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Thunderbolt Hood

If you are driving and encountering thunderstorm and lightning in the countryside, do you think it is safer to stay inside the car? What is Faraday Cage effect?

As you are aware, lightning strike is a natural phenomenon. A strong flash of light in the dark sky creates spectacular visual contrast. If you look at it from an aesthetic point of view, it is stunning. Modern Chinese writer and poet, Guo Moruo, also expressed the power of thunder and lightning in his Ode to Thunder and Lightning. In reality, people are scary about thunder and lightning because they could get injured or even killed if being hit. In fact, most of the lightning strikes on people could be prevented. A thunderstorm occurs when the electric charges in a cumulonimbus cloud accumulate and the potential difference between cloud and ground reaches a certain level, leading to a cloud-to-ground discharge. This is the lightning phenomenon we see. When you see lightning, the thunderstorm could be very close to you. Temperatures of the air near the lightning discharge path increase suddenly and hence the volume of air expands sharply, resulting in shock waves which bring about the loud thunder. Deriving from the speed of propagation of sound waves in air, if you happen to hear thunder 3 seconds after seeing a lightning strike, it indicates that the thunderstorm is about 1 kilometre away from you. If you are in an exposed area, you should find a safe place to shelter as soon as possible, but you should refrain from staying under a tree or near a mast as lightning tends to strike tall objects. You are more likely to be hit by lightning strike if you stand close to these objects. If you are driving and encountering thunderstorm and lightning in the countryside, do you think it is safer to stay inside the car?In the event of a thunderstorm, the safest way is to take shelter indoors. However, if you stay in the car, close the doors and windows tightly, and lower any external antenna to avoid attracting lightning, this can also prevent you from getting struck by lightning. It is because even if the vehicle is struck by lightning, the car case can act as a shield, and the current will flow through the car case and tires and then to the ground. The current will not pass through the people in the vehicle. What is the principle to keep the people safe in a metal case then? This is mainly due to the Faraday Cage effect.A "Faraday cage" is a cage made of metal or a good electrical conductor. An English physicist invented the cage which was named after his last name. In 1836, he discovered that the excess charges on a charged electrical conductor (such as metal) exist only on its surface (i.e., the outer case) and do not affect the objects enclosed by it. Therefore, the case of the electrical conductor can "protect" its interior. When driving in a thunderstorm and if the car is struck by lightning, due to the "Faraday cage" effect, the current of the lightning strike will flow through the metal case, the axles and the wet tires of the car to the ground. Therefore, when encountering thunderstorm and lightning while driving, it is basically safe to stay in the car. It should be noted that when a car is struck by lightning, the transient high-voltage electricity may damage the electronic equipment in the car and make it malfunction. This situation is similar to the case when a lightning strike hits an aircraft. If the safety conditions permit, it is best to park the car in a safe place by the road, away from trees and masts, turn off the engine and electronic audio system, close all windows and doors, and not to touch the window handles and others. Of course, before going out, you should take note of the weather information issued by the Observatory, and avoid outdoor activities in case of severe weather.

Thunderstorm and Lightning

March 2020

https://www.hko.gov.hk/en/education/weather/thunderstorm-and-lightning/00544-Thunderbolt-Hood.html

en

Why does a Thunder Sound the Way It does?

Thunder is the sound generated by lightning.The electric current in a lightning bolt heats the surrounding air to more than 20,000 degrees Celsius within a few milliseconds. This is even hotter than the surface of the sun. The sudden heating causes the surrounding air to expand violently and creates big sound waves, like what happens in a bomb explosion, along the entire length of the lightning bolt.

In this series, we will explore the science of thunder and lightning from different angles. To start with, let us talk about the peculiar sound of thunder.Thunder is the sound generated by lightning. The electric current in a lightning bolt heats the surrounding air to more than 20,000 degrees Celsius within a few milliseconds. This is even hotter than the surface of the sun. The sudden heating causes the surrounding air to expand violently and creates big sound waves, like what happens in a bomb explosion, along the entire length of the lightning bolt.We usually hear a loud boom followed by several seconds of rumble. The first loud sound comes from the part of the main lightning bolt closest to us and the following rumbles come from those that are farther away. The sound may bounce off the low hanging clouds, nearby hills or buildings to create a series of low and continuous echoes. Lightning also occurs inside a cloud. Since this type of lightning tends to be less powerful than cloud-to-ground lightning and much higher up in the sky, the sound of thunder is generally softer. 

Thunderstorm and Lightning

https://www.hko.gov.hk/en/education/weather/thunderstorm-and-lightning/00023-why-does-a-thunder-sound-the-way-it-does.html

en

Why Lightning Looks the Way It does - Crooked and Forked?

The propagation of the lightning channel follows a path of the least resistance, which is not a straight but a zig-zag line.

In the first episode we explored why thunder roars.  As thunder is always accompanied by lightning, lets now take a look at the spectacular display of lightning.Lightning is a release of electric charges from a cloud to the air and then to the ground.  Air itself is an electrical insulator but the abundance of charges inside the cloud can induce a very high voltage across the cloud and the ground.  This causes the air to become ionized and conductive.  A channel of ionized air emerges from the cloud at first and propagates towards the ground in steps, each of typically tens of meters long.  The propagation of the lightning channel follows a path of the least resistance, which is not a straight but a zig-zag line because impurity or moisture in the atmosphere causes the air to be ionized in different directions.  Sometimes the lightning channel finds more than one conductive path and branches out, giving lightning its forked appearance.Among all the branches of the lightning channel, the one which first reaches the ground establishes a conductive path all the way from the cloud to the ground, as if there is a very long metal rod connecting them together.  There will then be a sudden and massive flow of electric current between the cloud and the ground and all the branches of the lightning channel will be lit up, giving a bright flash across the sky.

Thunderstorm and Lightning

https://www.hko.gov.hk/en/education/weather/thunderstorm-and-lightning/00022-why-lightning-looks-the-way-it-does-crooked-and-forked.html

en

Why does Lightning always Come before Thunder?

A person on the ground sees the lightning flash before hearing the thunder because light at a speed of around 300,000,000 meters per second travels much faster than sound which moves at 340 meters per second. If one is 1,000 meters away from the thunderstorm, he/she would see the flash almost instantly after lightning occurs as it takes just a few microseconds, while the thunder arrives only after about 3 seconds (1,000 meters divided by 340 meters per second).

The God of Thunder was empowered by the Jade Emperor to punish bad people on the Earth.  Once, the God of Thunder mistakenly killed a kind-hearted woman.  The Jade Emperor, having looked into the matter, raised the woman from death and named her the Goddess of Lightning.  He also dictated that whenever the God of Thunder stroke the Earth, he must let the Goddess of Lightning release light first to distinguish the right from wrong and prevent injustice.  The above is just a Chinese legend.  In nature, a lightning flash and the associated thunder occur at almost the same time in a thunderstorm.  A person on the ground sees the lightning flash before hearing the thunder because light at a speed of around 300,000,000 meters per second travels much faster than sound which moves at 340 meters per second.  If one is 1,000 meters away from the thunderstorm, he/she would see the flash almost instantly after lightning occurs as it takes just a few microseconds, while the thunder arrives only after about 3 seconds (1,000 meters divided by 340 meters per second).As a rule of thumb, by counting the seconds between the flash and the thunder and dividing the number by 3, you can estimate your distance from the thunderstorm in kilometers.  For example, if you hear the thunder 9 seconds after seeing the flash, the thunderstorm should be about 3 kilometers away from you.  If you see a flash and hear a thunder clap almost simultaneously, the storm must be very near you. Seek shelter immediately.Lightning always comes with thunder.  If you see a lightning flash but cannot hear any thunder, the thunderstorm is most likely be quite far away from you.  The thunder clap must have been deflected up the sky.  The longer the sound travels, the more it will be deflected away from the ground.  Typically, thunder is seldom heard beyond 15 kilometers from the storm.

Thunderstorm and Lightning

https://www.hko.gov.hk/en/education/weather/thunderstorm-and-lightning/00021-why-does-lightning-always-come-before-thunder.html

en

Why We Cannot Hear the Thunder of a Distant Lightning?

Since air temperature decreases with height, the speed of sound also decreases with height. This means that for a sound wave traveling close to the ground, the part of the wave closest to the ground is traveling the fastest, while the part farthest above the ground is traveling the slowest. As a result, the wave changes the direction it propagates and bends upwards. The longer it travels, the more it will be refracted away from the ground. Typically, thunder is seldom heard by a ground observer beyond 15 kilometres from the storm.

In the last episode, we mentioned that one might see a lightning flash but would not hear any thunder if the storm was far away.  Let's take a look why this is so.Air is the medium for sound to propagate (you hear no sound from a vacuum).  The speed of sound in air depends on several factors including the air temperature.  Usually, sound travels faster in warm air than in cool air.For most of the time, the air near the ground is warmer than the air above (please refer to the article "The higher you climb, the colder it gets" for details).  Since air temperature decreases with height, the speed of sound also decreases with height.  This means that for a sound wave traveling close to the ground, the part of the wave closest to the ground is traveling the fastest, while the part farthest above the ground is traveling the slowest.  As a result, the wave changes the direction it propagates and bends upwards.  The longer it travels, the more it will be refracted away from the ground.  Typically, thunder is seldom heard by a ground observer beyond 15 kilometres from the storm.In the lower atmosphere, sometimes an inversion exists, i.e. the temperature increases with height. The sound of thunderstorm will then be refracted downward towards the ground. In some cases it can be rather focused, leading to an intense clap or several claps of thunder.

Thunderstorm and Lightning

https://www.hko.gov.hk/en/education/weather/thunderstorm-and-lightning/00020-why-we-cannot-hear-the-thunder-of-a-distant-lightning.html

en

What is a dry thunderstorm?

Dry thunderstorm refers to thunder and lightning that occur without bringing rain to the ground.

Thunderstorms are usually accompanied by rain.  However, not all thunderstorms are wet and this seemingly weird weather phenomenon is called dry thunderstorm.  Let's see how this happens.Dry thunderstorm refers to thunder and lightning that occur without bringing rain to the ground.  In fact, the thunder-bearing clouds do produce rain but the rain droplets have evaporated in the air before reaching the ground.  This is possible if the clouds are sufficiently high and the humidity of air between the clouds and the ground is low enough.  One example is lightning originating from the spreading top of a cumulonimbus cloud, which is often called the anvil cloud and is distant from the main body of thunderstorm where rain pours heavily.  The anvil cloud is so high that rain coming from it evaporates before reaching the ground.  However, lightning from the anvil cloud is capable of traveling through the dry air and reaching the ground.Dry thunderstorm is very dangerous to people on the ground as one may not be aware of the sudden occurrence of lightning in a rain-free area.  It is also the culprit of many wildfires.  Without any rain to wet the grasses and trees, dry thunderstorm can trigger a fire more easily and the flames can spread more quickly.

Thunderstorm and Lightning

https://www.hko.gov.hk/en/education/weather/thunderstorm-and-lightning/00019-what-is-a-dry-thunderstorm.html

en

What is cloud-to-cloud lightning?

Cloud-to-cloud lightning is the discharge between areas of cloud without the discharge channel reaching the ground. For most of the time, it occurs between oppositely charged portions of the same cloud but sometimes it takes place between two separate clouds. Since the discharge channel of cloud-to-cloud lightning may be obscured by the cloud, it may or may not be visible to an observer on the ground.

Lightning strokes are classified into different types according to their own characteristics.  The two most common types are cloud-to-ground lightning and cloud-to-cloud lightning.  Cloud-to-ground lightning is lightning discharge between a cumulonimbus cloud and the ground.  Of all types of lightning, cloud-to-ground lightning poses the greatest threat to people and facilities on the ground.Cloud-to-cloud lightning is the discharge between areas of cloud without the discharge channel reaching the ground.  For most of the time, it occurs between oppositely charged portions of the same cloud but sometimes it takes place between two separate clouds.  Since the discharge channel of cloud-to-cloud lightning may be obscured by the cloud, it may or may not be visible to an observer on the ground.  One may therefore be surprised to learn that cloud-to-cloud lightning is, in fact, the most frequently occurring type of lightning.  Roughly speaking, it occurs around ten times more often than cloud-to-ground lightning.Obviously, cloud-to-cloud lightning does not pose a threat to life and property on the ground.  However, this type of lightning takes place in the sky and sometimes passes through the clear air between clouds, therefore it is of great practical interest to those concerned with the safety of aircraft.

Thunderstorm and Lightning

September 2009

https://www.hko.gov.hk/en/education/weather/thunderstorm-and-lightning/00018-what-is-cloudtocloud-lightning.html

en

How to detect lightning?

