same friction effect. When a wave nears the shore, its bottom is slowed down so much that its top overruns it and falls with a crash, churning up a line of foam.

https://www.hko.gov.hk/en/education/earth-science/physics-in-daily-life/00232-why-are-waves-always-parallel-to-the-shore-on-approaching-the-seashore.html

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What is a storm surge?

Storm surge is a raised sea brought by tropical cyclones. The winds of a tropical cyclone are the main culprit in water level rise, while its low pressure contributes to a lesser extent. The geography of Hong Kong suggests that it is vulnerable to storm surges when winds come from the east or south. This could be visualized if we bear in mind that a tropical cyclone's winds blow in a counter-clockwise direction.

Last month, as Severe Typhoon Megi approached Hong Kong, there was fear about the storm surge it might bring. Simply put, a storm surge is a raised sea brought by tropical cyclones. The winds of a tropical cyclone (Figure 1) are the main culprit in water level rise, while its low pressure (Figure 2) contributes to a lesser extent. Over the past hundred years or so, there have been two big killers, one in 1906 and the other in 1937, causing a heavy toll of 15,000 and 11,000 respectively.The height of water brought by a storm surge depends on the water depth (i.e. bathymetry) and the shape of the coastline. It could be made worse if the arrival of the surge comes with the astronomical high tide. The geography of Hong Kong suggests that it is vulnerable to storm surges when winds come from the east or south. This could be visualized if we bear in mind that a tropical cyclone's winds blow in a counter-clockwise direction. Have a look at Figure 3, which shows the tracks of the past 20 tropical cyclones that brought the most significant storm surges to Hong Kong, based on available data. The record holder was Typhoon Hope in 1979, which raised the water by 3.2 metres at Tai Po Kau in eastern New Territories. Overall, the tracks indicated that either the storms hit Hong Kong directly or almost, or they swept past Hong Kong from the west or south. In comparison, Megi's track (Figure 4) was quite different. As it maintained a distance of 400 kilometres from Hong Kong during its passage, the water rose only 0.7 metre locally (Figure 5), of which 0.5 metre was associated with the northeast monsoon prevailing at the time. Thus Megi's storm surge was just 0.2 metre.As a matter of fact, the northeast monsoon had caused minor flooding in the past. The low-lying areas at Sheung Wan in western Hong Kong were once vulnerable spots when in winter time, the strong northeast monsoon combined with a high tide pushed the water up. The problem has largely disappeared after improvement in the infrastructure. Because of the havoc wreaked by storm surges, the Observatory made calculations and projections in the 1970s to mitigate the possible damage to new town developments. For this reason, places like Shatin in northeast New Territories had been reclaimed an additional 3 metres. In weather forecasting, the Observatory runs a storm surge model every time a tropical cyclone approaches the south China coast and alerts the public to the danger of storm surge if any.

November 2010

https://www.hko.gov.hk/en/education/aviation-and-marine/marine/00168-what-is-a-storm-surge.html

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How to measure tide level and storm surge

A number of instruments can be used to measure tide accurately. In the past, the Observatory used a float which could stay on the sea surface. With ever-improving technologies, the method of measurements continues to advance. Some modern instruments make use of the reflection of sound wave to measure the distance of the sea surface from the instrument to derive the height of the sea surface.

Tide is like a beautiful symphony. The ups and downs in tide following the change in gravitational force of the Sun and the Moon form the main melody. However, meteorological factors may cause changes in the rhythm. For example in the case of rapid flooding due to storm surge, it is just like a sudden booming sound of a drum which is shocking.A number of instruments can be used to measure tide accurately. In the past, the Observatory used a float which could stay on the sea surface, such as a hollow metal ball being placed inside the well of a tide gauge station. The metal ball will rise or fall with changes in sea level, driving the mechanical components connecting to the ball. The sea level can be determined based on the movement of the metal ball.With ever-improving technologies, the method of measurements continues to advance. Some modern instruments make use of the reflection of sound wave to measure the distance of the sea surface from the instrument to derive the height of the sea surface. Some others make use of a pressure transducer underwater to obtain the difference between water pressure and atmospheric pressure to derive the sea level.In addition to the storm surge, tropical cyclones may also bring overtopping waves. The height of the splashes caused by huge waves hitting the embankment has a lot to do with the shape of the coastline, orientation, depth of the seabed, and the structure of the embankment. To reduce the effect of waves to the tidal measurement, the fluctuation due to waves will be filtered out as much as possible. The tide stations currently managed by the Observatory include Quarry Bay, Tai Po Kau, Tsim Bei Tsui, Shek Pik, Tai Miu Wan and Waglan Island. These together with other tide gauge stations managed by other government departments and organisations are made available in real time under "Tidal Information" webpage on the Observatory's website. As of end 2018, data from 12 tide gauge stations have been made available. When Severe Typhoon Mangkhut affected Hong Kong in 2018, some of the tide gauge stations measured record-breaking maximum tide level. Apart from tide gauge station in Waglan Island, which was destroyed by high winds and huge waves at that time, the remaining 5 tide gauge stations managed by the Observatory registered record-breaking storm surges. As a result of global warming, the threat of severe storm surge brought by tropical cyclones will increase. We must stay vigilant and be fully prepared to face the threat of storm surge.

January 2019

https://www.hko.gov.hk/en/education/aviation-and-marine/marine/00523-how-to-measure-tide-level-and-storm-surge.html

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Considerations when selecting a site for tide gauge station

Apart from having a calibrated instrument, a good site is also important for accurate measurement of tide level. Which of the following you would consider as a potential site of tide gauge station?

Continuous and accurate monitoring of tide level is essential for predicting the astronomical tide level [1], monitoring storm surge [2] and tsunami. It is also irreplaceable when studying long-term sea level change. Moreover, tide data are used by mariners, coastal engineers, ferry operators, anglers and the general public for planning their activities. Last time, we introduced how to measure tide level [3]. Apart from having a calibrated instrument, a good site is also important for accurate measurement of tide level. Which of the following you would consider as a potential site of tide gauge station? A. Bay with complicated terrain B. Area subjected to erosion or with soft soil C. River estuary or drainage outlet D. Area with frequent marine activity or easy to accumulate marine debris A. Bay with complicated terrain Theoretically the water surface level is the same at any location in a tank of still water. However, the fact is that sea is not stationary. Tidal current and meteorological conditions [4] such as wind and atmospheric pressure make the sea level changing over time and with locations. The temporal and spatial variations will be more significant if the terrain is complicated. On the contrary, a site with good exposure to sea allows the measured tide and the predicted astronomical tide to be a good representative measurement of the area and its vicinity. For example, the tide gauge stations in Quarry Bay and Tai Po Kau can well represent the conditions in Victoria Harbour and Tolo Harbour respectively.B. Area subjected to erosion or with soft soil If the tide gauge station is built in an area subjected to erosion or above soft soil, the instrument may subside or tilt slowly, which will affect the accuracy of the tide level measurement. Impact due to minor and gradual settlement can be corrected by monitoring the settlement of the station regularly. If there is a geodetic reference station nearby, the reference level of the tide gauge station could be calibrated by performing land survey regularly. Nevertheless, setting up a station on solid rock is highly preferred.C. River estuary or drainage outlet In the event of heavy rain, rainwater will flow downstream along rivers or via surface runoff to the catchment. The water level near the catchment may fluctuate rapidly. Similarly, outflow of drains will pose similar undesirable impact. Moreover, the difference between rainwater and sea water density can possibly affect the accuracy of the measurement of certain tide gauge instruments.D. Area with frequent marine activity or easy to accumulate marine debris Vessels may induce large sea level oscillations. For instrument making use of the reflection of sound wave or radio wave to measure tide level, any boats or marine debris beneath the instrument will result in incorrect data. Besides, propeller turbulence and the marine debris can damage the instrument. The frequency of repairs and maintenance of the instrument will then increase accordingly.So the correct answer is none of the above. Apart from those factors mentioned above, factors like availability of electricity and data communication facilities, protection against damage by sea wave and extreme weather, adequate means of access for installation and maintenance, etc. also need to be considered. Thus it is not easy to find a good site for tide measurement. In reality, due to a number of site constraints, it is very often not possible to find a perfect site. Hence, it is very important to preserve a good and representative site.

January 2021

https://www.hko.gov.hk/en/education/aviation-and-marine/marine/00557-considerations-when-selecting-a-site-for-tide-gauge-station.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.

December 2009

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

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Response to Strike of Tsunami

Tsunami risk faced by Hong Kong mainly comes from the South China Sea. In particular, major earthquakes can occur in the Manila Trench to the west of Luzon. The severe tsunami waves thus triggered can reach Hong Kong in about 3 hours.

Hong Kong is an international financial centre. Many people would only think of the stock or property market when talking about earthquakes and tsunamis. It seems that a genuine tsunami will only occur at distant places or in disaster movies. However, will you not be affected by earthquakes and tsunamis at all? How would you protect yourself at the strike of tsunami if you are travelling in a place with a high risk of earthquake and tsunami?Most earthquakes in the world occur on the tectonic plate boundaries where energy released from time to time by friction and movement between plates will propagate out in the form of seismic waves. Not only will a strong submarine earthquake cause ground shaking, it may also trigger tsunami. When a major tsunami hits the coastal areas, sea level will rise rapidly, followed by huge waves like a water wall. It should be noted that the sea may retreat quickly as a precursor before the arrival of the tsunami. The strike of violent tsunami waves will hit and even destroy the coastal infrastructures. Floating debris like wreckage of ships and vehicles will also be damaging, causing great harm to life and property.Located more than 600 kilometres away from the circum-Pacific seismic belt, Hong Kong is not in an active seismic zone (Figure 1). Also, Taiwan and the Philippines serve as natural barriers, so that tsunami waves generated by strong earthquakes that occur in the Pacific would only cause limited sea level rise in Hong Kong. Sea level rises brought by tsunamis as recorded by the Hong Kong Observatory have never exceeded 0.3 meters since 1952. That said, tsunami risk faced by Hong Kong mainly comes from the South China Sea. In particular, major earthquakes can occur in the Manila Trench to the west of Luzon. The severe tsunami waves thus triggered can reach Hong Kong in about 3 hours. Fortunately, in the past 100 years or so, the strongest earthquake that occurred in the Manila Trench was one with a magnitude of 7.6 in 1934. Hong Kong also felt the tremor at that time but did not receive any tsunami reports. In fact, there is no record of a destructive tsunami ever occurred in Hong Kong, and the felt tremor events recorded since 1905 have not caused any casualties.An effective earthquake monitoring system will earn us time to deal with approaching tsunamis. When a tsunami occurs, please pay attention to the tsunami warnings issued by local authorities. Leave the shore, beach, or low-lying coastal areas as soon as possible and evacuate to inland areas for at least two kilometres or go to high ground for temporary shelter. If there is not enough time to evacuate or traffic conditions do not permit, you can get up to the upper floors of a reinforced concrete multi-storey building for temporary shelter until the tsunami warning is cancelled.Established by China, the “South China Sea Tsunami Advisory Centre” of the Intergovernmental Oceanic Commission of the United Nations Educational, Scientific and Cultural Organization (IOC-UNESCO) has been operational since November 2019. The Centre provides round-the-clock tsunami monitoring and advisory services for countries and territories around the South China Sea. The Observatory also operates a local seismograph network to monitor seismic activities in Hong Kong and neighboring areas. If Hong Kong is expected to be affected by a significant tsunami, i.e. water level rising by 0.5 metres or more above the normal tide, and the tsunami is forecast to reach Hong Kong within three hours, the Observatory will issue a Tsunami Warning to remind the public to take preventive measures. Please refer to “Tsunami Monitoring and Warning in Hong Kong” for details.In conclusion, Hong Kong’s geographical location is far from active seismic zones and abnormal rise in water levels caused by tsunamis in the past have been limited. The risk of Hong Kong being hit by a major earthquake or tsunami is not high. That said, the footprints of Hong Kong people cover the whole world, including areas of high earthquake and tsunami risks such as Japan and Indonesia. Everyone should get prepared for danger in times of peace and know how to deal with disasters. In fact, major tsunamis did occur in the past two decades, including the Japanese tsunami in 2011 and the South Asian Tsunami in 2004 (Figures 2 and 3). If you wish to teach children how to deal with disasters associated with earthquakes and tsunamis, you may make reference to the "Earthquake!" and "Get Up to High Ground" of the COPE Disaster Book Series, a set of illustrated books for children published by the United Nations Office for Disaster Risk Reduction. The Chinese version can be viewed on the website of the Hong Kong Jockey Club Disaster Preparedness and Response Institute. Figure 2  On 26 December 2004, a major earthquake near Sumatra, Indonesia triggered a tsunami (commonly known as the “South Asian Tsunami”) and destroyed the nearby Banda Aceh city. After the tsunami, only a few buildings were left. (Source: International Tsunami Information Center) Figure 3  The Great East Japan Earthquake on 11 March 2011 caused a tsunami. The sea water washed away the Ishinomori Manga Museum and Ishinomaki Orthodox Church in Ishinomaki City, Miyagi Prefecture (red arrow indicates the location on the map before and after the tsunami).(Source: International Tsunami Information Center)

April 2021

https://www.hko.gov.hk/en/education/aviation-and-marine/marine/00560-Response-to-Strike-of-Tsunami.html

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Tsunami Monitoring in Hong Kong

Most tsunamis are generated by submarine earthquakes. Hong Kong has not been seriously affected by any tsunami in recorded history, with the Philippines Islands and Taiwan acting as an almost complete breakwater for such tsunamis in the Pacific. Diffracted sea waves are much weaker and therefore the energy that arrives in Hong Kong has been fairly small.