We can confirm the occurrence of lightning through the detection of this electromagnetic flux, like the reception of radio broadcast. If we have several electromagnetic flux detectors in different locations, the detected direction and time of arrival of the electromagnetic flux generated from the same lightning stroke will be different for different detectors. Upon collecting these data, we can carry out computation using a computer to derive the location and time of occurrence of the lightning stroke.

Even in advanced societies today, lightning still poses serious threats to lives and properties.  Whether these are the operations or systems in the airport, or those related to power transmissions and communication networks, they are prone to damages by lightning which will bring inconvenience and even hazard to our daily lives.  If we are able to detect promptly the occurrence of lightning and inform the public and relevant organizations in a timely manner, it should greatly reduce the chance of accidents and property losses.  Hence, it boils down to the question of how we can accurately detect lightning?Apart from light and sound, lightning also generates a strong electromagnetic field.  The situation can be visualized as a long electric cable extending from the thunderclouds to the earth surface through which a huge current passes through this cable transiently.  From the basics of electromagnetism, this will generate a very large electromagnetic flux radiating outwards at the speed of light.  Thus, we can confirm the occurrence of lightning through the detection of this electromagnetic flux, like the reception of radio broadcast.  If we have several electromagnetic flux detectors in different locations, the detected direction and time of arrival of the electromagnetic flux generated from the same lightning stroke will be different for different detectors.  Upon collecting these data, we can carry out computation using a computer to derive the location and time of occurrence of the lightning stroke.  Details of the algorithms of the Lightning Location Information System will be discussed in the next issue of the e-bulletin.

Thunderstorm and Lightning

December 2009

https://www.hko.gov.hk/en/education/weather/thunderstorm-and-lightning/00017-how-to-detect-lightning.html

en

Observe the Observatory to Prevent Lightning Strokes

You may refer to the Observatory's Hong Kong Lightning Nowcast service to keep track of thunderstorm development.With the launch of this service, those engaging in outdoor activities are encouraged to refer more often to the Observatory's location specific lightning nowcast. For instance, before hiking one may first draw up a contingency plan with exit routes, then watch out for weather changes and regularly review the chance of thunderstorms in the next couple of hours during the trip. If necessary, he/she may alter the route or cancel the trip immediately to avoid accidents.

In a stormy murky night, out of nowhere an intense flash pierces through the sky, swiftly meandering and splitting into innumerable graceful branches, with tips forming over tips, to paint a magnificent drawing. A short while later comes a deafening blast. Rumble! Rumble! Sleeping beauties are all but awakened. This is lightning. This is thunderstorm.Lightning is a natural wonder, but is also a fast gunman who kills in seconds. Occasionally somebody in Hong Kong gets injured due to lightning strokes. The most effective preventive measure is to seek shelter indoors. It is thus imperative for those who work or have activities outdoor to know when thunderstorm may strike.In addition to looking out for Thunderstorm Warning and using Location-Specific Lightning Alert Webpage,you may now also refer to the Observatory's new "Hong Kong Lightning Nowcast" service to keep track of thunderstorm development. It makes use of the direction and speed of areas of thundery showers derived from "Short-range Warning of Intense Rainstorms in Localized System (SWIRLS)" along with real-time lightning location data collected by "Lightning Location Information System (LLIS)" to predict where lightning strikes. Through blending observations with forecasts, this service provides lightning nowcast up to one hour ahead at user's actual or selected location, enabling the public to get hold of the development and movement of thundery showers for better precaution.With the launch of this service, those engaging in outdoor activities are encouraged to refer more often to the Observatory's location specific lightning nowcast. For instance, before hiking one may first draw up a contingency plan with exit routes, then watch out for weather changes and regularly review the chance of thunderstorms in the next couple of hours during the trip. If necessary, he/she may alter the route or cancel the trip immediately to avoid accidents.To use "Hong Kong Lightning Nowcast" service, one can access the Observatory's mobile website, and then choose "Hong Kong Lightning Nowcast" under "Weather at Your Location" (Figure 1). Alternatively, one may also choose "Automatic Regional Weather Forecast" on the Observatory's website and click on the "lightning" icon (Figure 2). Lightning icons indicate whether there will be lightning within the designated 30-minute period of forecast. A red lightning icon indicates lightning within 10 km, while an amber icon indicates lightning within 15 km. Icons in the second 30-minute forecast period are depicted in hollow shapes to indicate lower certainty of forecast. You may also watch the related "Cool Met Stuff" episode (Chinese only) to better understand the usage of this product.

Thunderstorm and Lightning

August 2017

https://www.hko.gov.hk/en/education/weather/thunderstorm-and-lightning/00495-observe-the-observatory-to-prevent-lightning-strokes.html

en

How to determine the lightning location?

Lightning location information systems make use of the direction finding technology and time-of-arrival technology. When a lightning detector receives electromagnetic flux from lightning, it will automatically compute the direction and arrival time of the electromagnetic flux. Such information including the amplitude and polarity of the signals as well as the station code will be transmitted to the central computer for further calculation.

In 1970s and 80s, electronic technology advanced rapidly.  At that time, scientists were already capable of using electronic technology to detect the location of lightning discharge from cloud to ground.  There were essentially two common detection methods.  The first method was called direction finding technology, whereas the second one was called time-of-arrival technology.  With the implementation and wide applications of the global positioning satellite system and high speed microprocessors in late 1990s, new lightning location information systems were developed making use of the above two methods together, thereby greatly enhancing the detection efficiency and accuracy.As mentioned in the previous chapter, when a lightning detector receives electromagnetic flux from lightning, it will automatically compute the direction and arrival time of the electromagnetic flux.  Such information including the amplitude and polarity of the signals as well as the station code will be transmitted to the central computer for further calculation.  At the database of the central computer, the precise geographical positions of all stations are already stored for detailed analysis.  To trigger the analysis of lightning location, the minimum requirement is that there are at least valid data from two different stations received by the central computer within a few seconds.  Hence, the more data from different stations the central computer receives, the more accurate the location and time of occurrence of lightning can be determined.Stations closer to the location of lightning will receive the electromagnetic flux from the lightning earlier.  The central computer will compute the location and time of occurrence of lightning according to the direction and time of reception of the signals detected at different stations.  The computation process employs an iterative algorithm for calculating the precise location of the lightning.  The computation is principally based on trigonometry (please see Figure 1).  The distance of the lightning from each station is estimated from the time difference of reception of the electromagnetic flux at different stations.  After that, we can then draw a circle for each station with the estimated distance as the radius.  When all circles intersect closely at a point and the direction of that point from each station is consistent with the direction of the electromagnetic flux received at the station, the location of the lightning is then determined.

Thunderstorm and Lightning

March 2010

https://www.hko.gov.hk/en/education/weather/thunderstorm-and-lightning/00016-how-to-determine-the-lightning-location.html

en

Don't be a Lightning Rod

In thunderstorm situations, the lightning rod will attract nearby lightning and lead the electric current into the earth, hence protecting the building against lightning strokes. When thunderstorms occur, do not be a lightning rod. Do not stand in exposed areas, or under tall trees or poles, to avoid becoming a lightning rod that attracts lightning strikes.

The ice crystals inside a cumulonimbus cloud will collide with each other under vigorous turbulent flow and become electrically charged.  In general, the upper portion of the cloud is positively charged, while the middle and the lower parts are negatively charged.  The earth's surface beneath the cumulonimbus cloud will then be induced with positive charges.  When the electric voltage between the positive and negative charges rises to a certain extent, discharge will take place between clouds or between cloud and the earth's surface.  This is how lightning occurs.The distribution of electrical charges on the earth's surface depends on its topography.  More charges will be accumulated on a curved surface than a flat plain (see figure 1).  A tall building itself will be of the highest curvature on the ground.  When a cumulonimbus cloud gets close, more charges will be induced and accumulated on the building, increasing the chance of discharge.  Hence, lightning will tend to strike tall building.The same principle applies to the operation of a lightning rod, which is a metallic rod installed on the rooftop of a building with its bottom extending below the ground.  In thunderstorm situations, the lightning rod will attract nearby lightning and lead the electric current into the earth, hence protecting the building against lightning strokes. When thunderstorms occur, do not be a lightning rod.  Do not stand in exposed areas, or under tall trees or poles, to avoid becoming a lightning rod that attracts lightning strikes.  

Thunderstorm and Lightning

June 2011

https://www.hko.gov.hk/en/education/weather/thunderstorm-and-lightning/00015-dont-be-a-lightning-rod.html

en

Does lightning always strike the tallest object?

There is a common misconception that lightning only strikes the tallest object. Do not assume that you are safe from lightning even if you see something else tall in your vicinity.

Everyone knows that lightning is very dangerous.  However, there is a common misconception that lightning only strikes the tallest object.  Let us explore the facts behind this myth.Lightning stroke is initiated very high in thunderstorm clouds.  Objects on the ground have very little influence on its travelling path, until the stroke comes to a distance very close to the ground.  It is true that taller object is more susceptible to lightning because of shorter air gap between the lightning stroke and the object.  However, for any attraction effect to come into play, the lightning stroke has to be very close to the object.  The distance of influence is roughly equal to the height of the object.  For example, a 4-metre pole can only attract lightning strokes which are less than 4 metres away.  Therefore, do not assume that you are safe from lightning even if you see something else tall in your vicinity.

Thunderstorm and Lightning

September 2011

https://www.hko.gov.hk/en/education/weather/thunderstorm-and-lightning/00013-does-lightning-always-strike-the-tallest-object.html

en

Microburst Induced by Thunderstorms

The most violent form of downdraft from a thunderstorm is called the microburst.

Thunderstorms commonly occur in spring and summer in Hong Kong. Most thunderstorms form in cumulonimbus clouds which typically cover an area of a few kilometres in diameter with a vertical extent of ten kilometres or more.Thunderstorms are capable of producing a variety of intense and severe weather, including heavy rain, lightning, gust, hail, waterspout and tornadoes. Severe thunderstorms are associated with intense convection, resulting in violent descent of air, or downdraft, and heavy rain. The most violent form of downdraft from a thunderstorm is called the microburst - a strong downrush of winds which often radiate outward from intense thunderstorms as they strike the ground surface.  Microburst is short-lived, usually lasting for about 5 to 15 minutes, and they are relatively compact, usually affecting an area of 1 to 3 kilometres in diameter. An aircraft flying through a microburst would experience a sequence of rapid wind changes, namely headwind (wind blowing towards the aircraft), downdraft (wind blowing from above), then followed by tailwind (wind blowing from behind) (Fig. 1). Such rapid wind changes are hazardous to aircraft during landing and take-off. To overcome the adverse effect of the microburst, the pilot needs to take timely corrective action to ensure aircraft safety.In Hong Kong, a Terminal Doppler Weather Radar (TDWR) was installed at Tai Lam Chung (Fig.2) to detect windshear and microburst so as to enhance the safety of aircraft landing and taking off from the Hong Kong International Airport.  Equipped with sophisticated computer programs, the TDWR is able to automatically detect thunderstorm-induced microburst/windshear phenomena. The geographical distribution of windshear and microburst is displayed on graphical displays (Fig 3) for reference by air traffic control supervisors and aviation forecasters. The microburst/windshear alert information would be disseminated to pilots and aviation operators without delay.  These alerts are updated at least once every minute. 

Thunderstorm and Lightning

September 2011

https://www.hko.gov.hk/en/education/weather/thunderstorm-and-lightning/00025-microburst-induced-by-thunderstorms.html

en

Predict Lightning Positions by Tracking Thunderstorms: Airport Thunderstorm and Lightning Alerting System

ATLAS intelligently integrates real-time lightning location and weather radar data to provide amber and red lightning alerts automatically. The amber alert will be issued if C-G lightning is predicted within 5 km from the aerodrome; or when intense radar echo and C-G lightning are detected within 15 km and 5 km of range respectively. When CG lightning is affecting or is predicted to occur within a 1-km domain surrounding the airport island, the red lightning alert will be issued.

The Hong Kong International Airport (HKIA) is one of the busiest airports in the world.  In 2011, there were more than 300,000 aircraft landing at and departing from HKIA.  In rainy season, cloud-to-ground (C-G) lightning associated with thunderstorms may pose threats to ramp operators, and passengers embarking or disembarking outdoor.  For the safety of the airport personnel and the passengers while minimizing disruption to the airport operations as much as possible, the Hong Kong Observatory cooperated with the HKIA and developed the "Airport Thunderstorm and Lightning Alerting System" (ATLAS).ATLAS intelligently integrates real-time lightning location and weather radar data to provide amber and red lightning alerts automatically.  It uses a technique called "Tracking Radar Echoes by Correlation" (TREC) which calculates the motion vector, i.e. TREC vector, of every pixel on a radar image to predict the future lightning location.  The principle of TREC is summarized below.  The radar image is first divided into smaller boxes of equal size.  Two successive radar images (Time 1 and Time 2) are then compared.  Using Time 1 as basis, and by correlation, the corresponding box at Time 2 is found.  The relative positions of the paired-up boxes at Time 1 and Time 2 indicate the motion vector of radar echoes within the time period (Figure 1).  By extrapolation, it is able to calculate when the lightning will affect the airport.ATLAS provides amber and red lightning alerts.  The amber alert will be issued if C-G lightning is predicted within 5 km from the aerodrome; or when intense radar echo and C-G lightning are detected within 15 km and 5 km of range respectively.  When CG lightning is affecting or is predicted to occur within a 1-km domain surrounding the airport island, the red lightning alert will be issued (Figure 2).When the amber lightning alert is in force, all non-essential ramp activities will be suspended.  When the red lightning alert is on, to protect the personnel on the apron, all outdoor activities on apron including aircraft refueling, embarking and disembarking of passengers, baggage as well as cargo handling will be halted until lightning is over, which may lead to delays in baggage reclaim.

Thunderstorm and Lightning

June 2012

https://www.hko.gov.hk/en/education/weather/thunderstorm-and-lightning/00012-predict-lightning-positions-by-tracking-thunderstorms-airport-thunderstorm-and-lightning-alerting-system.html

en

Squall Lines and "Shi Hu Feng" - what you want to know about the violent squalls hitting Hong Kong on 9 May 2005

Shi Hu Feng, or Wind of the Stony Lake in general refers to those squally thunderstorms moving in suddenly from the north, it is the description of the gust associated with squall lines.

While its origin cannot be traced, the term of "Shi Hu Feng", or "Wind of the Stony Lake" has been used in southern China, particularly in the Pearl River Estuary, for a long time. It in general refers to those squally thunderstorms moving in suddenly from the north. From the meteorological point of view, Shi Hu Feng is the commoners' description of the gust associated with squall lines.A squall line is a cluster of severe thunderstorms or storm cells along a line. Squall lines travel fast wrecking havoc on its way. Apart from the heavy downpour and thunder, squall lines also bring sudden changes in the wind direction with an abrupt increase in wind speed. The severe gust associated with squall lines can exceed 100 kilometres per hour. Some of them even carry hails and tornadoes.Accompanying a southward moving cold front or trough of low pressure, squall lines usually affect southern China and its coastal area in late spring and early summer. Occasionally, squall lines are also formed near the outer rain band of the tropical cyclones approaching southern China. On the radar display, a squall line, which spans tens to hundreds of kilometers in length, manifests itself as a narrow band of intense rain area with a width of about ten to a few tens kilometers. Sometimes it also curves like a bow.There have been cases of squall lines causing casualties and economic damages when passing through Hong Kong. During the passage of one such squall line on 9 May 2005, gust up to 135 kilometres per hour was recorded in the Kwai Chung area. Some containers stacked up in container terminals there collapsed, resulting in one death and two others injured. There were over 100 reports of fallen trees and scaffoldings that day. Incidentally, a squall line also hit Hong Kong on the same day four years ago (9 May 2001), bringing down 50 containers in Kwai Chung.The violent winds and heavy rain of squall lines also pose a major threat to ocean- or river-going vessels. Squall lines were also blamed for the capsizing of passenger ferries in the Pearl River Estuary region in 1980, 1983 and 1985, claiming several hundred of lives.

Thunderstorm and Lightning

https://www.hko.gov.hk/en/education/weather/thunderstorm-and-lightning/00030-squall-lines-and-shi-hu-feng-what-you-want-to-know-about-the-violent-squalls-hitting-hong-kong-on-9-may-2005.html

en

Northwest "Shi Hu Feng" and Northeast "Shi Hu Feng"

The squall line that associated with a Northwest Shi Hu uall line of Northeast Shi Hu Feng will generally move from inland towards the northern or northeastern part of the New Territories.

"Shi Hu Feng" is the layman's description of the gust associated with squall lines. A squall line is a cluster of severe thunderstorms or storm cells along a line. Squall lines are fast moving and destructive, and will lead to sudden changes in the wind direction with an abrupt increase in wind speed. The severe gust associated with squall lines can exceed 100 kilometres per hour. Apart from heavy downpour and thunder, some of the squall lines even carry hail or hailstones and tornadoes. On the radar display, a squall line will normally appear as a narrow band of intense rain area, sometimes bow-shaped, with a width of about ten to a few tens of kilometers, but will extend from tens to hundreds of kilometers in length.In late spring and early summer, cold fronts or troughs of low pressure often drift south and affect southern China and its coastal area. Formation of Northwest Shi Hu Feng is probable when upper atmospheric disturbances propagate from west to east near the trough. A bow-shaped squall line that associated with a Northwest Shi Hu Feng will normally approach Hong Kong from the northwestern part of the Pearl River Estuary. The squall line will first affect the northwestern New Territories such as Lau Fau Shan and Yuen Long before sweeping towards other parts of the territory. For more detailed description of Northwest Shi Hu Feng, please refer to another article at the Educational Resources of the Observatory 'Squall lines and "Shi Hu Feng" - what you want to know about the violent squalls hitting Hong Kong on 9 May 2005'.Beside, when a tropical cyclone moves over the sea areas near Taiwan, the subsidence air associated with the tropical cyclone would often lead to fine and clear weather over the south China coastal region before the outer rainbands of the tropical cyclone start affecting Hong Kong.  Under prolonged sunshine, inland Guangdong will become very hot in the afternoon such high surface temperature may trigger the formation of thunderstorms.  Since the background flow in the middle levels of the atmosphere over the region would normally be north to northeasterlies under such situation, the thunderstorms formed inland will move towards Hong Kong.  When several severe thunderstorms or storm cells cluster into a squall line, the so-called Northeast Shi Hu Feng will then form.  The squall line will generally move from inland towards the northern or northeastern part of the New Territories before sweeping further south to affect other parts of the territory.  The episode of the squall line on 27 July 2008 is one typical case of Northeast Shi Hu Feng in Hong Kong (Figures 1 to 4). During the passage of the squall line, gust exceeding 100 kilometres per hour was recorded at Tai Mei Tuk and hail was also reported in some parts of the New Territories.

Thunderstorm and Lightning

June 2011

https://www.hko.gov.hk/en/education/weather/thunderstorm-and-lightning/00027-northwest-shi-hu-feng-and-northeast-shi-hu-feng.html

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The Looks of Water in Spring

In springtime, Hong Kong is occasionally affected by cold fronts followed by dry northerly winds. On the other hand, it is sometimes affected by warm and humid maritime airstream, causing very humid weather and even the occurrence of mist or fog. In Hong Kong, the most common type of fog in spring is advection fog. You may recall that February this year was more humid than usual, with the refrigerators, washing machines or walls "sweating", i.e. water droplets forming on the surfaces. This phenomenon is called "Huinan" in Putonghua by the locals, which means backing to the south.

We call the envelope of air surrounding the Earth the atmosphere. It is closely related to human and all kinds of living beings. Just as fish which cannot live without water, we all live in the bottom of the atmosphere and cannot leave it for a second. The atmosphere has various physical properties such as temperature, humidity, wind speed, air pressure and precipitation. Their variations can affect our lives and even existence. Through a series of articles, we will introduce the various elements or properties of the atmosphere in different seasons of the year. To begin our journey, let us start with the characteristics of water in springtime.The atmosphere is in motion all the time so that heat and moisture around the world can be exchanged. Hong Kong's climate is subtropical with different features in each of the four seasons. In springtime, Hong Kong is occasionally affected by cold fronts followed by dry northerly winds. On the other hand, it is sometimes affected by warm and humid maritime airstream, causing very humid weather and even the occurrence of mist or fog. In Hong Kong, the most common type of fog in spring is advection fog. During this time, as the water along the coast of Guangdong is still rather cool, the warm and humid air coming from the distant ocean will be cooled by the underlying water. This results in condensation of water vapour into droplets and hence formation of fog (see the figure).You may recall that February this year was more humid than usual, with the refrigerators, washing machines or walls "sweating", i.e. water droplets forming on the surfaces. This phenomenon is called "Huinan" in Putonghua by the locals, which means backing to the south. From the perspective of meteorology and physics, this happens when cold air recedes and another warm and humid maritime airstream comes in rapidly. As the surface temperatures of walls, floors and outdoor glasses still remain low, water vapour in the warmer air can easily condense into tiny water droplets. The low water-absorbing capacity of these surfaces favours the aggregation of tiny droplets into water droplets, which are visible to us. However, as temperatures rise later on, moisture begins to vaporize and the weather phenomena of "Huinan" will disappear gradually.

Weather Phenomena

June 2010

https://www.hko.gov.hk/en/education/weather/weather-phenomena/00039-the-looks-of-water-in-spring.html

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The Looks of Water in Summer

Heavy rain occurs most often during the summer months in Hong Kong and brings about flooding and landslips, causing inconvenience to people. Why do we have more rainstorms in the summer? The main physical conditions for heavy rain events are adequate supply of water vapour, strong uplift motion of surrounding air and an unstable atmosphere.

Heavy rain occurs most often during the summer months in Hong Kong and brings about flooding and landslips, causing inconvenience to people. Why do we have more rainstorms in the summer? The main physical conditions for heavy rain events are adequate supply of water vapour, strong uplift motion of surrounding air and an unstable atmosphere. These will be discussed one by one below.In general, moist air reaching Hong Kong could come from the south, originating from the South China Sea and even as far away as the Bay of Bengal. It could also be come from the east, originating from Western Pacific. In summer, solar heating heats up the continental landmass over mainland China faster than the seas. The differential warming leads to the summer monsoon and creates a steady wind blowing from the sea toward the land, bringing an abundant supply of moist air.Strong convective activity generally occurs in the summer. When solar radiation heats up the humid air near the earth surface, an abundance of water evaporates into vapour and rises. As water vapour rises through the atmosphere, it cools to the point where it condenses to form water droplets. A huge collection of these tiny suspended droplets forms clouds and becomes visible as cumulus or even cumulonimbus (see the figure). As the air continues to rise and cool, droplets within the clouds will continue to grow by coalescence. When the droplets grow too large and become too heavy to be supported by the surrounding updrafts, they will fall out of the cloud as rain. When the updraft is so strong that the cloud grows significantly allowing large raindrops to form, rainstorm is formed.The stability of the atmosphere depends on its humidity and temperature. A cold and dry atmosphere is generally more stable and inhibits uplift. On the contrary, moist and warm air is more unstable. In general, the lower levels of the atmosphere will be warmer and more humid, while the upper ones are cooler and drier during summer days. When moist and warm air parcel rises and condenses, significant amount of latent heat will be released to the surrounding air which in turn makes it less dense, facilitating uplift of the air. If cumulus clouds form and converge together, latent heat will gather and strengthen the cloud's updrafts. Further vacuuming up of humid air will promote a full development of cumulonimbus and also torrential rain. Besides, air flows like a fluid and often produces disturbance or vortex which enhances the uplifting of air and hence the development of heavy rain.Although heavy rain may cause inconvenience, it is beneficial to human beings in relieving droughts, for example. It could also help cool us during the hot summer. Meanwhile, in the threat of a global water shortage, collecting rain water is a smart and useful way to conserve this valuable resource.

Weather Phenomena

September 2011

https://www.hko.gov.hk/en/education/weather/weather-phenomena/00040-the-looks-of-water-in-summer.html

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The Looks of Water in Autumn

Why the weather in autumn is usually fine and clear? In summer, Hong Kong is mainly affected by a southerly airstreams from the sea and the weather is generally humid with rain. Towards autumn, the increasing prevalence of the northeast monsoon originating from the inland areas causes the atmosphere to dry out and the rainy season over northern, central and eastern China comes to an end.

In hot and moist summertime, we always look forward to an earlier arrival of autumn to bring us a pleasant relief. Autumn is the transition between summer and winter. Although there may still be occasional rain episodes, the weather is generally cool and dry. "Water" plays a less active and changeable role than it does in spring or summer. Let's see why the weather in autumn is usually fine and clear from a meteorological point of view.In Hong Kong, autumn is less pronounced than other seasons. However, in most parts of China including Beijing, Shanghai, Wuhan, Lanzhou, Urumqi and Lhasa, autumn is full of character. According to climatological record of these places, autumn is comfortable and on average rainy days are much fewer than those in the summer. After a rainy summer, the dust and impurities in the air are washed away. According to the physics of scattering, when the amount of dust and larger particles in the air is reduced, violet and blue light of shorter wavelengths will be scattered more than red light of longer wavelength (Fig.1). This makes the sky appear blue.In summer, Hong Kong is mainly affected by a southerly airstreams from the sea and the weather is generally humid with rain. Towards autumn, the increasing prevalence of the northeast monsoon originating from the inland areas causes the atmosphere to dry out and the rainy season over northern, central and eastern China comes to an end. The dry and cold air mass over northern China intensifies and spreads southward in phases, displacing the warm and humid air prevailing in the summer. As a result, clouds and moisture are cleared up and the weather becomes fine and conditions comfortable.Even in autumn, there can be disruptions to the pleasant weather, especially when weather systems are weak in intensity. The prevalence of weak and dry northeast monsoon brings to Hong Kong abundant sunshine, which in turn induces significant temperature contrast between land and sea, leading to the formation of land and sea breezes. The small scale moisture-laden sea breeze converges with the prevailing northeasterly winds. When the sea breeze strengthens in the afternoon, the convergence zone moves inland. Thunderstorms and showers would then occur over inland areas. On the contrary, when solar heating vanishes at night, temperatures over land area drop more rapidly due to radiation cooling. Sea breeze would subside and give way to land breeze. The convergence zone moves from inland towards the coast with the land breeze and brings showers to Hong Kong and the adjacent coastal areas (Fig.2).