Tsunami Monitoring in Hong Kong Most tsunamis are generated by submarine earthquakes.  Hong Kong has not been seriously affected by any tsunami in recorded history, with the Philippines Islands and Taiwan acting as an almost complete breakwater for such tsunamis in the Pacific. Diffracted sea waves are much weaker and therefore the energy that arrives in Hong Kong has been fairly small. Hong Kong Observatory monitors warning messages issued by the Pacific Tsunami Warning Centre (PTWC) and keeps a close watch on the sea-levels recorded by monitoring stations in the western Pacific, South China Sea and Hong Kong.  If necessary, the Observatory will issue a tsunami warning to the public. Sea level records of seven very minor tsunamis detected in Hong Kong:- Date Origin Moment magnitude Mw Maximum sea level change recorded in Hong Kong(Height above normal tide level)  1952/11/5 Kamchatka 9.0 0.15 m   1960/5/23 Chile 9.5 0.3 m   1985/3/4 Chile 7.9 less than 0.1 m   1988/6/24 Luzon Strait 5.7 0.3 m   2006/12/26 Over the sea near Southern Taiwan 7.1 less than 0.1 m   2010/2/28 Chile 8.8 less than 0.1 m   2011/3/11 Over the sea east of Honshu,Japan 9.0 0.2 m  Tsunami Warning If a severe earthquake in the South China Sea or the Pacific Ocean is expected to generate a tsunami resulting in a significant tsunami (i.e. a tsunami with a height of 0.5 metre above the normal tide level) in Hong Kong and the estimated time of arrival (ETA) of the tsunami at Hong Kong is within 3 hours, the Observatory will issue a Tsunami Warning to alert members of the public to take precautions. For significant tsunamis that are not expected to reach Hong Kong in 3 hours, the Hong Kong Observatory would issue a Tsunami Information Bulletin to notify members of the public.  In addition, the Observatory would also issue Tsunami Information Bulletins for insignificant tsunamis expected in Hong Kong. Precautionary Measures Stay away from shores, beaches and low-lying coastal areas. If you are there, move inland or to higher grounds. The upper floors of high, multi-storey, reinforced concrete building can provide safe refuge if there is no time to quickly move inland or to higher grounds. Do not engage in water sports. Vessels should stay away from the shore or shallow waters. If vessels remain moored in typhoon shelters, their moorings should be doubled and all personnel should leave the vessels and head for higher grounds. Please observe these precautions until the Observatory cancels the tsunami warning. Please stay tuned to the radio or television for further information. Most tsunamis are generated by submarine earthquakes.  Hong Kong has not been seriously affected by any tsunami in recorded history, with the Philippines Islands and Taiwan acting as an almost complete breakwater for such tsunamis in the Pacific. Diffracted sea waves are much weaker and therefore the energy that arrives in Hong Kong has been fairly small.Hong Kong Observatory monitors warning messages issued by the Pacific Tsunami Warning Centre (PTWC) and keeps a close watch on the sea-levels recorded by monitoring stations in the western Pacific, South China Sea and Hong Kong.  If necessary, the Observatory will issue a tsunami warning to the public.Sea level records of seven very minor tsunamis detected in Hong Kong:-If a severe earthquake in the South China Sea or the Pacific Ocean is expected to generate a tsunami resulting in a significant tsunami (i.e. a tsunami with a height of 0.5 metre above the normal tide level) in Hong Kong and the estimated time of arrival (ETA) of the tsunami at Hong Kong is within 3 hours, the Observatory will issue a Tsunami Warning to alert members of the public to take precautions.For significant tsunamis that are not expected to reach Hong Kong in 3 hours, the Hong Kong Observatory would issue a Tsunami Information Bulletin to notify members of the public.  In addition, the Observatory would also issue Tsunami Information Bulletins for insignificant tsunamis expected in Hong Kong.

https://www.hko.gov.hk/en/education/aviation-and-marine/aviation/00231-tsunami-monitoring-in-hong-kong.html

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What is Climate Projection?

What is climate projection? What are the climate challenges that mankind has to face for the rest of this century? Since future changes in man-made greenhouse gas emissions are difficult to predict, scientists must make assumptions of possible greenhouse gas emissions scenarios and simulate future changes in the atmosphere and oceans using computer climate models under the assumptions of these scenarios, i.e. making climate projection.

Ever since the Industrial Revolution, human activity has produced huge amounts of greenhouse gases, causing global climate to warm with increasing occurrence of extreme weather events. The impacts of human-caused climate change are clear. One may ask: what are the climate challenges that mankind has to face for the rest of this century? In fact, climate scientists around the world have done a lot of research on related issues and made future climate projections for Earth through the assessment reports regularly published by the Intergovernmental Panel on Climate Change (IPCC). To understand what climate projections are, one must first understand the difference between weather forecasts and climate projections.Weather forecastsBased on atmospheric and oceanic conditions observed at present or in the recent past, numerical weather prediction models are employed to forecast weather for the next few days, depicting day-to-day and even hour-to-hour variations. The forecast content can include temperature, humidity, air pressure, wind direction, wind speed, rainfall, etc. Parameters exhibiting little fluctuations (e.g, ocean temperature, atmospheric concentrations of greenhouse gases) on timescales of short-term weather do not have a large impact on the forecasts.Climate projectionsGlobal climate depends on the energy balance of Earth. For example, Earth will warm if the amount of energy entering Earth exceeds that leaving Earth. However, the amount of human-caused greenhouse gases emissions is not governed by physical laws, but is affected by many unpredictable factors, such as the development of countries around the world, the supply of low-carbon energy, the development of energy-saving technologies, the formulation and implementation of emissions reduction policies, lifestyle and behavioral habits, etc. Hence, scientists must make assumptions of possible greenhouse gas emissions scenarios and simulate future changes in the atmosphere and oceans using computer climate models under the assumptions of these scenarios, i.e. making climate projection (Figure 1). According to the current level of technology, computer climate models can simulate the changes in long-term average of atmospheric and oceanic parameters, but they still cannot effectively simulate year-to-year variations. Hence, climate projections are generally expressed in terms of 20-year averages. Projections can include air temperature, rainfall, ocean temperature, sea level, sea ice area, etc.IPCC released the Working Group I contribution to the Sixth Assessment Report (AR6) “Climate Change 2021: The Physical Science Basis” in August 2021. A new set of 5 scenarios taking into account various trends of greenhouse gas concentrations is employed in AR6 to generate climate projections for the 21st century. Taking temperature as an example, compared to 1850-1900, global mean surface temperature in 2081-2100 is very likely to be higher by 2.1-3.5℃ and 3.3-5.7℃ under the intermediate (SSP2-4.5) and very high (SSP5-8.5) greenhouse gas emissions scenarios respectively.Climate projections provide important reference for climate change adaptation planning. For example, infrastructure design standards are revised based on projections of rainfall and sea level rise in order to cope with more intense rainfall, higher sea levels, stronger tropical cyclones and the associated threats of storm surge in the future.For details of the projections, please refer to the “Global Climate Projections” and “Hong Kong Climate Projections” on the Observatory's website.

General Climatology

May 2023

https://www.hko.gov.hk/en/education/climate/general-climatology/00696-What-is-Climate-Projection.html

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What is Climatological Normal?

When reading weather reports or summaries such as the Monthly Weather Summary issued by the Hong Kong Observatory (HKO), you may sometimes see the clauses like "the monthly average temperature was above normal" or "the last year's rainfall was less than normal". So, what is the meaning of "normal" in these statements?

When reading weather reports or summaries such as the Monthly Weather Summary[1] issued by the Hong Kong Observatory (HKO), you may sometimes see the clauses like "the monthly average temperature was above normal" or "the last year's rainfall was less than normal". So, what is the meaning of "normal" in these statements?According to the relevant guidelines of the World Meteorological Organization (WMO) in 2017[2], the standard climatological normal is derived from the observations of meteorological data calculated from the average over a 30-year period (with the ending year of "0", such as 1981-2010, 1991-2020) and should be updated every 10 years. The reason for using 30-year data is that a set of climatological normals which is stable over a longer period of time is required as a standard for climate monitoring purposes. The 30-year climatological normals can generally fulfill this requirement. If the averaging period is not long enough, some shorter-term climate phenomena, such as the influence of El Niño or La Niña, will increase the instability of the climatological normals.Climatological normals have a wide range of uses, providing references on the recent climate for planning, design and applications in various sectors, such as agriculture, energy, engineering, water management, transportation, tourism, environment, and research. Normals are also used as references for climate monitoring and forecasting and serve as benchmarks for determining the anomaly of the current weather condition from the corresponding long-term average.In 2021, the HKO compiled a new set of 30-year climatological normals for Hong Kong based on the meteorological data measured at the Hong Kong Observatory Headquarters and other key meteorological stations from the period of 1991-2020 to replace the previous 1981-2010 climatological normals. The comparison between the 1991-2020 and 1981-2010 normals reveals that there are statistically significant increases in the annual mean, mean maximum, and mean minimum temperatures in Hong Kong. This is commensurate with the significant long-term warming trend in Hong Kong[3] due to global warming and local urbanization. For annual rainfall and relatively humidity, the changes between these two 30-year periods are not statistically significant.Members of the public can get access to information on the climatological normals for 1991-2020 and other previous normals at the Observatory's "Climatological Information Webpage". Comparison of the 1991-2020 and 1981-2010 normals Meteorological element 1991-2020normal 1981-2010normal Difference("1991-2020"–"1981-2010") Annual Mean Temperature(°C) 23.5 23.3 0.2* Annual Mean Maximum Temperature(°C) 26.0 25.6 0.4* Annual Mean Minimum Temperature(°C) 21.6 21.4 0.2* Annual Rainfall (mm) 2,431.2 2,398.5 32.7 Annual Mean Relative Humidity (%) 78 78 0 *Changes are statistically significant

General Climatology

February 2021

https://www.hko.gov.hk/en/education/climate/general-climatology/00559-What-is-Climatological-Normal.html

en

Definition of seasons

The definition of "meteorological seasons" refers to the partition of a year into four periods of roughly equal length. The merit of using "meteorological seasons" is that the length of the seasons and the start time of the seasons are generally unchanged. It will be simpler and more convenient for research analysis, results comparison and development of service products.

Hong Kong's climate is sub-tropical with different climatic characteristics throughout the year. For example, January and February are colder, July and August are hotter, June to August are rainier, while November and December are drier and less cloudy. Currently, the Hong Kong Observatory adopts "meteorological seasons" approach to divide the four seasons and provide climate data analysis and climate services in Hong Kong (e.g. seasonal predictions, trend analysis, ranking calculations, climate change studies, etc.). The definition of "meteorological seasons" refers to the partition of a year into four periods of roughly equal length (i.e. three months as a period). For the Northern Hemisphere, the warmest and coldest periods are respectively designated as Summer (June, July and August) and Winter (December, January and February), and the in-between periods as Autumn (September, October and November) and Spring (March, April and May). This definition is generally in line with the annual temperature cycle depicted by the 30-year (1981 - 2010) monthly mean temperatures in Hong Kong (Figure 1), i.e. June, July and August are usually the three hottest months, while December, January and February are the three coldest months in a year.The merit of using "meteorological seasons" is that the length of the seasons and the start time of the seasons are generally unchanged. It will be simpler and more convenient for research analysis, results comparison and development of service products. It is also easier for the public and special users to understand and use relevant data and products. Major climate centres in the world, such as Beijing Climate Center, European Centre for Medium-Range Weather Forecasts, Japan Meteorological Agency, National Oceanic and Atmospheric Administration, also adopt "meteorological seasons" in climate monitoring and some of their climate services.In addition to "meteorological seasons", some researchers and meteorological agencies also use different meteorological elements and indices to determine the "four seasons" and other specially defined seasons. For example, using running average of daily temperatures to determine the commencement date of the four seasons in a year, using rainfall amount to identify the beginning of the rainy season, and utilising the key features of the monsoon hydrologic cycle to assess the onset time of the monsoon seasons. This observational-based approach in defining seasons is generally more complicated and more commonly used for research purposes. Moreover, the start and end dates of a season as calculated by such approach usually vary significantly from year to year, and the same approach may not be directly applicable to different regions.

General Climatology

June 2020

https://www.hko.gov.hk/en/education/climate/general-climatology/00545-definition-of-seasons.html

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Methane - The second culprit in global warming

Methane is one of the three major greenhouse gases and the second most significant driver of global warming after carbon dioxide.

Methane is one of the three major greenhouse gases and the second most significant driver of global warming following carbon dioxide. In comparison to carbon dioxide, methane is more potent but has a much lower atmospheric concentration and stays in the atmosphere for a shorter duration. According to the Sixth Assessment Report (AR6) of United Nations Intergovernmental Panel on Climate Change (IPCC), global average surface temperature in 2010-2019 was about 1.1°C above pre-industrial levels, of which about 0.5°C was contributed by methane (Figure 1).The largest natural sources of methane include anoxic water and sediments of wetlands and freshwater lakes, gas and oil seeps, and mud volcanoes. The largest anthropogenic emissions sources are enteric fermentation from cattle and sheep in livestock production, manure treatment, landfills, waste treatment, rice cultivation and fossil fuel exploitation.Methane emissions have nearly doubled in the past two centuries and they are mainly driven by human since 1900. The present-day atmospheric concentration of methane was unprecedented in at least 800,000 years (Figure 2). Against the background of continued global warming, permafrost in the cryosphere is projected to continue to warm and thaw, releasing methane and carbon dioxide to the atmosphere and enhancing the greenhouse effect to form a vicious cycle.At the United Nations Climate Change Conference in 2021, over 100 countries pledged to reduce global methane emissions by 30% by 2030. To achieve the goals of Paris agreement, there is an urgent need to step up efforts to reduce emissions.