Weather Phenomena

December 2011

https://www.hko.gov.hk/en/education/weather/weather-phenomena/00041-the-looks-of-water-in-autumn.html

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The Looks of Water in Winter

Located at the southeastern edge of the vast Asian continent, Hong Kong's weather conditions in winter are also quite different from other sub-tropical regions. In winter, cold air mass accumulates over the central and northern regions of the Asian continent such as Siberia. As the continent cools down more quickly than the ocean, winter monsoon establishes itself, blowing from the land toward the sea, advecting the cold air from the northern region to the coastal areas and bringing cold weather to Hong Kong.

Hong Kong is situated in the sub-tropical zone at the latitudes similar to those of Hawaii. However, the temperature variation of Hong Kong within a year is larger than that of Hawaii. In Hong Kong, the monthly mean temperature during summer is different from that in winter by more than 10oC. Located at the southeastern edge of the vast Asian continent, Hong Kong's weather conditions in winter are also quite different from other sub-tropical regions. In winter, cold air mass accumulates over the central and northern regions of the Asian continent such as Siberia. As the continent cools down more quickly than the ocean, winter monsoon establishes itself, blowing from the land toward the sea, advecting the cold air from the northern region to the coastal areas and bringing cold weather to Hong Kong.When the cold winter monsoon reaches Hong Kong, local temperature will drop significantly. Under certain weather conditions, frost may appear over rural areas and on high ground. Frost is a weather phenomenon which can be in general classified into three types: hoar frost (mainly caused by radiation cooling, also known as radiation frost), advection frost and rime. In Hong Kong, hoar frost occurs more frequently than the others. Let's focus on this type of frost here. In the early morning of cold season, the thin layer of white ice crystals appearing on the grass or soil might be hoar frost. The occurrence of hoar frost not only depends on weather conditions, but also the nature of the object surface. When the object is chilled by radiation cooling, the surface temperature will drop rapidly and the temperature of surrounding air close to the object will remain relatively high. The air will be chilled and simultaneously releases the excessive water vapour. When the temperature drops to 0oC or below, water vapor condenses through deposition directly on the surface as ice crystals, i.e. the hoar frost. Deposition, the direct formation of solid phase of the substance from its gas phase without going through the liquid phase, is a kind of latent heat release process. Frost is a common example of deposition  (Fig.1).In addition to temperature of the surrounding air and the object itself, the amount of cloud will inhibit radiation cooling. Under cloudy conditions, radiation cooling will be weaker and is not conducive to the formation of frost. Therefore, hoar frost occurs mostly under clear nights when radiation cooling is the strongest. Wind also plays an important role in the formation of frost. If the wind is light, the air slowly flows through the cool surface and at the same time brings abundant supply of moisture which are conducive to the formation of frost. On the contrary, under strong wind condition, the air flows rapidly without good contact with the surface. The air at low levels will also mix up and is not favourable to the drop of temperature, which in turn inhibit the formation of frost.A cycle of four seasons passed. This is the last article of the series, I hope the readers can have a better understanding of the atmosphere and appreciate the changing weather in four seasons.

Weather Phenomena

March 2012

https://www.hko.gov.hk/en/education/weather/weather-phenomena/00042-the-looks-of-water-in-winter.html

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Ice pellets – a rarely observed winter precipitation phenomenon in Hong Kong

How does ice pellets form? What is the difference between ice pellets and snow? During the processing of ice pellet formation, water droplets melted from snow but yet to condense remain in the air aloft. These droplets, when in touch with cold objects, form ice and stick to them. This phenomenon, known as icing in aviation, can be dangerous to flying aircraft.

Ice pellets – a rarely observed winter precipitation phenomenon in Hong Kong WONG Sau-ha and CHEUNG Ping February 2017 Did you head north for a cold white Chinese New Year holiday as the January temperature in Hong Kong was warmer than normal? Indeed the chance of seeing snow in Hong Kong was very slim. But last year (2016), if you would still remember, under an intense cold surge, even though there was no snow in Hong Kong, we experienced an even rarer form of winter precipitation - ice pellets How does ice pellets form? What is the difference between ice pellets and snow? As temperature falls, water vapor in the air will condense into rain drops. When the temperature is sufficiently low below freezing, water vapor condenses directly into solid ice crystals and become snowflakes. In winter, snowflake will form aloft where temperature is very low and there is enough water vapor. In the atmosphere, temperature generally decreases with increasing height. As the snowflake falls to the ground under gravity, it may start to melt if the surrounding temperature is above freezing. If the snowflake does not melt or has not fully melted during its fall because the surrounding temperatures are cold enough, we can observe snow. However, temperature does not always decrease with increasing height. If there is a layer of warmer air in between the cold layers as in Figure 1, what happens? Snowflakes may first melt into raindrops in the warm layer, then the raindrops may condense once again, this time into ice pellets in the freezing layer below. Unlike snow that falls gently, ice pellets, being harder, will bounce up and down when they hit the ground or tough objects. On 24 January 2016, an intense cold surge brought very cold weather to Hong Kong. Ice pellets, being such a rare wintry precipitation, was first ever observed at the Hong Kong International Airport (HKIA) that afternoon. There were also reports of ice pellets and widespread icing on high grounds. Figure 2 shows ice pellets and icing observed near Tai Mo Shan Radar Station on that day. The tephigram in figure 3 depicted the vertical structure of the atmosphere in the morning of 24 January. The black and red lines show respectively the changes in air and dew point temperatures with height. One could see two cold layers (marked in blue) separated by a warmer layer (indicated by orange) at the lower part of the atmosphere, exactly what one would expect ice pellets to form. During the processing of ice pellet formation, water droplets melted from snow but yet to condense remain in the air aloft. These droplets, when in touch with cold objects, form ice and stick to them. This phenomenon, known as icing in aviation, can be dangerous to flying aircraft. Indeed on that day, a helicopter of the Government Flying Service had to suspend its rescue mission to go to Shek Kong for ice removal, and the Airport Meteorological Office also received several flights reporting icing. "Behind beauty lurks danger" applies particularly well for many weather phenomena. We must pay attention to safety when appreciating their wonders. For more information on wintry precipitation, please visit the Hong Kong Observatory's Blog and Education Resources. https://www.hko.gov.hk/en/blog/00000190.htm https://www.hko.gov.hk/en/blog/00000160.htm Figure 1   Schematic diagram of ice pellet formation Figure 2   Ice pellets (left) and Icing (right) observed at Tai Mo Shan Radar Station on 24 January 2016 Figure 3   Tephigram in the morning of 24 January 2016 Did you head north for a cold white Chinese New Year holiday as the January temperature in Hong Kong was warmer than normal? Indeed the chance of seeing snow in Hong Kong was very slim. But last year (2016), if you would still remember, under an intense cold surge, even though there was no snow in Hong Kong, we experienced an even rarer form of winter precipitation - ice pelletsHow does ice pellets form? What is the difference between ice pellets and snow?As temperature falls, water vapor in the air will condense into rain drops. When the temperature is sufficiently low below freezing, water vapor condenses directly into solid ice crystals and become snowflakes. In winter, snowflake will form aloft where temperature is very low and there is enough water vapor. In the atmosphere, temperature generally decreases with increasing height. As the snowflake falls to the ground under gravity, it may start to melt if the surrounding temperature is above freezing. If the snowflake does not melt or has not fully melted during its fall because the surrounding temperatures are cold enough, we can observe snow.However, temperature does not always decrease with increasing height. If there is a layer of warmer air in between the cold layers as in Figure 1, what happens? Snowflakes may first melt into raindrops in the warm layer, then the raindrops may condense once again, this time into ice pellets in the freezing layer below. Unlike snow that falls gently, ice pellets, being harder, will bounce up and down when they hit the ground or tough objects.On 24 January 2016, an intense cold surge brought very cold weather to Hong Kong. Ice pellets, being such a rare wintry precipitation, was first ever observed at the Hong Kong International Airport (HKIA) that afternoon. There were also reports of ice pellets and widespread icing on high grounds. Figure 2 shows ice pellets and icing observed near Tai Mo Shan Radar Station on that day. The tephigram in figure 3 depicted the vertical structure of the atmosphere in the morning of 24 January. The black and red lines show respectively the changes in air and dew point temperatures with height. One could see two cold layers (marked in blue) separated by a warmer layer (indicated by orange) at the lower part of the atmosphere, exactly what one would expect ice pellets to form.During the processing of ice pellet formation, water droplets melted from snow but yet to condense remain in the air aloft. These droplets, when in touch with cold objects, form ice and stick to them. This phenomenon, known as icing in aviation, can be dangerous to flying aircraft. Indeed on that day, a helicopter of the Government Flying Service had to suspend its rescue mission to go to Shek Kong for ice removal, and the Airport Meteorological Office also received several flights reporting icing."Behind beauty lurks danger" applies particularly well for many weather phenomena. We must pay attention to safety when appreciating their wonders. For more information on wintry precipitation, please visit the Hong Kong Observatory's Blog and Education Resources.https://www.hko.gov.hk/en/blog/00000190.htm https://www.hko.gov.hk/en/blog/00000160.htm

Weather Phenomena

https://www.hko.gov.hk/en/education/weather/weather-phenomena/00487-ice-pellets-v-a-rarely-observed-winter-precipitation-phenomenon-in-hong-kong.html

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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].

Weather Phenomena

September 2020

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

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Hong Kong at Freezing Point - Historical Perspective

Being in the sub-tropical zone, Hong Kong experiences a relatively warm climate. Only in relatively rare occasions in Hong Kong can local residents witness wintry weather phenomena, e.g. snow, commonly found in regions with cold climates. According to historical meteorological records, eyewitness reports and documentation from January 1948 to February 2014, weather phenomena such as frost, ice, rime, sleet, and snow have been reported during the winter months in Hong Kong.

Climatological Records of Special Winter Weather in Hong KongBeing in the sub-tropical zone, Hong Kong experiences a relatively warm climate. Only in relatively rare occasions in Hong Kong can local residents witness wintry weather phenomena, e.g. snow, commonly found in regions with cold climates. According to historical meteorological records, eyewitness reports and documentation from January 1948 to February 2014, weather phenomena such as frost, ice, rime, sleet, and snow have been reported during the winter months in Hong Kong. In this article, these five types of phenomena are referred generally as "special winter weather phenomena in Hong Kong". Figures 1 and 2 show respectively their yearly and monthly frequency distribution in increasing rarity: frost, ice, rime, sleet and snow. Among them, rime, sleet and snow were the least reported, with only a handful of cases over the past 66 years. Apart from the single frost case reported in November 1987, all other special winter weather occurred during the period from December to March. For the differentiation and identification of special winter weather phenomena, please check the companion article, Hong Kong at Freezing Point - Observation Perspective.In Hong Kong, snow was the rarest winter weather phenomenon, with only four reports received since 1948: on 2 February 1967, 13 December 1967, 29 January 1971 and 14 December 1975 (see Table 1). Brief flurry of slight snow was reported at Cape Collinson Training Center in the first case and at Tai Mo Shan near its summit in the other three cases.The Coldest Day in Hong KongBesides the four snow reports in post-war years, there was also a well-known ice and snow event in "Hong Kong" towards the end of the 19th Century. It happened in January 1893 in an era when "Hong Kong" was still confined to the area south of Boundary Street, Kowloon. Between 15 and 18 January, the weather in Hong Kong was exceedingly cold with temperatures at the Observatory headquarters plummeted to extreme levels. In the morning of 18 January 1893, the Observatory's air temperature reached 0.0oC. This is the lowest temperature reading since records began in 1884 and a record that has never been broken since then. On higher grounds, sub-zero temperatures were registered at places such as the Peak and the "Botanic Gardens" (renamed as Hong Kong Zoological and Botanical Gardens in 1975). According to eyewitness reports, significant icing appeared at the Peak on 16 and 17 January. The China Mail reported that "Peak residents speak of the accumulation of ice in the Hill district as having largely increased, and in many parts of the roads were positively dangerous, their surface being covered with a coating of ice on which it was almost impossible to get a firm foothold." It also reported that icicles weighted heavily onto the telegraph wires. In the then "Hong Kong Naturalist" journal, Mr. L. Gibbs wrote that "The rain froze as it fell and I well remember that by the time I reached Magazine Gap, a heavy ulster I wore was frozen stiff in front of me." [1]. Based on these vivid descriptions and the characteristics of different winter precipitations as described in the companion article Hong Kong at Freezing Point - Observation Perspective, the icing conditions at that time were likely due to freezing rain.Although no snow was observed at the Observatory, the first Director of the Observatory indicated in his report that the tops of the hills appeared to be covered by snow or hoar-frost. Moreover, snowstorms were reported in Guangdong north and east of "Hong Kong" (i.e. north of Boundary Street) [2-3]. Mr Charles Ford, the Superintendent of the Botanical Afforestation Department, recalled the stunning wintry scenery as follows [2] (Figure 3):"All the hills on the mainland and Lantao (Lantau) island were likewise white with ice, one of the hills (3147 feet) of Lantao having what appeared to be snow for some few hundreds of feet down from its summit. As early as the evening of January 15 the summit of Taimoshan (about 3300 feet) on the mainland had assumed a whitish appearance, presumably from ice or snow."The myth of snow in 1883There had been an old photograph circulating online, showing snow-covered streets and the "Foochow Club" and triggering speculation that heavy snow had occurred in Hong Kong in 1883, the year the Observatory was established. However, the "Foochow Club" in the photograph was in fact in Fuzhou, Fujian Province [4] and not in Hong Kong. Neither was there any record of snow occurrence in the Observatory's reports in 1883.

Weather Phenomena

April 2014

https://www.hko.gov.hk/en/education/weather/weather-phenomena/00480-hong-kong-at-freezing-point-historical-perspective.html

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Hong Kong at Freezing Point - Observation Perspective

How to identify different types of special winter weather phenomena? These phenomena predominantly occur from December to March. They differ in their formation processes, crystal structures and appearances, as well as the weather conditions under which they form.

The cold spells that affected Hong Kong in December 2013 and February 2014 stirred up a bit of snow frenzy among the local media and social network. Some sturdy souls even braved the chilly weather to make their way up Tai Mo Shan in the hope of seeing snow. The Observatory has also received a number of enquiries together with videos/photos about possible special winter weather phenomena in Hong Kong. We are very thankful to members of the public providing the Observatory with such observations of special weather phenomena.How to identify different types of special winter weather phenomena?Under intense cold surges and favourable atmospheric conditions, five types of special winter weather phenomena, namely frost, ice, rime, sleet and even snow, have been reported in Hong Kong in the past, according to the Observatory's records from January 1948 to February 2014 (their historical occurrences and statistical distributions are summarized in a companion blog article, Hong Kong at Freezing Point - Historical Perspective). These phenomena predominantly occur from December to March. They differ in their formation processes, crystal structures and appearances, as well as the weather conditions under which they form [1-5] (see Appendix I). Examples of frost, ice and rime observed in Hong Kong are shown in Figures 1(a), 1(b) and 1(c) respectively. A schematic diagram illustrating the typical atmospheric conditions for the formation of different types of precipitation in winter is shown in Figure 2.A Recent Case at Tai Mo ShanTo illustrate how winter precipitation types are identified and classified, let us take a look at a "suspected snow case" reported at around 11 p.m. on 10 February 2014 near the Tai Mo Shan Radar Station. During that night, an eyewitness saw and recorded a 2 to 3 minute long video clip of some suspected solid precipitation followed by liquid precipitation. The eyewitness also took a few photographs of icing phenomena on the ground as well as on the surface of leaves.The air temperatures measured at the Tai Mo Shan Automatic Weather Station were -1 to -2oC around the reporting time. There was no measurable rainfall at the station. The upper-air sounding data that evening indicated that the freezing level was about 3,400 m, above which snow could naturally form if other meteorological conditions were favourable. However, there was apparently a relatively warm and saturated layer between 1,200 and 2,500 m with temperatures ranging between 5 and 9oC. The microwave radiometer at the Hong Kong International Airport also detected a vertical profile with a colder layer (0.0 to 2.5 oC) at around 1 km in altitude and a warmer saturated layer above. Checking against the illustration in Figure 2, the vertical temperature profile analyzed for the evening of 10 February satisfied the atmospheric conditions favourable for sleet or freezing rain formation. This suggests that even with sub-freezing temperatures at the top of Tai Mo Shan, there was still a thick layer of relatively warm cloud aloft, and any snowflakes falling through that layer would have melted. The possibility of having snow reaching the ground was therefore considered slim. A more likely scenario would be for melting snowflakes or raindrops to re-freeze into ice pellets as they encountered freezing or sub-freezing temperatures near the surface of Tai Mo Shan. As such, the weather phenomena at Tai Mo Shan that night could well be very slight sleet (a mixture of rain and tiny ice pellets) together with slight icing/freezing rain on the ground.Past "Cold Cases" of Snow Reports?After World War II, the Observatory received four snow reports, namely on 2 February 1967, 13 December 1967, 29 January 1971 and 14 December 1975. They all happened years ago when instrumental observations were relatively scarce. Apart from surface air temperatures, little was known about the meteorological conditions where and when such rare phenomena happened. The snow reports in those days were assessed mainly based on witnesses' descriptions and weather data near the reporting times. Except for the case in 1975, all other three cases occurred at air temperatures above freezing. Were such phenomena unambiguously snowfall? Could some of these be melting snowflakes, slight sleet or other types of precipitations?From a physical point of view, snow cannot stay in solid state for long if the ambient temperature is well above freezing. But when the air temperature is just slightly above freezing point, snowflakes may still be kept in a partially melted or even unmelted state, if the ambient air is sufficiently dry. The upper-air sounding of the case on 13 December 1967 depicted this dry condition. For the two cases on 2 February 1967 and 29 January 1971, the ambient conditions were not so dry and less favourable for preserving snowflakes in solid state. As such the possibility for the observed precipitations being related to melting snowflakes or slight sleet cannot be dismissed. If there were time machines that could tunnel advanced equipment of modern era through time and back to the event locations, assessment should have been much easier.

Weather Phenomena

April 2014

https://www.hko.gov.hk/en/education/weather/weather-phenomena/00481-hong-kong-at-freezing-point-observation-perspective.html

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A rare spectacular sight of rime in Hong Kong

Water droplets from fog usually freeze to form white ice crystals to the windward side of tree branches or rocks where they grow gradually to ice feathers of rime. Under very cold weather conditions, when tiny, super-cooled water droplets in fog come into contact with solid objects at a temperature below the freezing point (0oC), they will freeze on the outer surface of the objects to form a deposition of white, rough ice crystals, called "rime".

A severe winter monsoon affected southern China between 15 and 18 December 2010. On December 16, the weather in Hong Kong was cold with some rain. Winds were fresh to strong northerlies, occasionally reaching gale force on high ground. Figure 1 shows the weather chart at 8 a.m. on December 16. The temperature at the Hong Kong Observatory dropped to 8.1 degrees in the morning while that at Tai Mo Shan fell rapidly to below the freezing point (0℃) (see Figure 2). As the weather was cloudy with rather strong winds on high ground (see Figure 3), the temperature at Tai Mo Shan fell from about 3 degrees at mid-night and lingered around 2 degrees below zero during the day (see Figure 2). During that period, there were spectacular rime formations near the Tai Mo Shan weather station, the sight of which were captured by Observatory staff.Rime and frost are formed under different weather conditions (cloudy or windy days do not favour the formation of frost (see another article on frost)). Under very cold weather conditions, when tiny, super-cooled water droplets in fog come into contact with solid objects at a temperature below the freezing point (0℃), they will freeze on the outer surface of the objects to form a deposition of white, rough ice crystals, called "rime"  (see Figure 5).   On that morning, it was cold and humid with low-hanging clouds at Tai Mo Shan just after raining.  Sufficient amount of moisture together with strong winds favour the formation of rime (see Figure 6).Water droplets from fog usually freeze to form white ice crystals to the windward side of tree branches or rocks where they grow gradually to ice feathers of rime (see Figure 7).  The length of the ice feathers of rime ranges from several centimeters to tens of centimeters, some even clump together or completely cover a shrub on the ground.Rime can have a variety of beautiful forms, making a magnificent decoration of snowy trees and silvery flowers with dazzling grace for the cold winter (see Figure 8).In fact, rime is not un-common in southeastern China and Taiwan. In this region, it is possible to appreciate the dazzling grace of rime on high ground during the passage of severe winter surges. According to its distribution, rime is commonly found on the windward side of hill slopes. Nevertheless, the beautiful shapes of rime cannot sustain for a long time as it would melt rapidly and disappear without trace under the sun. The moist environment at the hill top, together with the thick clouds and strong northerlies that help maintain the temperature at Tai Mo Shan to below the freezing point (see Figure 2),  provides a good weather condition for the growth of rime on that day. With all these favourable conditions, the Observatory staffs were lucky to have captured the spectacular views of rime in Hong Kong on December 16.

Weather Phenomena

December 2010

https://www.hko.gov.hk/en/education/weather/weather-phenomena/00036-a-rare-spectacular-sight-of-rime-in-hong-kong.html

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Cloudy morning or foggy night? High resolution upper-air observations help catch them right!

A radiosonde is used to measure and calculate wind direction, wind speed, temperature, humidity and pressure at various heights in the atmosphere, providing indispensable data for weather prediction.

Since 2005, the Observatory has been using an automatic upper-air sounding system (Fig. 1) to launch a weather balloon at around 8 a.m. and 8 p.m. each day at King's Park Meteorological Station, Kowloon. A radiosonde is attached to a weather balloon to measure and calculate wind direction, wind speed, temperature, humidity and pressure at various heights in the atmosphere, providing indispensable data for weather prediction.As information technology was not well developed in early years and transmission speed of communication link was relatively slow, stringent specifications were imposed on the format of upper-air meteorological data. Upper-air data were trimmed to show only data at some specific pressure levels. The data in between those specific levels had to be discarded. Tephigram showing upper-air observation data was rather coarse then (Fig. 2). As time progresses, radiosonde can now measure weather elements once every two seconds and transmission bandwidth can essentially allow dissemination of huge amount of data. Hence the quantity of upper-air observation data has been greatly increased, producing high-resolution tephigram to facilitate more detailed analysis by weather forecasters (Fig. 3).The webpage for Weather Information for Outdoor Photography, recently launched by the Observatory, also provides detailed upper-air weather information for reference by the public. The information enables those who are interested in taking pictures of clouds and fog to plan their filming activities. Figure 4 shows the low-level atmospheric observation data on the morning of 7 April 2016. It was foggy that morning with low clouds. The atmosphere was very moist below 1,000 m and it was nearly saturated (i.e. relative humidity close to 100%) between 500 and 600 m. Victoria Harbour was surrounded by fog and low clouds as seen from the weather photo taken at the Victoria Peak, just like a wonderland!

Weather Phenomena

July 2016

https://www.hko.gov.hk/en/education/weather/weather-phenomena/00482-cloudy-morning-or-foggy-night-high-resolution-upperair-observations-help-catch-them-right.html

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Sastrugi

A common sight in Polar Regions is the formation of ripple like features and ice sculptures on the snow plains and ice fields, known as sastrugi.

A common sight in Polar Regions is the formation of ripple like features and ice sculptures on the snow plains and ice fields.  From afar, they appear very similar in appearance to sand dunes on a beach.  Known as sastrugi they are formed by wind erosion and their size can be from a few centimeters to a few meters high.  Unlike the ripples in sand dunes, sastrugi ridges are usually parallel to the wind direction.Intricate ice sculptures can form as a result when softer snow is eroded away from more compact snow leaving complex shapes and jagged features (Photos (2) and (3)).

Weather Phenomena

March 2012

https://www.hko.gov.hk/en/education/weather/weather-phenomena/00139-sastrugi.html

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Frost in Hong Kong

Frost is ice that forms directly from water vapor in the ambient air and deposits on a cold surface. It is formed when the surface of an object is cooled to below the freezing point of water.