General Climatology

April 2022

https://www.hko.gov.hk/en/education/climate/general-climatology/00670-Methane-the-second-culprit-in-global-warming.html

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Why Carbon Dioxide is a Greenhouse Gas?

Carbon dioxide is the single most important greenhouse gas in the atmosphere. The annual mean concentration set a record high again in 2017, reaching 405.5 ppm which is about 46% above pre-industrial levels. A recent study showed that the present-day atmospheric concentration of carbon dioxide is likely the highest in the last three million years, which is a worrisome situation indeed.

Solar radiation reaches the Earth in the form of shortwave radiation and heats up the Earth's surface. The Earth's surface then emits infrared radiation (longwave radiation) in order to cool down. However, greenhouse gases in the atmosphere will absorb part of the infrared radiation emitted by the Earth, and then re-emit infrared radiation in all directions. Part of the infrared radiation will escape to space but part of it will go back to the Earth, heating up the Earth's surface and causing the greenhouse effect.The Earth's atmosphere consists of nitrogen (~78%), oxygen (~ 21%), argon (~0.9%) and trace gases such as carbon dioxide, water vapour, methane, etc. Nitrogen and oxygen together account for more than 90% of the atmosphere but they are not greenhouse gases. Carbon dioxide accounts for just about 0.04% but it is the single most important greenhouse gas in the atmosphere and also the principle control knob of Earth's temperature. Why carbon dioxide is a greenhouse gas? To answer this question, we first need to understand that infrared radiation is an electromagnetic wave. Laws of Physics require that molecular vibration must lead to a change in the relative distribution of charge for the gas to absorb electromagnetic radiation.Let us take nitrogen as an example for illustration (see left side of Figure 1). A nitrogen molecule consists of two nitrogen atoms (blue balls). The positive charges carried by the two nitrogen atoms are identical and symmetrical in distribution. The vibration of nitrogen atoms along the chemical bond does not change the relative distribution of charges. Therefore, nitrogen cannot absorb infrared radiation. Then how about carbon dioxide? (see right side of Figure 1) A carbon dioxide molecule consists of two oxygen atoms (red balls) and one carbon atom (grey balls). Three different modes of vibration are possible with such configuration. When the carbon atom moves along the chemical bond towards either one of the oxygen atoms, or moves up and down relative to the oxygen atoms, the relative distribution of the charges will be altered and hence carbon dioxide can absorb infrared radiation.Carbon dioxide is the single most important greenhouse gas in the atmosphere. The annual mean concentration set a record high again in 2017, reaching 405.5 ppm which is about 46% above pre-industrial levels. A recent study showed that the present-day atmospheric concentration of carbon dioxide is likely the highest in the last three million years, which is a worrisome situation indeed.

General Climatology

May 2019

https://www.hko.gov.hk/en/education/climate/general-climatology/00525-why-carbon-dioxide-is-a-greenhouse-gas.html

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What is Carbon Cycle?

What is carbon cycle? Carbon cycle is a biogeochemical cycle in which carbon is continuously exchanged and recycled among several natural reservoirs, including atmosphere, ocean, terrestrial biosphere, rocks and fossil fuels, where carbon is stored.

Carbon is the building block of life.  It is the basic element of all organic substances, from fossil fuel to DNA.  Carbon cycle is a biogeochemical cycle in which carbon is continuously exchanged and recycled among several natural reservoirs, including atmosphere, ocean, terrestrial biosphere, rocks and fossil fuels, where carbon is stored.The exchange of carbon between different reservoirs involves different processes.  For example, plants remove carbon dioxide (CO2) from the atmosphere by photosynthesis; plant and animal respiration returns carbon to the atmosphere as CO2 or as methane (CH4) under anaerobic conditions.  These processes are so tightly tied to the plant life cycle that the growing season can be seen by the way CO2 concentration fluctuates in the atmosphere seasonally.  In northern winter, atmospheric CO2 concentration climbs when plants are decaying.  During spring, plants start to grow again, taking up more CO2 from the atmosphere, and the CO2 concentration drops.  Compared to the amount of CO2 released into the atmosphere by combustion of fossil fuels in the past decades, the amplitude of this natural seasonal fluctuation of CO2 is much smaller (Fig. 1).CO2 is also exchanged continuously between the ocean and the atmosphere.  This exchange is largely controlled by sea surface temperatures, ocean acidity, circulating currents and the biological processes of photosynthesis and respiration by marine plants and animals.  Cold ocean waters favour the uptake of CO2 from the atmosphere.  In winter, cold waters at high latitudes, heavy and enriched with dissolved CO2, sink from the surface layer to deep ocean (see "What is thermohaline circulation?").  Upward moving currents in the tropics bring dissolved CO2 up from the depth to the surface ocean and release the gas back to the atmosphere.  Ocean plants and animals also contribute to the exchange of CO2 through photosynthesis and respiration.  Some of the carbon-rich dead organisms will sink to the bottom of the ocean and form layers of limestone sediment on the ocean floor, resulting in   removal of carbon from the atmosphere.  Ocean acidity acts in the way that an acidified ocean will have less ability to hold more CO2.  However, the pH value and the buffering capacity of the ocean can be restored by dissolution of limestone (calcium carbonates).  These processes have various time scales, ranging from decades to millennia. In an even longer time scale, CO2 and water together will form carbonic acid which can dissolve calcium or magnesium from rocks to form insoluble carbonates.  CO2 and rain water can also react with limestone to form soluble calcium bicarbonates over millions of years, a process called weathering.  Insoluble carbonates will eventually be washed into the ocean and settle on the ocean floor.  Those soluble calcium bicarbonates will also precipitate out from the ocean water and form layers of sediment.  The cycle continues as these layers of sediment are drawn into Earths mantle by subduction of tectonic plates.  Eventually, carbon will be released back to the atmosphere by volcanic activities. In an unperturbed natural carbon cycle, these exchanges between reservoirs are approximately in balance.  In fact, the atmospheric concentration of CO2 had been relatively stable between 260 to 280 ppm for 10,000 years before the industrial revolution.  Human activities such as burning of fossil fuels, deforestation and land use changes have perturbed the global carbon cycle since then.  To understand the fate of the fossil-fuel CO2 released in the atmosphere, scientists use models of global carbon cycling  for investigation.  It is found that although the ocean plays a key role in absorbing this greenhouse gas, a substantial portion of CO2 will stay in the atmosphere, awaiting much slower removal processes like weathering and deposition of sediment on the ocean floor.  Figure 2 shows the global carbon cycle for the 1990's.  According to the IPCC's Fourth Assessment Report, 50% of the additional CO2 in the atmosphere will be removed within 30 years, a further 30% will be removed in a few centuries and the remaining 20% may remain in the atmosphere for many thousands of years.  In other words, the climatic effect brought by the leftover CO2 will also persist for such a long period of time.

General Climatology

September 2012

https://www.hko.gov.hk/en/education/climate/general-climatology/00250-what-is-carbon-cycle.html

en

Onset of Southwest Monsoon - End of the Fog Season and Start of the Rain Season

The basic driving force of the southwest monsoon is the large-scale differential heating between landmasses and oceans. Starting from April, the Sun gradually moves north and the amount of sunshine will increase in the northern hemisphere. The Asian continent will be heated up faster than its neighboring oceans. As air over the land becomes lighter and rises, a low pressure center will eventually form over the continent. The relatively warm and moist air from the ocean will then in general flow towards this low pressure centre, thus leading to the onset of the southwest monsoon.

Onset of southwest monsoon - end of the fog season and start of the rain season Southwest monsoon The basic driving force of the southwest monsoon is the large-scale differential heating between landmasses and oceans. Starting from April, the Sun gradually moves north and the amount of sunshine will increase in the northern hemisphere. The Asian continent will be heated up faster than its neighboring oceans. As air over the land becomes lighter and rises, a low pressure center will eventually form over the continent. The relatively warm and moist air from the ocean will then in general flow towards this low pressure centre, thus leading to the onset of the southwest monsoon. Fig. 1 shows the monthly mean of prevailing wind direction at Waglan Island between 1971-2000. It clearly indicates that the prevailing wind direction in Hong Kong veers from easterlies to southwesterlies between May and June. Fig. 1. Prevailing wind direction at Waglan Island (1971-2000). The end of the fog season The fog season in Hong Kong usually begins in February and ends in April. When warm moist maritime air moves over the relatively cool sea surface near the coast, air temperature will gradually fall to the dew point. Water vapour in the air will condense into very small water droplets which lead to the formation of fog (this is commonly called advection fog). Since the air is warmer than the sea water underneath, the atmosphere is relatively stable and helps confine the water vapour to the lowest level of the atmosphere. As the season proceeds, the sea water will also warm up gradually such that the atmosphere over the ocean will become more unstable. Hence, the onset of the southwest monsoon often marks the end of the fog season. Climatology shows that the Observatory rarely observes fog in May, on average less than 0.2 day in that month (Fig. 2). Fig. 2. Climatology of number of days with fog being observed at the Observatory (1971-2000). The start of the rain season The southwest monsoon will bring in warm moist air and the warming up of sea water will fuel the development of convective activities. Coupling with other favorable meteorological conditions like low level convergence or upper level divergence, the southwest monsoon could sometimes lead to heavy rain. Climatology shows that the amount of rainfall increases significantly in May (Fig. 3). This correlates well with the onset of the southwest monsoon. Fig. 3. Climatology of monthly mean rainfall (mm) recorded at the Observatory (1971-2000) Reference Meteorology Today: An Introduction to Weather, Climate and the Environment. Author: C. D. Ahrens. Brooks/Cole Pub Co. The basic driving force of the southwest monsoon is the large-scale differential heating between landmasses and oceans. Starting from April, the Sun gradually moves north and the amount of sunshine will increase in the northern hemisphere. The Asian continent will be heated up faster than its neighboring oceans. As air over the land becomes lighter and rises, a low pressure center will eventually form over the continent. The relatively warm and moist air from the ocean will then in general flow towards this low pressure centre, thus leading to the onset of the southwest monsoon. Fig. 1 shows the monthly mean of prevailing wind direction at Waglan Island between 1971-2000. It clearly indicates that the prevailing wind direction in Hong Kong veers from easterlies to southwesterlies between May and June.The fog season in Hong Kong usually begins in February and ends in April. When warm moist maritime air moves over the relatively cool sea surface near the coast, air temperature will gradually fall to the dew point. Water vapour in the air will condense into very small water droplets which lead to the formation of fog (this is commonly called advection fog). Since the air is warmer than the sea water underneath, the atmosphere is relatively stable and helps confine the water vapour to the lowest level of the atmosphere. As the season proceeds, the sea water will also warm up gradually such that the atmosphere over the ocean will become more unstable. Hence, the onset of the southwest monsoon often marks the end of the fog season. Climatology shows that the Observatory rarely observes fog in May, on average less than 0.2 day in that month (Fig. 2).The southwest monsoon will bring in warm moist air and the warming up of sea water will fuel the development of convective activities. Coupling with other favorable meteorological conditions like low level convergence or upper level divergence, the southwest monsoon could sometimes lead to heavy rain. Climatology shows that the amount of rainfall increases significantly in May (Fig. 3). This correlates well with the onset of the southwest monsoon.

General Climatology

https://www.hko.gov.hk/en/education/climate/general-climatology/00261-onset-of-southwest-monsoon-end-of-the-fog-season-and-start-of-the-rain-season.html

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A Brief Introduction to Standardized Precipitation Index (SPI)

Standardized Precipitation Index (SPI) is a normalized index representing the probability of occurrence of an observed rainfall amount when compared with the rainfall climatology at a certain geographical location over a long-term reference period. SPI enables rainfall conditions to be quantified over different time scales, facilitating the analyses of drought impact on various water resource needs.

Deficit of rainfall over a period of time at a certain location could lead to various degrees of drought conditions, affecting water resources, agriculture and socio-economic activities.  Since rainfall varies significantly among different regions, the concept of drought may differ from places to places.  As such, for more effective assessment of the drought phenomena, the World Meteorological Organization (WMO) recommends adopting the Standardized Precipitation Index (SPI) to monitor the severity of drought events.In simple terms, SPI is a normalized index representing the probability of occurrence of an observed rainfall amount when compared with the rainfall climatology at a certain geographical location over a long-term reference period.   Negative SPI values represent rainfall deficit, whereas positive SPI values indicate rainfall surplus.  Intensity of drought event can be classified according to the magnitude of negative SPI values such that the larger the negative SPI values are, the more serious the event would be.  For example, negative SPI values greater than 2 are often classified as extremely dry conditions.Moreover, SPI enables rainfall conditions to be quantified over different time scales (e.g. 3-, 6-,12-, or 24-month rainfall), facilitating the analyses of drought impact on various water resource needs.  For example, SPI-3 measures rainfall conditions over a 3-month period, the anomalies of which impact mostly on soil water conditions and agricultural produce; while SPI-24 measures rainfall conditions over two years, as prolonged droughts can give rise to shortfalls in groundwater, stream flow, and fresh water storage in reservoirs.An advantage in using SPI is that only rainfall data are needed for its computation.  SPI can also be compared across regions of different climatic zones.  For more information about the methodology and applications of SPI, please refer to the WMO guideline.

General Climatology

September 2013

https://www.hko.gov.hk/en/education/climate/general-climatology/00254-a-brief-introduction-to-standardized-precipitation-indexspi.html

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Upper air Ozone Monitoring by the Observatory

Ozone exists in the entire atmosphere and its concentration changes with locations, altitudes and seasons. Since 1993, the Observatory has been conducting upper-air ozone measurement. A weather balloon with an ozone sensor kit is launched regularly at the King's Park Meteorological Station to monitor the variation of ozone concentration above Hong Kong.

When we talk about "Ozone", what is the first thing that comes to your mind?  Many people may immediately think of "Ozone layer" in the stratosphere.  "Ozone layer" is where most of the ozone molecules accumulate at and typically located at around 20 to 30 km above the Earth's surface, blocking the ultraviolet rays (UV-C) which can directly damage our skin and increase the risk of skin cancer (Figure 1).In fact, ozone exists in the entire atmosphere and its concentration changes with locations, altitudes and seasons.  Since 1993, the Observatory has been conducting upper-air ozone measurement.  A weather balloon with an ozone sensor kit is launched regularly at the King's Park Meteorological Station to monitor the variation of ozone concentration above Hong Kong (Figure 2).  As the Observatory is a member of the Global Atmospheric Watch (GAW) programme of the World Meteorological Organization (WMO), the data collected are disseminated to the World Ozone and Ultraviolet Radiation Data Centre (WOUDC) for monitoring and analysis.Figure 3 shows the average concentration of ozone in the upper air of Hong Kong in the past 20 years (1994-2013).  It is noticed that the ozone concentration increases rapidly above 20 km from the ground and peaks at around 30 km altitude.  This is the typical altitude of ozone layer in the tropical region.  The ozone concentration in the troposphere is much lower than that in the stratosphere, but it possesses prominent variations throughout the year.  Generally speaking, this tropospheric ozone contributes to around 15% of the total ozone in the atmosphere.  Its concentration is generally higher in spring (March to May) and is believed to be due to changes in the global circulation of the atmosphere.  Besides, weather elements like wind, moisture, sunshine duration and stability of the atmosphere can also affect the concentration of ozone.  Moreover, the ozone concentration near the surface can even be affected by human factors.For instance, on a sunny day under the influence of a continental airstream, the ozone concentration in the lower atmosphere could build up significantly.  Figure 4 shows an example of a tropical cyclone case of Typhoon Haikui over eastern China on 8 August 2012, bringing a northerly continental airstream to Hong Kong.  On that day, the observed ozone concentration below 3 km altitude could rise to more than double of its long-term average (Figure 5)!

General Climatology

April 2015

https://www.hko.gov.hk/en/education/climate/general-climatology/00452-upper-air-ozone-monitoring-by-the-observatory.html

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What are the Ozone conditions now in Hong Kong?

What are the ozone conditions in Hong Kong? What is ozone? What is Ozone layer? How does Ozone protect us? Is ozone harmful or beneficial to human body? How can we protect the Ozone layer? How to measure ozone in atmosphere?

What is "Ozone"? What is "Ozone layer"? Oxygen is a chemical element (symbol O) ranking number 8 in the periodic table and has an atomic mass of 16.  When two oxygen atoms combine they form oxygen gas (symbol O2), which makes up one fifth of the volume of air and is the most essential element for life.  Ozone (symbol O3) is formed from three oxygen atoms and is a product of ultraviolet rays acting on oxygen in the air.  As ozone is very unstable, the extra oxygen atom is easily lost causing ozone to turn into the more stable oxygen gas.  On the other hands, rays of ultraviolet light in sunlight makes new ozone molecules and a certain concentration of ozone is maintained in the atmosphere and settles in a layer between 20 and 30 kilometers from the ground known as the "Ozone layer".How does Ozone protect us? How can we protect the Ozone layer?Ultraviolet rays can damage our skin and increase the risk of skin cancer.  It also affects our immune system and damages our eyes, leading to cataracts or even the loss of sight.  The ozone layer provides a shield (see figure 1) by keeping out over 90 percent of ultraviolet rays thus keeping us out of harm's way.Studies indicate CFCs destroy ozone.  One molecule of CFC can destroy about a hundred thousand ozone molecules and reduce the ozone in the atmosphere leading to the occurrence of ozone holes at both the north and south poles. It would allow more ultraviolet light to reach the ground and affect the Earth's ecology. Therefore, we must at all cost reduce the use of sprays solution, refrigerant, air-conditioners and other products which contain CFCs. Furthermore, if we use such products, we have to arrange their periodic inspection and service to avoid accidental release of the CFCs.Ozone is harmful or beneficial to human body?Ozone is a colourless gas but has a pale blue colour and a slight odour of freshly cut grass at higher concentrations.  At first breath, ozone gives a feeling of freshness but it can become uncomfortable when a large amount is inhaled.  Although ozone has purifying, sterilizing and deodorizing power, it is hazardous to our eyes and lungs, even seriously damaging the lungs if ozone is inhaled excessively and causing death.  Because of industrial and vehicular emissions, the air in urban areas is often polluted.  A high concentration of ozone is formed under sunlight by photochemical effect at the lower atmosphere.  This is "bad" ozone. However, ozone at high altitudes protects all living things on Earth by absorbing ultraviolet rays from the sun.  In addition, a thin layer of ozone brought down from aloft after a thunderstorm is refreshing.  This is known as "good" ozone.How to measure Ozone in atmosphere?The Hong Kong Observatory has been measuring ozone concentrations in the upper atmosphere since 1993.  A meteorological balloon with an ozone sensor kit (see Figure 2) is launched every week.  The balloon can rise to above 30 kilometers with the ozone sensor measuring the ozone concentration continuously as it rises through the atmosphere.  The ozone sensor kit contains a measuring solution and a pump to pass air through the solution (see figure 3).  This process enables the determination of the ozone concentration.  The Hong Kong Observatory regularly disseminates the ozone concentration data to the World Ozone Data Centre for monitoring and analysis.What are the Ozone conditions in Hong Kong? Figure 4 illustrates the ozone data (green curve) over Hong Kong on July 2008.  It can be seen that the ozone concentration significantly increases above the level of 100 hPa (about 16 kilometers high) in the atmosphere.  The maximum ozone concentration occurs between 20 and 30 kilometers high (area between the two white lines shown in figure 4), which is known as the ozone layer.Although the ozone layer over Hong Kong protects us by absorbing the ultraviolet rays, it does not mean that it will not change in the future.  Thus, we have to treasure our environment and preserve the nature, to protect ourselves and our next generation.

General Climatology

https://www.hko.gov.hk/en/education/climate/general-climatology/00256-what-are-the-ozone-conditions-now-in-hong-kong.html

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Last time it snowed in Hong Kong

With intense winter monsoon bringing very cold air mass from the north, sub-zero temperatures could sometimes occur on high ground and in the New Territories, where there were reports of frost, ice, rime or even slight snow. From 1967 to 1975, there were four days with slight snow reported in Hong Kong.

Hong Kong's climate is sub-tropical, tending towards temperate for nearly half the year. During the winter season, it is not uncommon for temperatures to drop below 10oC in the urban areas, and the lowest temperature ever recorded at the Observatory was 0oC on 18 January 1893.  With intense winter monsoon bringing very cold air mass from the north, sub-zero temperatures could sometimes occur on high ground and in the New Territories, where there were reports of frost, ice, rime or even slight snow[1-3].  From 1967 to 1975, there were four days with slight snow reported in Hong Kong (see table).  Although frost and ice still occasionally occurred in winter, there was no more report of snow ever since.

General Climatology

https://www.hko.gov.hk/en/education/climate/general-climatology/00247-last-time-it-snowed-in-hong-kong.html

en

Volcanoes, weather and climate

Carbon dioxide emitted by volcanoes to the atmosphere is also one of the natural factors contributing to variations in the ancient climate. However, various studies have shown that, in the last century, the annual amount of carbon dioxide released by human activities far exceeded that released by terrestrial and submarine volcanoes.

Volcanic eruption is one of the most stunning natural phenomena on the Earth. Although images of the mushroom ash clouds and volcanic lightning are spectacular and impressive, volcanic eruptions do have significant impacts on humans. Volcanic ash and lahars (i.e. mudflow from a volcano) can pose safety threats to people living on the ground and ash clouds can seriously disrupt traffic in the air. Recent examples include the volcanic eruptions in Iceland in 2010 and in Chile in June 2011.Volcanic activities can also affect weather and climate. Large amount of gases and particles can be released to the atmosphere during eruptions, thereby affecting short term weather and long term climate. Generally speaking, the influence of volcanic eruptions on weather or climate depends very much on the severity and duration of the eruption, the chemical composition of the ash plume, and the location (latitude) of the volcano. For small scale eruptions, the ash plume is usually well contained in the troposphere (a layer of air stretching from the ground to about 10-16 km aloft, depending on latitude). Direct impacts on regional weather could include low visibilities and reduced diurnal temperature variation. Since most of the ash and gases ejected into the troposphere will likely be washed out by rain within a couple of weeks (Figure 2(a)), the impact on global weather and long term climate will be minimal[1]. Powerful eruptions are capable of blasting sulphur-rich gases, mainly in the form of sulphur dioxide, into the stratosphere (a layer of air extending from the top of the troposphere to roughly 50 km above ground). The sulphur dioxide will react with water molecules under sunlight to form sulphate aerosols (tiny particles of sulphuric acid). High winds in the stratosphere will subsequently spread the aerosols across the hemisphere or even the whole globe (depending on the latitude of the volcano) in the following weeks. This layer of aerosols could remain in the stable stratosphere for a couple of years and reduce the sunlight reaching the troposphere below, leading to tropospheric cooling (Figure 2(b)). This cooling effect is particularly significant in the tropics and the summer hemisphere [1]. Although volcanic aerosol in the stratosphere can produce cooling at the surface from a global average perspective, its impact on the Northern Hemisphere winter is indirect and complex. In gist, the associated changes in the atmospheric circulation of the Northern Hemisphere could cause a positive phase of the Arctic Oscillation [2], bringing warmer winters to some parts of the Northern Hemisphere including northern Eurasia and parts of North America. Interested readers could refer to the relevant references for detailed discussions on the processes [1, 3]. The eruption of Mount Pinatubo in the Philippines in June 1991 is one of the examples of major volcanic eruptions leading to a decline in the global average temperature. The peak altitude of the ash plume of this eruption reached about 34 km. The ash and aerosol of the eruption lowered the global average temperature by about 0.5oC in the following 18 months. It took a couple of years for the global average temperature to return to the pre-eruption level (Figure 3). Other past major eruption events show that the cooling effect of individual volcanic eruption on global climate is usually short-lived when compared with the long term warming trend due to enhanced greenhouse effect [4, 5].Carbon dioxide emitted by volcanoes to the atmosphere is also one of the natural factors contributing to variations in the ancient climate. However, various studies have shown that, in the last century, the annual amount of carbon dioxide released by human activities far exceeded that released by terrestrial and submarine volcanoes. The estimated annual amount of anthropogenic (i.e. human-induced) carbon dioxide emission in 2010 is about 35 gigatons, which is more than 100 times the estimated global volcanic carbon dioxide emission (about 0.26 gigaton per year) [6].The study of volcanic eruption impact on global climate allows scientists to better understand the relative contributions of natural factors and human-induced factors on the global warming trend observed in the 20th century. Scientists have used state-of-the-art computer climate models to simulate past climate variations. Their model simulations captured the short term cooling effect of major volcanic eruptions in the past century generally well. Most importantly, when only natural factors (such as volcanic and solar activity) are considered, the models cannot simulate the rapid warming in the second half of the 20th century (blue line in Figure 4(b)). The warming can only be explained if human-induced factors (notably the emission of greenhouse gases and aerosols) are included in the model simulations (red line in Figure 4(a)). Therefore, it is very unlikely that the 20th century warming can be explained solely by natural causes. Climate model simulations show that most of the global warming observed over the last 50 years is very likely due to human activities [7].

General Climatology

August 2011

https://www.hko.gov.hk/en/education/climate/general-climatology/00246-volcanoes-weather-and-climate.html

en

An Introduction to Phenology

Phenology is primarily a subject of studying the words of our nature, covering areas like the study of the growth, blossom and withering of plants, the timing of bird migration and insect activities, etc., with a view to understanding the change of climate and its impact on the ecosystem. The influence of the Greenhouse Effect on the Earth is becoming more apparent. Phenological observations are essential as they can enhance our understanding of the impact of global warming on the biosphere. Entering the 21st Century, the influence of the Greenhouse Effect on the Earth is becoming more apparent. Phenological observations are essential as they can enhance our understanding of the impact of global warming on the biosphere.