After a clear winter night in northern China, the grass is usually covered with a layer of frost crystals, sparkling under the rising sun in the early morning. People in northern China often call this phenomenon as "the descent of frost". In the Chinese lunar calendar, there is a solar term called "Frost's Descent" on the 23rd or 24th day of October each year. It is the last solar term in autumn.  After this day, the activity of cold air becomes frequent over northern China and frost is likely to occur in the early morning. "Frost's Descent" is a bit misleading because although rain and snow fall from the sky, we never see frost falling from the sky - it forms near the ground.Frost is ice that forms directly from water vapor in the ambient air and deposits on a cold surface. It is formed when the surface of an object is cooled to below the freezing point of water.  Frost consists of spicules of ice which grow out from the surface of an object. The formation of frost is an example of deposition, which means that water vapor is directly frozen into ice crystals without going through the liquid phase. There are many different types of frost depending on how it forms and the appearance of frost crystals, such as hoar frost, advection frost, frost flowers, etc.  In Hong Kong, hoar frost is the most common type of frost in winter. Hoar frost refers to the white ice crystals deposited on the ground or exposed objects such as the rim of leaves, cable masts and even car windshields. It is formed due to radiation cooling at night. So hoar frost is also known as radiation frost. With clear sky and light wind conditions, the ground or exposed objects can be cooled to below the freezing point by radiation cooling at night, and hoar frost will then form on the object surfaces below freezing point. The effect of radiation cooling depends both on cloud amount and wind force; cloudy or windy conditions are not conducive to the formation of frost overnight. Air temperatures recorded by the Observatory are normally above 0oC when frost occurs in Hong Kong, since observations are taken at about 2 meters above the ground, where it can be warmer than ground temperature.Hong Kong's climate in winter is seldom bitterly cold. The minimum temperatures fall below 10℃ in urban areas rather occasionally. However, it is not uncommon for frost to occur on high ground and in the northern part of the New Territories. Figure 2 shows the number of days with reported frost in Hong Kong since 1955. Frost is usually observed between December and February, which is the coldest period in Hong Kong. As the tissues of crops covered with frost can be damaged by freezing temperatures, ground frost is an economic hazard to people growing fruit and vegetable. Therefore, the Observatory issues the frost warning to farmers whenever ground frost is likely to occur. 

Weather Phenomena

December 2010

https://www.hko.gov.hk/en/education/weather/weather-phenomena/00037-frost-in-hong-kong.html

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Ice crystal icing - a threat to aircraft engine

If an aircraft flies through an airspace with a high concentration of ice crystal, ice crystals will be sucked into the jet engine and melt on the warmer engine surface to form a thin layer of liquid water. This "sticky" layer facilitates the buildup of ice by trapping more ice crystals, affecting the operation of the jet engines. Even worse, when the ice sheds inside the engine, the engine may be damaged or encounter temporary power loss. This may affect aviation safety.

In 2014，we presented an article here on aircraft icing[1]. In that article, we also mentioned about a phenomenon called "Ice Crystal Icing" (ICI). In fact, what is ICI?A commercial aircraft normally flies at an altitude of 30,000 feet or above except during the ascent and descent phases. At this altitude, the temperature is normally below minus 40 degrees Celsius. The moisture content in the air is very low and generally in the form of ice crystals. If an aircraft flies through an airspace with a high concentration of ice crystal, ice crystals will be sucked into the jet engine and melt on the warmer engine surface to form a thin layer of liquid water. This "sticky" layer facilitates the buildup of ice by trapping more ice crystals, affecting the operation of the jet engines. Even worse, when the ice sheds inside the engine, the engine may be damaged or encounter temporary power loss. This may affect aviation safety. In the field of aviation, this is known as "Ice Crystal Icing" (ICI) or "High Ice Water Content" (HIWC).Where will high concentration of ice crystal occur?There is ongoing research on the origin of HIWC. Mainstream hypothesis suggested that HIWC is associated with updraught inside strong convection. Water droplets freeze when they are carried by updraught from lower atmosphere to upper air where air temperatures are very low. When the updraught is strong, as is usually the case in tall cumulonimbus, the water droplets cool rapidly and some of the ice crystals formed can be very small in size. They are carried by the upper air wind to downwind locations. Thus the chance of having high ice water content is higher at the top of a tall cumulonimbus and its vicinity (Figure 1).Does the pilot know where the HIWC area is?Thunderstorm and strong convection are often associated with cumulonimbus. Pilots generally use the on-broad weather radar to monitor the distribution of cumulonimbus ahead of the flight and avoid flying through airspace with cumulonimbus However, the on-board radar is not very effective in detecting small ice crystals downwind of cumulonimbus, nor can the pilot notice the presence of high concentration of ice crystals visually, hence making the high ice water content phenomenon an invisible hazard to aircraft.How to expose the "invisible"?We currently use meteorological satellites to identify areas with high concentration of small ice crystals. By making use of the small differences in the optical property of small ice crystals and water droplets in different infrared bands, and through analysing multiple infrared channel signals received by meteorological satellite, we can identify areas of high ice water content (Figure 2).

Weather Phenomena

February 2018

https://www.hko.gov.hk/en/education/aviation-and-marine/aviation/00501-ice-crystal-icing-a-threat-to-aircraft-engine.html

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Tornado in the “eye” of LIDAR

A LIDAR emits infrared light and receives the light reflected from suspended particulates in the air. It makes use of Doppler effect to measure the speed of movement of suspended particles to derive the wind speed.

“A tornado was spotted at Chek Lap Kok at around 4pm today (25/9) and sustained for around 20 minutes. The occurrence was captured by both the cameras installed in the airport and the LIDAR…” This was the Facebook post of the Observatory on 25 September 2020, but how does a LIDAR detects tornado?A LIDAR emits infrared light and receives the light reflected from suspended particulates in the air. It makes use of Doppler effect to measure the speed of movement of suspended particles to derive the wind speed. However, basing on Doppler effect, the LIDAR can only measure the radial velocity of a tornado, i.e. the component of wind “moving towards or away” from the LIDAR, but not the component moving sideways, as shown in Figure 1. Compared with the Terminal Doppler Weather Radar, LIDAR has a higher spatial resolution and is able to provide more detailed observations on tornadoes that are smaller in size. In addition, LIDAR is able to detect in light rain as the measurement is based on the movement of suspended particles.When compared with human observations, a LIDAR can more readily determine the direction of rotation of the tornado. Take the tornado on 25 September as an example. From the perspective of the LIDAR, the wind on the left hand side of the tornado is blowing outwards while it is blowing inward on the right hand side. Thus, it is clear that the tornado is rotating in a clockwise direction (Figure 2).

Weather Phenomena

January 2021

https://www.hko.gov.hk/en/education/weather/weather-phenomena/00555-tornado.html

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Waterspout

A waterspout is a rapidly rotating columnar vortex containing water droplets, linking between the base of a convective cloud and the water surface. It’s wind speed is often lower than that of a tornado which occurs over land. As a waterspout resembles the shape of a dragon and it sometimes suck up water, it also have a Chinese common name called “Dragon sucking water”.

A waterspout is a rapidly rotating columnar vortex containing water droplets, linking between the base of a convective cloud and the water surface. It’s wind speed is often lower than that of a tornado which occurs over land. As a waterspout resembles the shape of a dragon and it sometimes suck up water, it also have a Chinese common name called “Dragon sucking water”.Waterspout falls into two major categories: the first category is the more common one called “fair-weather waterspout”. This type of waterspout occurs in relatively fair weather, and is not directly associated with thunderstorms. Studies have shown that the downdrafts originated from convective clouds will spread out as they reach the sea surface, creating cold outflows. This type of waterspout tends to form in the convergence zone between the cold outflows where cool air moves over warm water (Figure 1). A vortex first develops on the sea surface and then extends upwards gradually. When a funnel is formed, the waterspout has reached maturity. Besides, fair-weather waterspout has a relatively short life cycle and moving track. It generally does not persist longer than 20 minutes. It dissipates rapidly after making landfall, and rarely penetrates far inland.The secondary category is called “tornadic waterspout”. Tornadic waterspout has the same formation mechanisms and characteristics as land-based tornado. It is often associated with severe thunderstorms, and accompanied by severe weather such as high winds and hail. It sometimes develops over land and then moves to the sea. It differs from fair-weather waterspout in that it develops downwards in a thunderstorm. The funnel cloud appears at the initial stage, which then gradually extends towards the land or sea surface.Waterspouts generally occur in the tropics and subtropics. They have peak frequency in the morning and a secondary maximum in the late afternoon. Though waterspouts are relatively rare in Hong Kong, they have frequent occurrence in certain regions. For example, the Florida Keys in the U.S.A. might have the highest number of waterspouts in the world, with over 400 per year.The Observatory received reports on waterspout sightings on three days in June 2018, with two sightings over the waters in the vicinity of Cheung Chau (Figures 2 and 3). Both cases of waterspout were not accompanied by thunderstorms. In Hong Kong, waterspouts are most commonly seen during the rainy season between May and October (Figure 4). From 1959 to 2017, there was a total of 35 cases of waterspouts sighted in Hong Kong.

Weather Phenomena

August 2018

https://www.hko.gov.hk/en/education/weather/weather-phenomena/00510-waterspout.html

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Dust Devil

“What is dust devil? Recording of dust devil observed at the Hong Kong International Airport and introduction to this kind of special weather phenomenon.

Dust devil (also known as dust whirl or sand whirl) resembles tornado at first glance. While both weather phenomena are in the form of whirling column, the wind strength of dust devil is weaker and normally causes less serious damage. The formation of a tornado is generally accompanied by mature cumulonimbus or even thunderstorms, whereas dust devils occur under generally fine weather condition.On 14 July 2021, dust devil was observed over the construction site of the Hong Kong International Airport (HKIA) at about 2 pm. The related video is as follows:Formation of dust devil is usually associated with heated and rapidly rising air close to the ground surface. Some favourable meteorological and geographical conditions for the formation of dust devil are listed below: 1. Clear sky or less clouds: Abundant sunshine heats up the air near the ground which is favourable to generation of rapidly rising air. 2. Weak background wind: The whirling structure of rising air will be destroyed if the background wind speed is too high, making it difficult to maintain the dust devil. 3. Flat terrain: Flat ground surface can absorb heat energy from the Sun more effectively. If the surface is sandy or dusty, the sand and dust drawn into the whirling air render the dust devil more visible. On the day of dust devil observation, a ridge of high pressure affected the south China coastal areas. The weather was mainly fine in Hong Kong. At around 2 pm, the cloud amount observed at the HKIA was 5 oktas (8 oktas for overcast sky). Winds were light to moderate with air temperatures rising to 34 degrees Celsius. The meteorological conditions basically fulfilled the conditions for the formation of dust devil. It is worth to mention that dust devil can rotate clockwise or anticlockwise. This is because in such a small scale, the effect of Coriolis force cannot come into play.

Weather Phenomena

October 2021

https://www.hko.gov.hk/en/education/weather/weather-phenomena/00663-Dust-Devil.html

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Listening to the “Rain” Symphony

Have you ever heard of the musical symphony of raindrops? In this article, we explain why sound of raindrops can be extensively diverse in nature and how they compose a piece of rain music.

Have you ever heard of the musical performance of raindrops? In fact, raining is just like a concert, playing the music endlessly and dynamically. Maybe you don't believe that there are different types of rain sounds. Indeed, as the surroundings and weather conditions change, the rain sounds are entirely different.Generally speaking, there are three sources of raindrop sound: 1. When raindrops are falling through the air, the air flow around the raindrop brings air vibration and results in continuously gentle whispering sound; 2. As raindrops hit the ground or other object surfaces, vibration is triggered with distinctive pattering sound (Sound 1 Top); 3. After raindrops fall and hit a puddle, air bubbles are produced and contract instantaneously. Sound wave is generated at the moment of restoration of water surface (Figure 1), creating the characteristic ‘plink’ sound (Sound 1 Bottom).The composition of rain music is thus based on these different types of sound.However, there are far more variations in raindrop sound than we can imagine. Sizes and falling speeds of raindrops, and also different types of the impact surfaces contribute to the diverse range of pitches and dynamics of every note and phrase in the score. For example, vibration frequencies of leaves and stones are not the same, leading to differences in the sounds of raindrops on them. Even though all puddles are accumulation of water, the sound of raindrops on them is still not identical. Since the surface tension of water surface is affected by many factors such as the temperature and impurities, the larger the surface tension, the louder the ‘plink’ sound that it can produce. Moreover, when raindrops of different sizes are impacting the water surface, they generate air bubbles with various diameters. The air bubble produced is like a beaten drum, the larger the raindrop, the lower the pitch is in general. Finally, all these variables, together with the differences in the falling speeds of raindrops, create the melodic rhythms and constitute a piece of rain music.Although rain music is composed of three types of sound in general, every type is a natural and ever-changing musical instrument. Next time when it is raining, let’s listen to the sound of raindrops and enjoy the unique “rain” symphony performed by the nature.

Rain

April 2022

https://www.hko.gov.hk/en/education/weather/weather-and-life/00673-Listening-to-the-Rain-Symphony.html

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Why does it Rain?