"The duck knows first when the river becomes warm in spring", "A falling leaf heralds the coming of autumn". The above two Chinese sayings describe the phenological phenomena of the onset of spring and autumn respectively.Since the ancient times, humans have been taking phenological observations for various agricultural purposes. In the historical Hellenistic period two thousand years ago, people in Athens have already started making phenological observations. During the Zhou and Qin periods in China, phenological calendar was in place to guide rulers determining when to order peasants ploughing the land. In Japan, the time of cherry blossoms (Sakura) each year was recorded and such practice could be dated back to the early 9th Century in the Tang dynasty. Up till now, phenological observations of cherry blossom have been taken for over 1,200 years, the longest record in the world. From the 18th to 20th Centuries, very organized networks for systematic phenological observations have been developed in China, Europe, United States and Russia. These observations are very useful for the development of the science of Phenology.Phenology is primarily a subject of studying the words of our nature, covering areas like the study of the growth, blossom and withering of plants, the timing of bird migration and insect activities, etc., with a view to understanding the change of climate and its impact on the ecosystem. Phenological observations have to be taken at fixed locations, fixed time and based on the same kind of natural activity. The recording format should also be standardized for easy comparison. Different regions have different phenological patterns depending on their latitudes, longitudes and heights. Even at the same location, the phenological pattern observed in the past may be quite different from the present. According to past studies, it may exhibit periodic variations and can even be affected by solar activities.Entering the 21st Century, the influence of the Greenhouse Effect on the Earth is becoming more apparent. Phenological observations are essential as they can enhance our understanding of the impact of global warming on the biosphere. Looking around, numerous incidents have occurred which may reflect such impact. For examples, several years ago large herds of reindeers living near the Arctic Circle were drowned during their migration in autumn and winter, suggesting that the cause was due to the cracking of the ice layer on the icy lake which might not have grown thick enough; a tendency for early occurrence of cherry blossoms in Japan; large areas of pine forest in the mid-latitude regions were damaged by pine beetles which hatched out earlier and reproduced in vast quantity due to warm winter or hot summer; it was even observed that the blossoms of Kapok trees during winter in Hong Kong had changed with many green leaves still attached to the trees, while the trees used to be rather "bald" in the old days (see Figure 1). These observational facts have reminded us that our natural environment might have silently entered a stage of irreversible change.

General Climatology

March 2013

https://www.hko.gov.hk/en/education/climate/general-climatology/00369-an-introduction-to-phenology.html

en

A new frontier for aviation meteorology - Support carbon reduction in the air

The Hong Kong Observatory provides more specific forecast of thunderstorms over the airspace near the airport (in particular the flight paths and the holding points). The services would facilitate air traffic controllers to reschedule flight routes early to reduce diversion of aircrafts and waiting time of aircraft circling in the air. This, in turn, helps to reduce fuel consumption and carbon dioxide emissions.

In 2007, the United Nations Intergovernmental Panel on Climate Change (IPCC) indicated in its Fourth Assessment Report that the observed global warming since the mid-20th century was very likely due to the observed increase in anthropogenic greenhouse gas concentrations. Carbon dioxide is a major greenhouse gas in the atmosphere. According to the latest information, the concentration of carbon dioxide nowadays is the highest among the past hundreds of thousands of years. The concentration of carbon dioxide has increased more than one third of that in the 18th century before the industrial revolution.Apart from industry, power plants, cars and other human activities which emit carbon dioxide on the ground, aircraft in air will also emit carbon dioxide due to combustion of fossil fuels.  According to estimation of the aviation industry, a flying aircraft emits 49 kg of carbon dioxide per minute on average. Hence, a long haul flight travelling 12 hours will emit a total of 35 tonnes of carbon dioxide. Although the carbon dioxide produced by the aviation industry currently contributes only two to three percent of the total global greenhouse gas emissions, with the growth of air traffic in the future, the aviation industry is already aware of the need to tackle the challenge of climate change early. World Meteorological Organization, International Civil Aviation Organization and the aviation industry are now working together. Apart from conducting research on using more environmental-friendly fuel as well as more efficient and low emission aircraft engines, ways are being explored to provide more accurate and precise aviation weather forecast to facilitate air traffic control to use the flight paths and air space more effectively to reduce flight diversions and holding of aircraft due to severe weather conditions.  This will help to reduce fuel consumption and carbon dioxide emissions.  Currently, weather information are provided to air traffic controllers, pilots and airlines mainly through highly-condensed text messages using special codes like the Aerodrome Forecast (TAF) (Figure 1). TAF only covers aerodrome area (within around 8 km). Such weather codes were developed back in the mid-20th century due to the then limitations in international data transmission. In recent years, the demand for weather services for aviation has increased, in particular more precise and accurate forecast is demanded for larger terminal areas (commonly up to hundreds of kilometres), which are not covered by the scope of the existing Aerodrome Forecast.Severe weather impacting airport and aircraft in flight is becoming increasingly important.  For example, if the flight path (especially the arrival and departure corridors near the airport) or the designated areas which aircraft wait for their turn to land (also known as "holding point") is affected by thunderstorms, aircraft may need to be diverted to other airports or to circle in the air for considerable time if the flight path has not been adjusted earlier. With increasing number of flights, the effect of severe weather conditions on flight management increases and may have serious impact on the air traffic. Severe weather not only cause delay of flight schedule, but also increase flight time of aircraft, which means more fuel consumption and carbon dioxide emission as well as an increase of cost for airlines.In Hong Kong, the Observatory in cooperation with the Civil Aviation Department is developing new aviation weather forecasting services to provide more specific forecast of thunderstorms over the airspace near the airport (in particular the flight paths and the holding points). The services would facilitate air traffic controllers to reschedule flight routes early to reduce diversion of aircrafts and waiting time of aircraft circling in the air.  This, in turn, helps to reduce fuel consumption and carbon dioxide emissions.Internationally, the Observatory has participated in the work of an Expert Team of the World Meteorological Organization to enhance the provision of aviation weather information. We have obtained the support of the aviation industry on the development strategy of fostering terminal weather services to meet the needs to increase the efficiency of air traffic management in the 21th century, thereby reducing fuel consumption and carbon dioxide emissions. In addition to the development of significant convection forecast near the terminal region, research are being conducted on provision of information on weather conditions impacting flight safety and flight efficiency, such as icing, turbulence, high-altitude wind and temperature forecast.All in all, no matter whether you live in the city or countryside, are on ground or in the air, climate change affects every one of us and is a serious challenge for this and the future generations.  Cooperation among different sectors, governments and the public is needed. The aviation industry and meteorological community have already taken part in the carbon reduction action. We hope that the carbon reduction can gain wider support from all over the society. Let's work together to create a better future for our earth and for our next generations!

General Climatology

June 2012

https://www.hko.gov.hk/en/education/climate/general-climatology/00251-a-new-frontier-for-aviation-meteorology-support-carbon-reduction-in-the-air.html

en

What is microclimate ?

Microclimate generally refers to the specific climatic conditions within a small area (such as street, park, riverside, etc). Due to the influence of the surrounding terrain, orientation and density of buildings, weather conditions during the time as well as other factors, the climatic characteristics at an area may differ from those prevailing over the surrounding large region.

You may possibly be aware that the temperature, humidity and other weather elements can be very different even within the same area.  This is the so-called microclimate.  Microclimate generally refers to the specific climatic conditions within a small area (such as street, park, riverside, etc).  Due to the influence of the surrounding terrain, orientation and density of buildings, weather conditions during the time as well as other factors, the climatic characteristics at an area may differ from those prevailing over the surrounding large region.  Therefore, the weather elements at different points of location can somehow be rather different even though they are within the same area.Take a more prominent example, winds over roads at the city centre or the leeward side of high-rise buildings are generally weaker.  Together with other factors such as traffic and heat from structures, the area becomes stuffy and heat accumulates, leading to higher temperatures.  On the other hand, temperatures over coastal or windward areas are usually relatively lower.

General Climatology

August 2018

https://www.hko.gov.hk/en/education/climate/general-climatology/00509-what-is-microclimate-.html

en

Endless wildfires

Very often, hot weather is accompanied by wildfires (also known as bushfires and forest fires), especially in dry environment. In June 2019, more than a hundred intense and long-lived wildfires occurred in the Arctic Circle. In the same year, the Amazon forest experienced the most active fire season since 2010. Yet, the most significant wildfire in 2019 was the Australian bushfires which occurred in the second half of the year. The bushfires began in Queensland in September 2019, spreading south to New South Wales and Victoria, and gradually became under control in March 2020.

According to the analysis of the World Meteorological Organization, 2019 was the second warmest year on record globally, with the past five years, 2015-2019, being the five warmest on record. Under global warming, extreme hot weather events ravaged different parts of the world. For example, Australia registered the hottest summer on record between the end of 2018 and early 2019. In December 2019, a high temperature near 50°C was recorded in southern Australia. Europe could not be spared and was hit by heatwaves twice within a month between late June and late July 2019. In the first heatwave, France registered a record breaking temperature of 46°C. The second heatwave was more pervasive, breaking high temperature records in Germany, the Netherlands, Belgium, Luxembourg and the United Kingdom. During the two heatwaves, more than 1,400 excess deaths as compared with the average were observed in affected regions. Very often, hot weather is accompanied by wildfires (also known as bushfires and forest fires), especially in dry environment. In June 2019, more than a hundred intense and long-lived wildfires occurred in the Arctic Circle. In the same year, the Amazon forest experienced the most active fire season since 2010. Yet, the most significant wildfire in 2019 was the Australian bushfires which occurred in the second half of the year. The bushfires began in Queensland in September 2019, spreading south to New South Wales and Victoria (Figure 1), and gradually became under control in March 2020. The bushfires have killed many people, destroyed hundreds of homes and burned large areas of land, causing massive devastation to ecosystems and the environment. Analyses showed that the bushfires had burned 21 per cent of Australia’s forests, far exceeding similar records in other continents over the past 20 years. In addition, scientists estimated that 800 million animals were killed by the fires in New South Wales alone, and the total number of animals killed exceeded 1 billion for the whole nation. Is this extreme bushfire event a random event, or its chance of occurrence has been enhanced by climate change? We can get some clues from the climate report published by the Bureau of Meteorology, Australia. The temperature of Australia has risen by 1°C since 1910, leading to an increase in the frequency of extreme heat events. In southeastern Australia, there has been a decline of around 11% of April-October rainfall since the late 1990s, and there is a long-term increasing trend in the Forest Fire Danger Index (Figure 2). 2019 was the warmest and driest year on record for Australia, setting the scene for the bushfires. In fact, many studies have pointed out the link between climate change and Australian bushfires. For instance, climate change made the 2018 Queensland bushfires four times more likely; and climate change will make firestorms more likely in southeastern Australia. In light of the Australian bushfires, scientists have reviewed recent research results and drawn the conclusion that human-induced climate warming has already led to a global increase in the frequency and severity of fire weather, increasing the risks of wildfire. Wildfires not only pose direct threat to lives and properties but also release pollutants detrimental to human health and ecosystems (Figure 3). In addition, carbon dioxide released by wildfires exacerbates the greenhouse effect. Natural factors that trigger wildfires (e.g. lightning) are beyond human control. However, reducing greenhouse gas emissions to mitigate climate change impact could be under human's control. The threats of climate change on the human daily lives and the ecosystems are imminent. We must make every effort to combat climate change proactively.

Climate Change

June 2020

https://www.hko.gov.hk/en/education/climate/climate-change/00547-endless-wildfires.html

en

Oceans under Climate Change

Human activity has emitted huge amounts of greenhouse gases into the atmosphere, intensifying the greenhouse effect and affect marine ecosystems severely. This article will describe how climate change affecting the oceans.

Human activity has emitted huge amounts of greenhouse gases into the atmosphere, intensifying the greenhouse effect and causing more heat energy to accumulate on Earth. About 90% of the excess energy is absorbed by the oceans, causing the oceans to warm and severely affecting marine ecosystems. To cope with the warming environment, marine organisms need to migrate to cooler regions. Since the 1950s, marine species have been shifting poleward at an average rate of about 60 km per decade.Since the 1980s, marine heatwaves have roughly doubled in frequency and become longer and more intense, causing recent mass mortalities in coral reefs, kelp forests, seagrasses and mangroves. Corals have high ecological value as coral reefs provide food and habitat for a variety of sea life. However, ocean warming causes bleaching and even death of corals. The Great Barrier Reef, for example, underwent mass bleaching four times between 2016 and 2022.Ocean warming reduces the solubility of oxygen in seawater and strengthens upper ocean stratification, hence weakening the exchange of oxygen between the atmosphere and the ocean interior and reducing dissolved oxygen in the oceans. Ocean warming exacerbates deoxygenation in some regions, even leading to massive mortalities.Approximately a quarter of all anthropogenic carbon dioxide emissions has been absorbed by the surface ocean. As the surface seawater absorbs more carbon dioxide, its pH value drops. This is a process called ocean acidification. Since the pre-industrial era, the pH value of global surface waters has decreased by 0.1, representing about a 26% increase in acidity. Ocean acidification severely affects the calcification process of marine organisms which require calcium for the build-up of shells and structures.Sea level rise has accelerated due to thermal expansion of seawater and melting of glaciers and ice sheets on land. The average rate of global mean sea level rise has more than doubled between 1993 and 2022. Rising sea level impacts many coastal ecosystems, including mangroves and shallow coral reefs. It also makes low-lying coastal areas more prone to flooding. Freshwater for drinking and irrigation may be affected if seawater intrudes groundwater.Ocean warming favours the development of tropical cyclones. The global proportion of major tropical cyclone occurrence has increased with global warming. Tropical cyclones bring high winds, heavy rain and storm surges. Against the backdrop of rising mean sea level, the threat of storm surge to coastal areas increases accordingly.If global warming level exceeds 2°C by the end of this century, the risks of extinction, extirpation and ecosystem collapse are projected to escalate rapidly. Even if warming is limited to less than 2°C, coral reefs are still at risk of widespread decline, and marine biodiversity near the equator and in the Arctic is expected to decline. Climate change also alters many ecosystems in the oceans, impacting regional fisheries and the associated food supply.To mitigate the impacts of climate change on the oceans, countries around the world must promptly step up their efforts to reduce carbon emissions.