The water droplets in the sky come together to form clouds. Some clouds are light and puffy, while some are dark and heavy. Convection or rising air keeps the water droplets suspended in the air. However, the water droplets fall when they become too large and heavy to remain suspended in the air. Water falling from clouds in the form of rain, snow, hail or other forms is called precipitation.

On a warm humid day, if we take a bottle of beverage from the refrigerator, we may notice that water will appear on the bottle surface.  This is due to the property that cool air holds less moisture than warm air.  In this case, water vapour invisible to human eye in the air is cooled on the surface of the bottle.  Condensation takes place as the water vapour turns into visible water droplets.In nature, water over ocean or on the surface of the earth evaporates into the atmosphere and forms water vapour.  It becomes cooler as it rises in the atmosphere.  Like the aforementioned case, the water vapour condenses on the surface of condensation nuclei such as dirt, dust and salt.The water droplets in the sky come together to form clouds.  Some clouds are light and puffy, while some are dark and heavy.  Convection or rising air keeps the water droplets suspended in the air.  However, the water droplets fall when they become too large and heavy to remain suspended in the air.  Water falling from clouds in the form of rain, snow, hail or other forms is called precipitation (Figure 1).It is interesting to note that if the lower atmosphere is dry, the precipitation may evaporate before it reaches the ground.  This explains why rain echoes could sometimes be identified a few kilometers above the ground on the radar image, but no rainfall is recorded at the surface.

Rain

June 2011

https://www.hko.gov.hk/en/education/weather/rain/00044-why-does-it-rain.html

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The shape of raindrops

Water molecules like to stick with each other and minimize the surface area of the water body. A sphere has the smallest surface area for a given volume. Thus, small raindrops (with diameter < 2 mm) are usually spherical in shape, as shown in the light rain icon used by the Observatory.

What do raindrops look like? Probably 💧 After all, emoji cannot get it wrong, isn't it? Raindrop falls due to gravity. Falling raindrop, however, does not take that shape due to the physical property of surface tension. This is because water molecules like to stick with each other and minimize the surface area of the water body. A sphere has the smallest surface area for a given volume. Thus, small raindrops (with diameter < 2 mm) are usually spherical in shape, as shown in the light rain icon used by the Observatory (Figure 1). How about larger raindrops (with diameter of 3-6 mm)? In addition to surface tension, the shape of larger raindrops is affected by deformation by airflow. As a large raindrop falls through the sky, air flows around the raindrop, causing pressure to its bottom slightly higher than that to its top. Its bottom hence gets flattened out, making it look like a custard bun. The effect of airflow is smaller on small raindrops as smaller raindrop falls slower. How about even larger raindrops? It turns out that as raindrops grow beyond a certain size, they will become aerodynamically unstable and break into smaller raindrops when they fall. The shape of the raindrop affects the appearance of the rainbow. It will be further discussed in the near future.

Rain

December 2019

https://www.hko.gov.hk/en/education/weather/rain/00533-the-shape-of-raindrops.html

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Why is there More Rain in Tai Mo Shan?

The excessive rain over Tai Mo Shan is called orographic rain. When moist air hits a hill, it is blocked and forced to lift up along the slope. Owing to decrease in air pressure with height, the ascending air mass expands in reduced air pressure.

The rain season in Hong Kong is usually from April to September, with moisture mainly coming from the seas south of us.  Although Hong Kong is a small place, it is hilly with few flat land.  The major range of hills runs from the northeast to the southwest.  Situated in the central part of the New Territories, Tai Mo Shan is the highest peak in Hong Kong.  Lantau Peak (Fung Wong Shan) and Sunset Peak (Tai Tung Shan) on the Lantau Island, together with Ma On Shan in the eastern part of the New Territories are also the major peaks in Hong Kong.  According to the annual mean rainfall distribution map of Hong Kong (Figure 1) that was plotted using the rainfall recorded by rain gauges over the territory, the regional difference of annual mean rainfall is large.  This is closely tied to the terrain of Hong Kong.  For example, the annual rainfall recorded at Waglan Island is less than 1,800 millimetres while that at Tai Mo Shan is over 3,000 millimetres.  The excessive rain over hilly area is called orographic rain.When moist air hits a hill, it is blocked and forced to lift up along the slope.  Owing to decrease in air pressure with height, the ascending air mass expands in reduced air pressure.  This process consumes energy and the temperature of air mass will drop.  When the air mass cools gradually to the dew point temperature, it will become saturated and condense, turning into cloud and rain.  This kind of rain is called orographic rain (Figure 2).  This explains why the rain gauges located near the hilly areas of Hong Kong record more rainfall.  Generally speaking, orographic rain mainly occurs over the windward side of the hill.  On the leeward side, the air mass has already lost much of its moisture as precipitation.  It will become warmer and drier as it descends along the slope on the leeward side, resulting in less rainfall on the leeward slope.  However, as the hills in Hong Kong are not very high, there is still enough moisture for precipitation even after the orographic clouds have climbed over the ridge.  Therefore, the rainfall on the windward slope does not differ much from that on the leeward slope.  Nevertheless, the difference between the rainfall recorded over hilly region and flat land is significant.In other parts of the world, such as the South Island of New Zealand in the Southern Hemisphere, orographic rain makes the area famous for its diverse landscapes.  The Southern Alps is a mountain range running along the South Island with a dozen of main peaks over 3,000 metres above mean sea level.  It divides the landscape of the South Island along its length.  The windward side over the western part of the Southern Alps has much rainfall and is covered by dense temperate rainforests, while the eastern region receives much less rainfall and comprises mainly grasslands.It is worth noting that in some days without rain, the rain gauge at Tai Mo Shan sometimes records about 1 millimetre of daily rainfall.  This is probably due to the condensation of moisture in the atmosphere or in the low clouds directly on the rain gauge.  However, this only accounts for a very small portion of the annual total rainfall.

Rain

December 2013

https://www.hko.gov.hk/en/education/weather/rain/00045-why-is-there-more-rain-in-tai-mo-shan-.html

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The rainstorm development mechanisms in Hong Kong

Entering the rainy season, Hong Kong people are no strangers to the sight of torrential rain. Depending on the intensity of the heavy rainfall, the Observatory will have to issue rainstorm warning and other warnings relating to heavy rain from time to time. But do you know why some torrential rains are more severe and longer-lasting than the others?

Entering the rainy season, Hong Kong people are no strangers to the sight of torrential rain. Depending on the intensity of the heavy rainfall, the Observatory will have to issue rainstorm warning and other warnings relating to heavy rain from time to time. But do you know why some torrential rains are more severe and longer-lasting than the others? Let us take a look at the favourable weather conditions for the development of rainstorms and some mechanisms that trigger rainstorms.One of the main mechanisms that trigger heavy rain is strong convective activity in the atmosphere. As indicated by the grey line in Figure 1, there is an updraft that rushes upwards from the ground or the low level to the high level. If combined with the unstable atmospheric condition (i.e., hot below and cold above) and the abundant moisture transport at the low level, rain will occur. To put it simply, if heat and moisture are sufficient, then more intense and sustained convective activity will lead to more rainfall recorded on the ground. In spring and summer, it is not difficult to have the right atmospheric thermal and moisture conditions for rainstorm development along the South China coast including Hong Kong. On this premise and simplifying the discussion, the following will only focus on the dynamics of weather conditions that are favourable for the development of strong convections.The converging airflow (i.e. convergence[2]) near the ground or at low level can force the air to rise. As for how high this "forced" upward air current can rise often depends on the supporting mechanisms at the mid to high levels. Upper-air disturbance[3] (refers to the large-scale perturbation of the airflow in the near horizontal direction in the mid to high-level atmosphere) is one of them, and the high-level divergence (i.e. the outward-flowing air) is the second. As depicted by the streamlines, the airflow of the disturbance is propagated like a wave, and the airflow ahead of the wave will rise, and the airflow on the rear side of the wave will sink (Figure 2). Since the atmosphere is a continuous fluid, when the upper airflow diffuses to the surroundings in a certain airspace, it will suck the lower level air upwards below the airspace to fill up the air "lost" due to high-level divergence. As shown in Figure 1, air can climb to the top of the atmosphere and form convections if the air near the ground or at low level is "forced" upward by convergence, mid-level disturbances, and high-level divergence. If these favourable atmospheric conditions continue for a long period of time, updrafts can intensify and support the development of rainstorms.On cooler and drier days during autumn and winter, the above atmospheric conditions and triggering mechanisms are often difficult to occur at the same time, so there are significantly less extensive and continuous rainstorms. Having said that, this does not mean that there will be no heavy rain in autumn and winter. In addition, during days with light winds in early autumn, the intensity and impact of local showers and thunderstorms caused by the convergence of sea breeze and high temperature weather cannot be underestimated[7]. In any case, with climate change, Hong Kong will have fewer and fewer cold days. In recent years, rainstorm warnings have been issued in January and February during winter. We should never relax our awareness of rainstorms.

Rain

September 2020

https://www.hko.gov.hk/en/education/weather/rain/00549-rainstorm-development-mechanisms.html

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Interesting Statistics on Rainstorm Warning Signals

According to the statistics from 1992 to 2010, the average numbers of "Amber", "Red" and "Black" rainstorm warning signals issued each year are 21.3, 4.6 and 1.2 respectively. June is the month with the highest number of rainstorm warning signals issued. The average numbers of "Amber", "Red" and "Black" rainstorm warning signals issued in June are 5.3, 1.5 and 0.3 respectively. No rainstorm warning signals has ever been issued in January, February and December.

The rain season in Hong Kong normally lies between April and September. Troughs of low pressure would affect the south China coastal areas in May and June and bring heavy downpours to Hong Kong. These rainstorms will cause traffic disruption, floodings, landslips and even casualties. In 1992, the Observatory first operated a colour rainstorm warning system for different intensity of rainstorms to alert the public. The rainstorm warning signals were classified as "Green", "Amber", "Red" and "Black" colours. In 1998, the rainstorm warning system was revised to adopt a 3-level warnings classified as "Amber", "Red" and "Black" rainstorm warning signals. According to the warning signal definitions, "Amber" rainstorm warning signal represents heavy rain has fallen or is expected to fall generally over Hong Kong exceeding 30 millimetres an hour; "Red" rainstorm warning signal represents heavy rain has fallen or is expected to fall generally over Hong Kong exceeding 50 millimetres an hour and "Black" rainstorm warning signal represents heavy rain has fallen or is expected to fall generally over Hong Kong exceeding 70 millimetres an hour.According to the statistics from 1992 to 2010, the average numbers of "Amber", "Red" and "Black" rainstorm warning signals issued each year are 21.3, 4.6 and 1.2 respectively. June is the month with the highest number of rainstorm warning signals issued. The average numbers of "Amber", "Red" and "Black" rainstorm warning signals issued in June are 5.3, 1.5 and 0.3 respectively. No rainstorm warning signals has ever been issued in January, February and December. The earliest issuance for "Amber", "Red" and "Black" rainstorm warning signals in a year were 5 March in 2009, 3 April in 2000 and 19 April in 2008 respectively while the latest issuance were 21 November in 2006, 18 October in 2002 and 2 September in 2001 respectively. The highest number of "Black" rainstorm warning signals issued in a year was 3 times, happened in 2000 and 2006. The shortest break between two "Black" rainstorm warning signals was recorded on 23 and 24 August 1999 in which the separation of these two rainstorm warning signals was less than 17 hours. The longest durations for "Amber", "Red" and "Black" rainstorm warning signals being in effect were 17.4, 13.2 and 5.8 hours respectively.There is a tendency for "Red" and "Black" rainstorm warning signals to occur at late night or in the early morning. In fact, the effective time for "Red" rainstorm warning signal during the period from midnight to noon is about 1.5 times of those from noon to midnight and that for "Black" rainstorm warning signal is even about 2.5 times higher. One of the possible reasons is that the low-level advection of warm moist air from the South China Sea towards the coastal region tends to occur at late night or early morning during the summer time which reduces the atmospheric stability and favours the formation of heavy rain. Furthermore, the rainstorms affecting Hong Kong are usually caused by troughs of low pressure, tropical cyclones, southwest monsoon and cold fronts. By analysing the rainstorm cases of "Red" and "Black" rainstorm warning signals, most of them were attributed to troughs of low pressure, about 59%. About 24% of the cases were attributed to tropical cyclones. Southwest monsoon and cold fronts contributed 6% and 4% respectively.In recent years, the heaviest rainstorm in Hong Kong was the one on 7 June 2008. Under the influence of an active trough of low pressure, a rain band swept across Hong Kong from west to east in that morning. Due to the rapid development of the rain band, the Observatory first issued "Amber" rainstorm warning signal at 5:15 am, followed by "Red" rainstorm warning signal in 40 minutes later and finally "Black" rainstorm warning signal at 6:40 am. The "Black" rainstorm warning signal last for 4 hours and 20 minutes till 11:00 am, making it the 4th longest duration of "Black" rainstorm warning signal being in force. During the period of the "Black" rainstorm warning signal, the total rainfall recorded at the Observatory for one hour between 8 and 9 am was 145.5 millimetres, which was the highest record of one hour rainfall since 1884.