Climate Change

May 2023

https://www.hko.gov.hk/en/education/climate/climate-change/00695-Oceans-under-Climate-Change.html

en

Corals facing the double whammy brought about by climate change

Human activities have released a large amount of greenhouse gases into the atmosphere, enhancing the greenhouse effect and trapping extra heat on the Earth. More than 90 percent of the extra heat has been absorbed by the oceans, leading to ocean warming which, in turn, causing coral bleaching. The colours of corals originate from the symbiotic algae living in the coral structure. As sea water warms, corals tend to reject the symbiotic algae and turn themselves white.

Corals are of high value to ecosystems and human. Coral reefs provide food and habitat for marine creatures, supporting the associated ecosystems. Coral reef structure along the coastal waters buffers against the threat of high waves whipped up by winds and storms, protecting the lives and property of coastal communities. However, climate change caused by human activities has brought about a double whammy to corals.Human activities have released a large amount of greenhouse gases into the atmosphere, enhancing the greenhouse effect and trapping extra heat on the Earth. More than 90 percent of the extra heat has been absorbed by the oceans, leading to ocean warming which, in turn, causing coral bleaching. The colours of corals originate from the symbiotic algae living in the coral structure. As sea water warms, corals tend to reject the symbiotic algae and turn themselves white. Since the major food source for corals comes from the photosynthesis products generated by the symbiotic algae, bleaching corals will become fragile and more vulnerable to diseases, and may even ultimately lead to their demise.Besides, about 30 percent of the carbon dioxide emitted by human activities has been taken up by the oceans, leading to ocean acidification. The carbonate ions in more acidic sea water will be reduced, seriously affecting the calcification process and skeletal growth of corals.The World Meteorological Organization announced that the atmospheric concentration of carbon dioxide had soared to 403.3 ppm in 2016, the highest level in the last 800,000 years. With an unabated increase in greenhouse gases, global warming continues. The average frequency of coral bleaching events has increased by four times in the past 40 years. According to the assessment of the United Nations Educational, Scientific and Cultural Organization, all World Heritage coral reefs are likely to disappear by 2100 unless there is a drastic reduction in carbon dioxide emission.

Climate Change

April 2018

https://www.hko.gov.hk/en/education/climate/climate-change/00507-corals-facing-the-double-whammy-brought-about-by-climate-change.html

en

Will Permafrost Thawing Increase Global Warming?

Global warming is causing the permafrost thawing in Lena-Delta, Siberia, Russia.

Permafrost is any ground that stays frozen year-round. In some areas, permafrost exists for thousands of years. The Arctic is a big climate-sensitive carbon pool on Earth where the carbon storage is twice of that in the current atmosphere. As the permafrost thaws, organic matter decomposes and releases greenhouse gases, such as carbon dioxide and methane into the atmosphere. This causes a positive feedback that accelerates global warming.According to the Sixth Assessment Report published by the Intergovernmental Panel on Climate Change in 2021, greenhouse gases equivalent to 14-175 billion tonnes of carbon dioxide can be released from the permafrost per 1℃ increase in global temperature. Comparing to about 40 billion tonnes of carbon dioxide emitted by human activities in 2019, emissions released by the permafrost should not be taken lightly. Fortunately, computer model simulations as of today do not reveal any tipping points of runaway global warming caused by permafrost thawing. However, emissions released by permafrost would continue to increase with climate warming and such trend could last hundreds of years.According to Emissions Gap Report 2021 published by the United Nations, even if the unconditional emissions reduction pledges of countries worldwide are implemented, there is only 66% chance to limit global warming (relative to pre-industrial levels) to 2.7℃ by the end of the century, and 90% chance to limit warming to 3.3℃ only. The possible outcome of global warming by the end of this century is far above the 2℃ or 1.5℃ temperature targets in the Paris Agreement. To achieve the climate goals of the Paris Agreement, global effort to reduce emissions must be increased significantly.

Climate Change

April 2022

https://www.hko.gov.hk/en/education/climate/climate-change/00671-Will-Permafrost-Thawing-Increases-Global-Warming.html

en

Who has Stolen Our Permafrost Tundra?

In geology, permafrost is defined as soil that is at or below 0°C for at least two consecutive years. Some researchers speculated that the crater was formed due to the explosive release of methane gas that was trapped inside the icy permafrost.

In July 2014, a big crater of around 30 metres wide was discovered in the Yamal Peninsula in northern Russia (Figure 1)[1].  The crater was located in the permafrost tundra area within the Arctic region.  Permafrost tundra generally refers to treeless plain areas where the soil underneath is either permafrost or permanently frozen soil.  In geology, permafrost is defined as soil that is at or below 0°C for at least two consecutive years[2].  Some researchers speculated that the crater was formed due to the explosive release of methane gas that was trapped inside the icy permafrost.  As the permafrost warmed up and started melting, gas pressure caused by the building up of methane increased until it was high enough to displace the overlying soil, forming the crater.  More craters were found in the Yamal Peninsula and other regions in northern Russia[3] subsequently.Most permafrost is located at high latitudes near the Arctic and Antarctic regions.  It is also found at high altitudes (alpine permafrost) in lower latitude regions.  Permafrost occupies about 24% of the land in the Northern Hemisphere[4].  Permafrost distributes over vast areas of northern Russia, Canada, Alaska and Greenland.  In permafrost tundra, the vegetation is mainly composed of small shrubs, grasses, mosses and lichens.One severe threat to permafrost tundra is global warming.  When the permafrost melts, it releases methane and carbon dioxide, both of which are greenhouse gases.  According to the Fifth Assessment Report made by the Intergovernmental Panel on Climate Change (IPCC), there is high confidence that climate change is causing the permafrost warming and melting in high latitude regions and in high-elevation regions[5].  As a result, the permafrost tundra areas are shrinking in size, which has been observed in some regions like the northern Russia and Tibetan Plateau[6].  The permafrost melting has caused soil above to sink and become uneven, triggering land subsidence.  This has significantly affected buildings, bridges, roads and railways that are built on the permafrost (Figure 2).  In addition, as the permafrost continues to warm, shrubs are invading the tundra area, replacing the lichens and other tundra vegetation there.  In Alaska, lichens are an important food source for caribous and the loss of lichens can lead to the decline in the growth and abundance of these animals[7].  Though the reasons for the occurrence of the big crater in northern Russia are yet to be determined, scientists are worried about the amount of methane and carbon dioxide released to the atmosphere due to permafrost melting may further accelerate global warming, in particular methane is an even more powerful greenhouse gas compared to carbon dioxide.  The level of methane in the atmosphere has increased and exceeded the pre-industrial level by around 150% since 1750[8].  Currently, there are yet large uncertainties regarding the amount of methane and carbon dioxide released through this permafrost melting mechanism[9].  However, this rising trend has to be closely monitored taking into account the factor of permafrost melting.To enhance upper-air measurements, HKO acquired a portable upper-air sounding system (PUPAS) in late 2014. PUPAS comprises a small receiver, an amplifier, a processor, a sonde checker, antennas and a tripod. Each component can basically be fitted inside a suitcase (Figures 2 and 3). The system is easily transportable by one or two persons to collect meteorological data (Figure 4) at different locations or even on board of a ship, thus enhancing the mobility and the application of upper-air sounding.

Climate Change

April 2015

https://www.hko.gov.hk/en/education/weather/monsoons/00453-who-has-stolen-our-permafrost-tundra.html

en

Return Period: “Once in N Years”?

Understand the basic concept and estimation method of return period.

Sometimes we may hear such a description for an extreme weather event: “once in N years”. Does that really mean we would encounter such an event once every N years?What is return period?The number “N” actually refers to a statistical concept called “return period”. The calculation of return period is shown in the figure below (Figure 1): first, from the existing data, we calculate the probability of a certain event to happen within a certain period of time (rate of occurrence, μ) through statistical methods, and we take its reciprocal (1 ÷ μ). Then we would know the average time separation between occurrences of such event. This is called the return period.The choice of statistical methods would affect the estimation of return periods. Taking the fictitious rainfall data in Figure 1 as an example, the time series on the far left shows 2 occurrences of exceeding 100 mm within that 50 years. If we estimate the rate of occurrence in the simplest way, that would be once in every 25 years, so the return period would be 25 years; but if we first estimate the long-term theoretical rainfall distribution from the limited data, and represent it with a smooth curve before we carry out the estimation of the return period, we would get what is being shown in Figure 1: the return period is estimated to be 50.0 years.Furthermore, we should also note that geographical and climatic factors vary in different regions, so even for the same event, the occurrence rate as well as the corresponding return period in different regions can be very different.To correctly understand return periodWe should be careful and understand return period correctly: return period is a long-term average, an expected value derived from probability. Even if the return period is N years, it does not mean that the event would eventually occur within N years. In particular, the occurrence of extreme events is quite random, and the time between each occurrence may vary greatly: it may happen not even once in the N-year period, but it may also happen several times in another N-year period. It is just that: from a statistical perspective, it happens once in N years on average. Taking the fictitious rainfall data in Figure 1 as an example, the time series on the far left shows that hourly rainfall over 100 mm occurred in the 2nd year and the 20th year with an interval of 18 years, but such event did not occur again in at least the following 30 years. While our estimation of the return period is 50 years, we could only say that the first two occurrences being only 18 years apart is a coincidence.Although return period is not a guarantee of when an event would occur, it does provide an objective figure to show the rareness and extremeness of an event, which is useful for deciding engineering standards, risk management, and academic research. As a long-term average, return period is a representative figure. The longer the return period, the rarer and more extreme the event.The estimation of return period for extreme eventsThe rare occurrence of extreme events also poses a challenge on the estimation of its return period. As the data we used to estimate the rate of occurrence has a limited length of available period, the data for extreme events are prone to be insufficient, which would in turn affect the accuracy of the estimation. Some very extreme events may even have never happened since the start of record. In that case, we could only estimate by extrapolation from existing data through statistical methods, or by extracting the data of future predictions from numerical models. As the data for extreme events are few from the beginning, one or two additional extreme events may already have a large impact on the estimation results (See Figure 2 below). In other words, estimates for extreme events are very sensitive to new observations, so it is not surprising that the estimated return periods may change dramatically as more observations are made.Climate change will also lead to changes in return periodsThe background environment has been changing under climate change. Just as the old saying in the stock market goes, “past performance is no guide to future performance”. We could expect the return periods of various events would also change. If we use past data in our estimation, the estimated return periods would inevitably lag behind, affecting its representativeness. On the other hand, numerical climate models generally predict that extreme weather events would become more extreme and frequent in the future under climate change, implying shortening of return periods as time evolves.Extreme events are becoming more frequent, and this is a challenge that we all must be prepared to face in the midst of climate change. For more information on climate change, please visit our “Climate Change” webpage.

Climate Change

April 2022

https://www.hko.gov.hk/en/education/climate/climate-change/00672-Return-Period-Once-in-N-Years.html

en

Will the Earth Return to Ice Age Again?

Human influence on the climate is clear. Atmospheric greenhouse gases concentrations have increased rapidly since industrialisation began. As the single most important greenhouse gas in the atmosphere, carbon dioxide has reached concentration about 50% higher than preindustrial levels. A study even suggests that present-day atmospheric carbon dioxide level is unprecedented in the past 23 million years. Will the Earth enter a glacial period again and repeat the climate history?

As Winston Churchill said, “The longer you look back, the farther you can look forward”, climate change research relies on studying past climate in order to project future climate. Through reconstructing paleoclimate, we can examine how climate changed over longer time scales. Paleoclimate data can also be used to compare with simulations of climate models, hence aiding to improve model capability. Through modelling paleoclimate with climate models, we can investigate driving forces of past climate change, providing insights into future climate change. Since the 1950s, climate models have advanced substantially and have become a key tool in climate change research.Paleoclimate reconstructions indicate that the Earth’s temperature had been like a roller coaster going through cold (glacial) and warm (interglacial) periods (Figure 1). Scientists generally believe that the wax and wane of glacial and interglacial periods were due to changes in the Earth’s orbital geometry (Milankovitch Cycles), which led to variations in intensity and latitudinal distribution of incoming solar radiation. During the previous interglacial period, summer incoming solar radiation at Northern Hemisphere was 10% higher than present-day level and climate model simulations suggested that the Arctic summer was 5℃ warmer. As we are currently in an interglacial period, one may think that global warming of the last century is natural. However, can the warming of the 20th century be explained by natural factors?According to the IPCC Fifth Assessment Report, the mean global warming rate was about 0.3℃ to 0.8℃ per thousand years during the transition from the previous glacial period to the present interglacial period. However, instrumental data shows that the average rate of global temperature increase from 1880 to 2020 is 0.08℃ per decade. In other words, the warming rate over the last century is at least ten times faster than that during the inception of the present interglacial period. If the world continues on the high greenhouse gas concentration pathway, global temperature will be 3.7℃ higher than the average of 1986-2005 by the end of the 21st century, implying a temperature increasing rate about 50 times faster than that during the inception of the present interglacial period. Human influence on the climate is clear. Atmospheric greenhouse gases concentrations have increased rapidly since industrialisation began. As the single most important greenhouse gas in the atmosphere, carbon dioxide has reached concentration about 50% higher than preindustrial levels. A study even suggests that present-day atmospheric carbon dioxide level is unprecedented in the past 23 million years. Will the Earth enter a glacial period again and repeat the climate history? In fact, human-caused carbon emissions have postponed the next ice age by 100,000 years as pointed out by a study. Based on the above information, it is more likely that human is making new climate history. “Anthropocene” has become a popular scientific term to indicate the significant human impact on the establishment of a geological epoch.

Climate Change

April 2021

https://www.hko.gov.hk/en/education/climate/climate-change/00561-Will-the-Earth-Return-to-Ice-Age-Again.html

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Cold winter in 2009/2010 - Global cooling?

Whether the cold weather is inconsistent with global warming? Is heavy snowfall inconsistent with global warming? The ocean has a long memory. The warming of the atmosphere produced by human activities will reside in the memory of the ocean for a much longer time.

Just before the arrival of the Year of the Tiger (in early February 2010) when people from all over China packed the public transportation homeward for the new year reunion, there were severe snowstorms in many Chinese provinces including Shanxi, Hebei, Henan, Shandong, etc., causing 24 highways to be closed to traffic.  Across the Pacific Ocean, Washington, D.C. recorded a new record for snowfall in December 2009, paralyzing the city for a few days.  Many people living in the mid-latitudes complained of the bitter cold weather and snow not seen in many years.  So, is global warming giving way to global cooling?  Well, not quite. 2009 still ranks as one of the hottest 10 years on record globally.  So, how can we explain this paradox?There are two issues here - the cold weather and the severe snowstorms.  So, more specifically, the questions here are (i) "whether the cold weather is inconsistent with global warming", and (ii) "is heavy snowfall inconsistent with global warming?"First of all, it must be emphasized that global warming is about the long term trend of rising temperature for the earth as a whole.  So, it should never be mixed up with one or two individual weather events occurring in a certain part of the earth.The answer to the first question is explained by Arctic Oscillation - a climate pattern that determines the transport of cold air from the Arctic to the mid-latitudes.  In the winter of 2009/2010, the pressure pattern between the Arctic and the mid-latitudes was such that it favoured the cold Arctic air to intrude into the mid-latitudes, as clearly captured by the NASA satellite (MODIS) picture showing the warmer-than-usual Arctic and the colder-than-usual mid-latitudes.While the mid-latitudes and the sub-tropical regions might experience colder/cooler weather, the earth as a whole had not got colder.  Since global warming is about rise of temperature for the entire earth, there is actually no contradiction between the cold weather we experienced recently and global warming.  A vivid explanation about artic oscillation is also available in the YouTube video (in Chinese) produced by the Hong Kong Observatory. As for the second question - is heavy snowfall inconsistent with global warming, one has to understand that it requires two things to create heavy snowfall: moist air and cold air.  There is plenty of cold air in some areas because of the present state of Arctic oscillation.  But where does the moist air come from?  One obvious source is of course the ocean.  This winter, the entire South China Sea and the East China Sea are much warmer than usual, as clearly depicted in the NASA sea surface temperature map below:Here, red again represents warmer-than-usual sea water and vice versa for blue.  The warm seas provide moist air that rides over the denser and colder air, giving rise to snow in the mid-latitudes and rain in the south, like Hong Kong.  No wonder it was always rainy weather whenever cold air reached Hong Kong this winter.This condition is not only consistent with global warming, but it is likely to occur more frequently in some places because of global warming.  The ocean has a long memory.  The warming of the atmosphere produced by human activities will reside in the memory of the ocean for a much longer time.  Nowadays, on average, the oceans are warmer than they were decades ago.  So, chances are that in winter in the future, we can expect the warm oceans to feed warm moist air to the continent more frequently, waiting for the burst of cold air from the north to create the snowstorms that many places witnessed these past couple of months.

Climate Change

March 2010

https://www.hko.gov.hk/en/education/climate/climate-change/00268-cold-winter-in-20092010-global-cooling.html

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Can the warming of the 20th century be explained by natural factors?

Can the warming of the 20th century be explained by natural factors? The natural factors affecting climate include solar activity, volcanic activity, the Earth's orbital variation, etc.

Through a question and answer approach, the Climate Change FAQs will explain some basic knowledge and facts of climate change in layman terms in order to enhance the public's understanding of the causes of climate change, its impacts and what we can do to mitigate its effects.Q. Can the warming of the 20th century be explained by natural factors?A. The natural factors affecting climate include solar activity, volcanic activity, the Earth's orbital variation, etc.Solar activity causes changes in solar energy output and affects the Earth's energy balance and climate.Recent satellite observations confirmed solar irradiance has an 11-year cycle related to sunspots. However, there is no increasing trend in solar irradiance in the last few decades, while global temperatures have increased significantly. Therefore, solar activity is not the main cause of the climate warming in the 20th century. Actually since the Industrial Revolution, increased manmade greenhouse gases have far more impact on the climate change than the variation of the Sun's irradiance. Volcanic eruptions eject large amount of dust and suspended particulates high into the atmosphere, temporarily shielding the Earth, reflecting sunlight back to space. This will decrease the solar energy received by the Earth's surface, causing short-term climate cooling. The Earth's orbital variation brings itself closer or further away from the sun in periods of hundreds of thousands of years, which could be related to the past ice-ages and very-long-term changes in the climate. However, they do not have much impact on the climate change observed over the centennial time scale in the past century. Climate models cannot reproduce the warming observed in recent decades when only natural factors are considered. According to model simulation, we should have observed a decreasing trend in the global average temperature in the last few decades if only natural factors are considered, but we have observed a significant increasing trend in the global temperature. On the other hand, models can simulate the observed temperature changes in the 20th century when human factors, such as greenhouse gas emissions, are included. Therefore, it is very unlikely that the 20th century warming can be explained only by natural causes. Climate modeling results show that most of the global warming observed over the last 50 years is very likely due to human activities.

Climate Change

June 2012

https://www.hko.gov.hk/en/education/climate/climate-change/00277-can-the-warming-of-the-20th-century-be-explained-by-natural-factors.html

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New record low for Arctic sea ice

The long term decrease in Arctic sea ice could have complex effects on the climate of the Northern Hemisphere. Since sea ice is a very effective reflector to sunlight, declining summer Arctic sea ice will expose more sea surface of the ocean to solar radiation. As water is less reflective than ice, the ocean will absorb more solar energy, resulting in a vicious cycle of warming up of the sea water and further reduction of sea ice.

Recently, one of the hottest news in climate change blogosphere is on the new record low for the Arctic sea ice cover in late August 2012. The National Aeronautics and Space Administration (NASA)[1] and National Snow and Ice Data Center (NSIDC)[2 ] both have announced that the daily Arctic sea ice extent on 26 August 2012 shrunk to a new low of 4.10 million square kilometres, breaking the previous record set on 18 September 2007 (Figure 1). Against the background of global warming, the Arctic sea ice extent has been decreasing over the past few decades. Besides the shrinking in area, recent studies also indicated that the sea ice in Arctic is getting thinner with less multi-year ice. This would make the sea ice more prone to melting out in summer. The long term decrease in Arctic sea ice could have complex effects on the climate of the Northern Hemisphere. Since sea ice is a very effective reflector to sunlight, declining summer Arctic sea ice will expose more sea surface of the ocean to solar radiation. As water is less reflective than ice, the ocean will absorb more solar energy, resulting in a vicious cycle of warming up of the sea water and further reduction of sea ice (Figure 2). A warmer Arctic Ocean will also warm up the air and release more moisture to the atmosphere over the Arctic region. This would subsequently disturb the high and mid-latitude atmospheric circulation, increasing the chance of the occurrence of extreme weather events in both summer and winter in the mid-latitude regions of the Northern Hemisphere[3-4]. Since the Arctic sea ice usually reaches its annual minimum around mid-September, the sea ice extent is expected to drop further in September 2012 and the minimum extent for 2012 will likely be even lower than that of 26 August 2012 (Figure 3). This is indeed a worrying trend of global warming. The Fourth Assessment Report of Intergovernmental Panel on Climate Change released in 2007 predicted that the Arctic might become ice-free in summer by 2100. Taking into consideration of the recent trend, some modelling studies now suggest that the Arctic Ocean could be nearly ice-free in summer as early as 2030[5], much sooner than previously thought!

Climate Change

September 2012

https://www.hko.gov.hk/en/education/climate/climate-change/00275-new-record-low-for-arctic-sea-ice.html

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The Impacts of Climate Change on Human Health

This article describes how climate change affects human health. As the climate warms, heatwaves are becoming more frequent and more intense. The risks of heat-related illnesses such as dehydration, heatstroke, heat exhaustion and cardiovascular diseases also escalate as a result. Heatwaves can lead to hospitalizations and even deaths.

Climate change affects our physical and mental health, both directly and indirectly.As the climate warms, heatwaves are becoming more frequent and more intense. The risks of heat-related illnesses such as dehydration, heatstroke, heat exhaustion and cardiovascular diseases also escalate as a result. Heatwaves can lead to hospitalizations and even deaths. During 1998-2017, over 166,000 people died globally due to heatwaves. In the summer of 2022, more than 61,000 people in Europe died from heat-related causes. During 1980-2017, the 150 most populated cities on Earth experienced a 500% increase in exposure in extreme heat events. Moreover, studies have shown that heatwaves can lead to stress, anxiety, mood and behavioral disorders, and even suicide.Geographical regions suitable for the transmission of mosquito-borne diseases are expanding due to increasing global temperatures. According to the World Health Organization, the number of dengue fever cases increased significantly from 500,000 in 2000 to 5.2 million in 2019. Global warming is one of the factors contributing to the growth of global dengue fever burden since 1950. Malaria incidences are also observed to shift to higher altitudes as the climate warms.Anthropogenic climate change has led to an increase in the frequency and intensity of heavy rainfall over most land areas in recent decades. Rising temperatures and heavy rainfall increase the risks of diarrhoeal diseases such as cholera, particularly in areas with poor sanitation conditions. High air and water temperatures, as well as longer summer seasons increase the chances of food poisoning too.Under global warming, heatwaves, heavy rainfall, floods, and droughts are becoming more frequent and more severe, impacting agricultural productivity and freshwater availability. Ocean warming, acidification and deoxygenation affect fisheries. Declined food availability can lead to malnutrition and even famine.By 2050, it is projected that climate change-related health impacts such as heat, malnutrition, malaria, and diarrhea will cause an additional 250,000 annual deaths globally compared to the average of 1961-1990. If we can actively save energy and reduce greenhouse gas emissions in our daily lives, achieving global carbon neutrality as early as possible, the impacts of climate change on human health can be reduced.

Climate Change

May 2024

https://www.hko.gov.hk/en/education/climate/climate-change/00716-The-Impacts-of-Climate-Change-on-Human-Health.html

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Under the influence of global warming, are tropical cyclone activities changing?

Under the influence of global warming, are tropical cyclone activities changing? Tropical cyclone is one of the most destructive weather systems on Earth. The possible change in tropical cyclone activity in a changing climate is a matter of great concern to the public and decision makers.

Through a question and answer approach, the Climate Change FAQ will explain some basic knowledge and facts of climate change in layman terms in order to enhance the public's understanding of the causes of climate change, its impacts and what we can do to mitigate its effects.Q: Under the influence of global warming, are tropical cyclone activities changing?A: Tropical cyclone is one of the most destructive weather systems on Earth. The possible change in tropical cyclone activity in a changing climate is a matter of great concern to the public and decision makers. According to a study conducted by an expert team of the World Meteorological Organization, it remains uncertain whether the changes in tropical cyclone activities based on the records of different basins in the last century or so, have exceeded the natural variability. This is because the trend detection is complicated by the large fluctuations in the frequency and intensity of tropical cyclones and the limitations in the availability and quality of global historical records. Looking into the future, theory and climate model simulations suggested that, if 21st century warming occurs as projected, the global frequency of tropical cyclones is expected to either decrease or remain unchanged. There will be some increase in the mean maximum wind speed of the tropical cyclones, although increases may not occur in all regions. The rainfall rates associated with tropical cyclones are likely to increase too. In western North Pacific and the South China Sea (0-45oN, 100-180oE), analysis of the observational data shows that the annual number of tropical cyclones decreases from about 35 in the 1960s to about 27 after 2000. Locally, the annual number of tropical cyclones landing within 300 km of Hong Kong has decreased from about 3 in the 1960s to about 2.5 in the 2000s, but the trend is not statistically significant.

Climate Change

September 2012

https://www.hko.gov.hk/en/education/climate/climate-change/00276-under-the-influence-of-global-warming-are-tropical-cyclone-activities-changing.html

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Onset of Southwest Monsoon - End of the Fog Season and Start of the Rain Season

The basic driving force of the southwest monsoon is the large-scale differential heating between landmasses and oceans. Starting from April, the Sun gradually moves north and the amount of sunshine will increase in the northern hemisphere. The Asian continent will be heated up faster than its neighboring oceans. As air over the land becomes lighter and rises, a low pressure center will eventually form over the continent. The relatively warm and moist air from the ocean will then in general flow towards this low pressure centre, thus leading to the onset of the southwest monsoon.

Onset of southwest monsoon - end of the fog season and start of the rain season Southwest monsoon The basic driving force of the southwest monsoon is the large-scale differential heating between landmasses and oceans. Starting from April, the Sun gradually moves north and the amount of sunshine will increase in the northern hemisphere. The Asian continent will be heated up faster than its neighboring oceans. As air over the land becomes lighter and rises, a low pressure center will eventually form over the continent. The relatively warm and moist air from the ocean will then in general flow towards this low pressure centre, thus leading to the onset of the southwest monsoon. Fig. 1 shows the monthly mean of prevailing wind direction at Waglan Island between 1971-2000. It clearly indicates that the prevailing wind direction in Hong Kong veers from easterlies to southwesterlies between May and June. Fig. 1. Prevailing wind direction at Waglan Island (1971-2000). The end of the fog season The fog season in Hong Kong usually begins in February and ends in April. When warm moist maritime air moves over the relatively cool sea surface near the coast, air temperature will gradually fall to the dew point. Water vapour in the air will condense into very small water droplets which lead to the formation of fog (this is commonly called advection fog). Since the air is warmer than the sea water underneath, the atmosphere is relatively stable and helps confine the water vapour to the lowest level of the atmosphere. As the season proceeds, the sea water will also warm up gradually such that the atmosphere over the ocean will become more unstable. Hence, the onset of the southwest monsoon often marks the end of the fog season. Climatology shows that the Observatory rarely observes fog in May, on average less than 0.2 day in that month (Fig. 2). Fig. 2. Climatology of number of days with fog being observed at the Observatory (1971-2000). The start of the rain season The southwest monsoon will bring in warm moist air and the warming up of sea water will fuel the development of convective activities. Coupling with other favorable meteorological conditions like low level convergence or upper level divergence, the southwest monsoon could sometimes lead to heavy rain. Climatology shows that the amount of rainfall increases significantly in May (Fig. 3). This correlates well with the onset of the southwest monsoon. Fig. 3. Climatology of monthly mean rainfall (mm) recorded at the Observatory (1971-2000) Reference Meteorology Today: An Introduction to Weather, Climate and the Environment. Author: C. D. Ahrens. Brooks/Cole Pub Co. The basic driving force of the southwest monsoon is the large-scale differential heating between landmasses and oceans. Starting from April, the Sun gradually moves north and the amount of sunshine will increase in the northern hemisphere. The Asian continent will be heated up faster than its neighboring oceans. As air over the land becomes lighter and rises, a low pressure center will eventually form over the continent. The relatively warm and moist air from the ocean will then in general flow towards this low pressure centre, thus leading to the onset of the southwest monsoon. Fig. 1 shows the monthly mean of prevailing wind direction at Waglan Island between 1971-2000. It clearly indicates that the prevailing wind direction in Hong Kong veers from easterlies to southwesterlies between May and June.The fog season in Hong Kong usually begins in February and ends in April. When warm moist maritime air moves over the relatively cool sea surface near the coast, air temperature will gradually fall to the dew point. Water vapour in the air will condense into very small water droplets which lead to the formation of fog (this is commonly called advection fog). Since the air is warmer than the sea water underneath, the atmosphere is relatively stable and helps confine the water vapour to the lowest level of the atmosphere. As the season proceeds, the sea water will also warm up gradually such that the atmosphere over the ocean will become more unstable. Hence, the onset of the southwest monsoon often marks the end of the fog season. Climatology shows that the Observatory rarely observes fog in May, on average less than 0.2 day in that month (Fig. 2).The southwest monsoon will bring in warm moist air and the warming up of sea water will fuel the development of convective activities. Coupling with other favorable meteorological conditions like low level convergence or upper level divergence, the southwest monsoon could sometimes lead to heavy rain. Climatology shows that the amount of rainfall increases significantly in May (Fig. 3). This correlates well with the onset of the southwest monsoon.

Climate Change

https://www.hko.gov.hk/en/education/climate/general-climatology/00261-onset-of-southwest-monsoon-end-of-the-fog-season-and-start-of-the-rain-season.html

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The monsoons and climate change

If research work on the subject is any guide, the relationship between climate change and the monsoons are anything but simple.

People asked us what climate change may mean for the Asian monsoons. Well, if research work on the subject is any guide, the relationship between climate change and the monsoons are anything but simple.First, what causes the monsoons in East Asia? It is all because of the huge land mass there. In the blog of 30 April 2010, it was explained that among all materials, water is practically the most difficult to heat up. So, during summertime with plenty of sunshine, the air over the Asian land mass heats up faster than that over the ocean. This creates a temperature difference and brings a southwest wind from the ocean towards the land. This is more or less how the summer monsoon comes about, although in East Asia, the peculiar topography of the Himalayas and the Tibetan plateau has an influence on it.The opposite happens in the case of winter monsoon. As the cool season progress, the Sun moves south and the amount of sunshine decreases. The air over the Asian land mass cools down faster than that over the ocean. The resulting temperature difference brings a northeast monsoon from the land towards the ocean. To illustrate how the air temperature changes over land and ocean during the year, let us compare the monthly average air temperature of Hong Kong with that of Honolulu (Hawaii) over the North Pacific. The two places have similar latitudes, but Honolulu being an island in the Pacific is strongly influenced by the ocean all year round. From Figure 3, one can see that Honolulu's temperature changes much less than Hong Kong. Tracking of air arriving at Hong Kong gives us an idea of where the air comes from. Figures 4 and 5 below present these tracks for July 2010 and December 2009 respectively. One can see that most of these were from the southwest in summer and from the northeast in winter. Studies show that both the summer and winter monsoons affecting China have weakened over the past few decades. Scientists think that there are several reasons for such a decrease: global warming, regional and global changes in atmospheric circulation, changes in solar radiation, as well as human-induced aerosols (i.e. tiny particles suspended in the atmosphere, generated by human activities such as burning and use of fossil fuel).In the context of global warming, the last few decades saw more pronounced warming over land than over ocean, particularly for high-latitude regions. The reduced temperature contrast between land and sea gives a weaker winter monsoon. Explaining the weakening of the summer monsoon is a little more complicated. This hinges on the summer cooling observed in central China over the past few decades. Such cooling may be linked to an increase in human-induced aerosols which reduces the incoming sunshine. The reduced temperature contrast between land and sea results in a weaker summer monsoon, which in turn means the southwest wind may not reach as far north as in the past. The end result is the shifting of the summer rain belt from north to south. More rain in central China translates into a further cooling of the land surface, and subsequently an even weaker monsoon. 

Climate Change

October 2010

https://www.hko.gov.hk/en/education/weather/monsoons/00072-the-monsoons-and-climate-change.html

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Is the Melting of Glaciers in West Antarctica Unstoppable?

Is the melting of glaciers in West Antarctica unstoppable? sing satellite data over the past two decades, researchers found that the grounding lines of the glaciers were all retreating inland rapidly, probably caused by warmer ocean currents under the floating ice.

In May 2014, scientists claimed that the glacier loss in the Amundsen Sea sector of West Antarctica had passed the point of no return (see blog Point of No Return), implying that the glaciers would eventually melt away.The glaciers of concern sit on bedrock lying below sea level and flow towards the ocean across the grounding line, at which point they become floating ice shelf over the sea water. Using satellite data over the past two decades, researchers found that the grounding lines of the glaciers were all retreating inland rapidly, probably caused by warmer ocean currents under the floating ice. With the glacier beds below sea level and sloping downward in the inland direction, the more the grounding line retreats, the more vulnerable the ice becomes to the erosive effect caused by the incursion of warm ocean currents. Normally, ice outflow to the ocean increases with ice thickness at the grounding line. Here, the ice thickness at the grounding line increases as the latter continues to retreat, implying more ice outflow to the ocean. The only way to stop the vicious cycle of retreating grounding line and melting glaciers is to have some high obstructions rising from the bedrock at some point, which is exactly the natural blockage currently lacking in the landscape of West Antarctica.

Climate Change

September 2014

https://www.hko.gov.hk/en/education/climate/climate-change/00442-is-the-melting-of-glaciers-in-west-antarctica-unstoppable.html

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Milankovitch Cycles

In the early 20th century, Milutin Milankovitch, a Serbian astronomer, proposed that the coming and going of ice ages on Earth were closely related to three orbital geometric parameters of Earth's revolution around the Sun.

In the early 20th century, Milutin Milankovitch, a Serbian astronomer, proposed that the coming and going of ice ages on Earth were closely related to three orbital geometric parameters of Earth's revolution around the Sun. The first parameter is the shape of Earth's orbit around the Sun. The orbit changes from nearly circular to elliptical in a periodical manner and the whole cycle takes about 100,000 years. The orbital changes will affect the amount of solar energy reaching the Earth in different seasons.The second parameter is the tilt of Earth's rotational axis. The axial tilt varies between 22.1 and 24.5 degrees in a cycle of around 40,000 years. Changes in this parameter will not alter the total amount of solar energy reaching the Earth but will affect the latitudinal distribution of insolation.The third parameter is the precession of Earth's rotational axis, i.e. the wobbling of Earth's axis. A full cycle of the wobbling takes about 26,000 years. Changes in this parameter will also affect the latitudinal distribution of insolation.According to Milankovitch, the impact of these parameters on the amount of insolation reaching the high latitudes in the Northern Hemisphere, where most of the ice and snow on Earth are found, is particularly important. Variation in ice and snow cover can lead to a positive feedback mechanism. For example, when the amount of insolation reaching the northern high latitudes decreases, summer heat is not sufficient to melt all the ice and snow precipitated in the preceding winter, leading to an overall increase of ice and snow in the course of the year. Increasing ice and snow will reflect more sunlight back into space, thereby reducing the amount of heat absorbed on Earth. This will set up a viscous cycle that supports further growth of ice and snow. Persistent increase in ice and snow year after year will eventually push the Earth into an ice age.Milankovitch's theory was finally accepted in the late 20th century after thorough examinations by scientists.

Climate Change

April 2017

https://www.hko.gov.hk/en/education/climate/climate-change/00490-milankovitch-cycles.html

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Where is the coldest place in Hong Kong on average? Why?

Where is the coldest place in Hong Kong on average? Why doesn't the air temperature at hilltop become close to that on the ground level?

We know that air temperature varies in different seasons and at different time of the day, and its variation depends on a number of factors such as cloudiness. Yet if we put these factors aside and simply look at the yearly average, do you know where in Hong Kong is the coldest? The answer can be found in the Observatory's "Regional Climate of Hong Kong" web page. Figure 1 depicts the average annual air temperature of Hong Kong at different locations. We can see that the average annual temperature recorded by the automatic weather station at Tai Mo Shan is about 6℃ lower than most other places. Besides, average temperatures at Ngong Ping and The Peak are also significantly lower. What do these places have in common that make them so special?You should have realized that the automatic weather stations at Tai Mo Shan, Ngong Ping and The Peak are all located near hilltop (with altitudes of 955m, 593m and 406m respectively). As explained in the article "The higher you climb, the colder it gets", temperatures at higher locations are generally lower than those below. This is because air mostly does not absorb sensible heat directly from the sun. Instead, the ground first absorbs the solar energy and is heated up. The ground in turn heats up the air just above it. The air becomes warmer and lighter, and moves up in the atmosphere. In this way, energy from the sun is transferred from the ground upwards. However, you may then ask the following question: "We all know that air keeps flowing and warm air tends to rise. When the warm air is transported from the ground to the hilltop, why doesn't the air temperature at hilltop become close to that on the ground level?"To answer this question, one should first know that the air pressure decreases with height (see "Introduction to Air Pressure" for details). When we are taking an elevator in a skyscraper, we may sense discomfort in our ears. This is because our eardrums feel the drop in air pressure as we move up. When hot air is raised from the ground to a higher level, it will expand as the surrounding air pressure is lower. It is a physical phenomenon that as air expands, its temperature drops.  In this way, the air reaching a higher level will be at a lower temperature than that of the air near the ground (Figure 2).In everyday life, you may have experienced the opposite phenomenon, namely, increase in air pressure results in higher temperature. If you try to pump a bike tire, you are sending in air into the tire making the pressure inside the tire increase. After you finish pump, try to feel the temperature of the tire. You should feel that the tire has become warmer.Therefore, even if warm air near the ground rises to the hill, the drop in pressure would cause the air to expand and cool down. As a result, temperatures on the hilltop are lower than those on the ground most of the time.

Climatological Information of Hong Kong

December 2012

https://www.hko.gov.hk/en/education/climate/climatological-information-of-hong-kong/00245-where-is-the-coldest-place-in-hong-kong-on-average-why.html

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Record-breaking Summer Temperatures in Hong Kong

Why do the summer temperature records get broken so easily? The summer mean temperature in Hong Kong exhibited a significant rising trend in the last hundred years or so.

In 2015, Hong Kong recorded a summer (June – August) mean temperature of 29.4 degrees, the highest since 1884 and breaking the previous record set only last year in 2014. Why do the summer temperature records get broken so easily?The summer mean temperature in Hong Kong exhibited a significant rising trend in the last hundred years or so. In the early 20th century (1901-1930), summer mean temperature ranged between 26.8 and 28.4 degrees (blue bars in Figure 1). However, summer mean temperatures have risen to a range between 27.8 and 29.4 degrees (red bars in Figure 1) in the past three decades (1986-2015). In other words, the distribution of summer mean temperatures has apparently shifted towards the high side by about one degree. While summer mean temperatures never exceeded 29.0 degrees in the early 20th century, it has become quite common to see such high summer mean temperatures nowadays. Naturally, record-breaking temperatures would occur more easily against the backdrop of a hotter summer as a result of a warming climate. The situation is like rolling a loaded dice in which the probability of getting a '6' has been deliberately increased. The world is moving towards the trajectory of a high greenhouse gas concentration scenario. If no effective mitigation measures are taken to reduce greenhouse gas emissions, we can expect more record-breaking summer temperatures in the future.

Climatological Information of Hong Kong

October 2015

https://www.hko.gov.hk/en/education/climate/climatological-information-of-hong-kong/00471-recordbreaking-summer-temperatures-in-hong-kong.html

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