Rain

June 2011

https://www.hko.gov.hk/en/education/weather/rain/00043-interesting-statistics-on-rainstorm-warning-signals.html

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Extreme Rainfall Events in Hong Kong

The Hong Kong Observatory has strengthened the dissemination of information about rainstorm starting from the rain season of 2024, especially when the rainfall intensity far exceeds the rainfall criteria of the Black Rainstorm Warning Signal, i.e. 70 millimetres (mm) in an hour. In such scenario, Special Weather Tips will be issued to the public through HKO's website and the push notification of the "MyObservatory" mobile app about the exceptional circumstances associated with the rainstorm and the need to be on high alert. For this purpose, the Observatory has introduced the following two new terms: "Severe Rainstorm" and "Exceptionally Severe Rainstorm"

Under global warming, the occurrence of extreme weather will become more frequent and Hong Kong will be in no exception. On 7 and 8 September 2023, extreme rainfall occurred in Hong Kong, breaking three rainfall records (Note 1) and causing major damages to the city. In fact, the rainfall intensity during that rain episode far exceeded the general understanding of the public about rainstorms. To address this, the Hong Kong Observatory (HKO) has strengthened the dissemination of information about rainstorm starting from the rain season of 2024, especially when the rainfall intensity far exceeds the rainfall criteria of the Black Rainstorm Warning Signal, i.e. 70 millimetres (mm) in an hour. In such scenario, Special Weather Tips will be issued to the public through HKO's website and the push notification of the "MyObservatory" mobile app about the exceptional circumstances associated with the rainstorm and the need to be on high alert. For this purpose, the Observatory has introduced the following two new terms: • Severe Rainstorm (SvR): rainfall intensity exceeding 100 mm in an hour; and • Exceptionally Severe Rainstorm (ESR): rainfall intensity exceeding 140 mm in an hour. According to the above definitions, there were 103 days in which SvR were recorded over different parts of the territory since the automatic rain gauge network of HKO established in 1984. Among the 103 days, 9 of them even reached the ESR level (Table 1) during the past 40 years. In terms of monthly distribution, SvR (ESR included) occurred more often in May, June and August (Figure 1). A revisit to the hourly rainfall records of the Observatory Headquarters since 1884 (Note 2) revealed that there were eight days with SvR, among which two exceeded the threshold for ESR. The first ESR over the Observatory Headquarters occurred on 7 June 2008 when the Black Rainstorm Warning Signal was in force whereas the second occurrence was right on 7 September 2023 when the "Black Rainstorm of the Century" took place. It was still a hot topic of scientific research concerning the root causes and triggering conditions of ESR events. Amongst the past cases in Hong Kong, the coastal areas of Guangdong were mostly under the influence of a trough of low pressure. As discussed in educational article "The rainstorm development mechanisms in Hong Kong", the low-level flows from the air masses with contrasting characteristics on the northern/southern sides of the trough will converge along the trough axis continuously, triggering convective development. If the low-level convergence couples with other favourable atmospheric conditions, including sufficient moisture transport, unstable atmosphere, mid-level disturbances, as well as upper-level divergence, severe convection or even rainstorm is likely to occur. Back on 7 June 2008, the rainstorm began to affect Hong Kong in the morning and the Observatory had issued the Black Rainstorm Warning Signal for a total of 4 hours and 20 minutes. At the Observatory Headquarters, 145.5 mm of rainfall was recorded during the one-hour period from 8 to 9 a.m., breaking the highest one-hour rainfall record at that time. Over other parts of the territory, 200 mm of rainfall was generally recorded that day with more than 300 mm over Lantau Island and the urban areas, leading up to widespread floodings, 39 landslip reports and road blockage (Figure 3). The latter severely affected the traffic conditions with the road transportation between the urban areas and the airport once interrupted. A large number of flights were delayed as a result. Overall, more than 10 people were injured during to the passage of the rainstorm. The rainstorm of 7 September 2023 began to affect Hong Kong in the evening and the Observatory issued the Black Rainstorm Warning Signal for the longest duration in history totalling 16 hours and 35 minutes, until the afternoon on the next day. At the Observatory Headquarters, 158.1 mm of rainfall was recorded during the one-hour period from 11 p.m. to 12 midnight on 7 September, breaking the previous record that was kept for just 15 years (see the 2008 case above). From 7 to 8 September, many parts of the territory recorded more than 400 mm of rainfall with the Eastern District and Southern District of Hong Kong Island even exceeded 800 mm. There were many landslip reports and 60 flood reports received over the territory (Figure 4). Power and water outages occurred in some places. During the passage of that rainstorm, there were at least two casualties and over 140 people injured. Since the Industrial Revolution, excessive resource consumption led to the continuous rise in the concentration of greenhouse gases, exacerbating global warming which in turn increases the water-holding capacity of the atmosphere. The latter provides more favourable conditions for the development of rainstorms, leading up to the possibility of more frequent occurrence of extreme rainfall events. Rainstorms can initiate and dissipate very rapidly with high randomness in movement and downpouring location. As such, early warning of rainstorms remained a major challenge. At present, a more practical and effective strategy to assess the rainfall distribution in Hong Kong is to conduct high-density and high-frequency detections by weather radar and rain gauge networks, followed by rainfall nowcast for the next few hours using technologies such as Artificial Intelligence. When the rainfall intensity reaches the SvR or ESR levels, the Observatory will alert the public through Special Weather Tips to pay attention to the exceptional weather change and take preventive measures accordingly. The Special Weather Tips issued at 8:40 a.m. on 4 May 2024 presents a real example: "... In the past one hour, exceptionally severe rainstorm affected Tseung Kwan O, over 140 millimetres of rainfall have been recorded. It is expected that exceptionally severe rainstorm will affect some areas in the eastern part of the territory. Members of the public should stay on high alert." To alleviate global warming and its associated impacts, everybody are reminded to conserve resources, reduce waste, and take action in daily life for the betterment of the earth. Note 1: The rainstorm from 7 to 8 September 2023 broke the one-hour, two-hour, twelve-hour rainfall records of the Hong Kong Observatory since records begun in 1884. The three new records were 158.1, 201.0 and 605.8 millimetres respectively. Note 2: The one-hour rainfall records of the Observatory Headquarters refer to the hourly rainfall measured at the clock hours. The one-hour rainfall records of other automatic rain gauges refer to the maximum running 60-minute rainfall record in a day.

Rain

May 2024

https://www.hko.gov.hk/en/education/weather/rain/Education_Extreme_Rainfall_Events_in_Hong%20Kong.html

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An Abrupt Change of Weather due to the Passage of a Severe Cold Front

An area of high pressure associated with cold air normally forms over the Tibetan Plateau, being of one of the sources of cold air in winter. Cold air will basically follow an anticyclonic outflow to spread southwards over China.

The difference in heat absorption between land and sea is the major reason for the formation of typical monsoon during winter and summer.  In addition to the dynamical effect due to terrain, the Tibetan Plateau over western China has a significant effect on the monsoon.  An area of high pressure ass℃iated with cold air normally forms over the Tibetan Plateau, being of one of the sources of cold air in winter.  Cold air will basically follow an anticyclonic outflow to spread southwards over China.  The winter monsoon in the Northern Hemisphere generally prevails from December to February.  There will still be outbreaks of northeast monsoon in March, or even up to May or June, but the strength and frequency decreases as season progresses.Below is an example on the passage of a severe cold front and its ass℃iated winter monsoon bringing an abrupt and significant change of the weather in Hong Kong on 13 March 2009.  The surface weather chart in figure 1 delineates a cold front over southern China on the morning of that day.  The cold front was expected to reach Hong Kong in the afternoon on the same day.  Before the arrival of the cold front, the weather in Hong Kong was generally calm with light breeze, mainly from the west and temperatures rose to 25℃ in parts of Hong Kong in the early afternoon.  With the passage of the cold front after 3:10 p.m., winds at the Hong Kong International Airport (HKIA) turned from westerly of about 5 knots (Figure 2) to north-northwesterly of over 25 knots (Figure 3) within 5 minutes.  Gale force winds were recorded in the western parts of the New Territories and the Lantau Island.  Temperatures at the HKIA dropped from about 24℃ at 3:15 p.m. to about 17℃ at 4:00 p.m. (i.e. a drop of 7℃ in 45 minutes) and further down to about 14℃ before midnight (Figure 4).  There was a sharp rise of air pressure from 1014 hPa at 3 p.m. to 1024 hPa at 9 p.m. (i.e. an increase of about 10 hPa in 6 hours) (Figure 5).  The onset of an intense winter monsoon will bring a significant change of weather within a short period of time.  Forecasters will provide not only the expected timing of the arrival of the cold front, but also the effect on the weather elements before and after the passage of the cold front to the public.  The public should take note of the weather forecast especially when there are significant weather changes.  The Hong Kong Observatory will issue the Cold Weather Warning to alert members of the public to the cold weather and its ass℃iated adverse health effect.

Hot and Cold Weather

December 2010

https://www.hko.gov.hk/en/education/weather/hot-and-cold-weather/00048-an-abrupt-change-of-weather-due-to-the-passage-of-a-severe-cold-front.html

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The Wind Chill Effect

You will feel colder in a windy day than what the thermometer measures. This effect is termed wind chill. This effect, termed "wind chill", will be more significant with increasing wind speed.

Your body always works hard to try to maintain a body temperature close to 37oC.  However, air movement will carry heat away from your body so that you will feel colder in a windy day than what the thermometer measures.  This effect, termed "wind chill", will be more significant with increasing wind speed.In some countries like U.S. and Canada, something called the "wind chill temperature" is reported along with the actual temperature measured.  It was created based on scientific research involving human volunteers and computer modeling, as well as medical understanding of how the body loses heat when exposed to cold.  The table below shows the wind chill temperatures for various air temperatures and wind speeds based on the formula used in both U.S. and Canada.  For example, if the air temperature is 4oC and the wind is blowing at 30 km/h, your face will feel as cold as -1oC in a calm day without any wind.One point to note, however, is that the wind chill temperature does not apply to inanimate objects.  An inanimate object will not cool below the actual air temperature in wind.  For example, no matter how hard the wind blows, a pile of water on the ground will never freeze if the air temperature is kept above the freezing point.

Hot and Cold Weather

March 2011

https://www.hko.gov.hk/en/education/weather/hot-and-cold-weather/00047-the-wind-chill-effect.html

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The 2008 South China snow storm and the cold weather of Hong Kong - a retrospect

How did the snow storm occur? Why was the weather cold and cloudy in Hong Kong during the same period? Why was there no freezing rain in Hong Kong?

The south China snow storm in 2008 In early 2008, south China suffered from a severe snow storm. The snow storm occurred mainly in provinces like Guizhou, Hubei, Hunan and Jiangxi (Figure 1). Because of the relatively drier air mass in the north, precipitation was less in northern China than in southern China. Despite the colder temperatures, provinces in northern China was comparatively free of snow. There was extensive freezing rain in southern China (Fig.1), rendering the road surface icy and slippery, and causing traffic accidents or disruption. It also froze on power lines, significantly increased their weight and causing the power transmission tower to crash down. There were serious electricity blackouts in many cities.The formation of freezing rainThe formation process of freezing rain begins when snow flakes fall from clouds at high altitudes (Fig.2). When the snow flakes fall through a layer of warm air with temperature higher than the freezing point, they completely melt and form rain droplets. As the rain droplets continue to fall and enter a layer of air with subfreezing temperatures, the rain drops do not freeze instantly but become supercooled.  When the supercooled rain drops eventually reach any subfreezing object on the ground, they will freeze immediately.  This kind of precipitation is called freezing rain.Conditions favourable for formation of freezing rain Freezing rain usually forms when a subfreezing air stream meets with a warm air mass. By virtue of their different densities, the subfreezing cold air stream undercuts the warm air stream which is displaced aloft (Figure 2). An inversion layer will then be established.  Lack of condensation nuclei in the atmosphere is also an essential condition for freezing rain to form.How did the snow storm occur? The extensive snow storm was related to the La Nina event, during which sea surface water over the central and eastern tropical Pacific was persistently cooler than normal. This caused the atmospheric circulation to deviate from its normal pattern, and specifically, outbreaks of cold air became more frequent. While the cold air stream spread towards the south incessantly, warm moist air originated from the South China Sea and even as far away as in the Indian Ocean was continually transported to southern China.  The cold and warm air streams met over southern China and in the areas south of Chang Jiang, causing long period of precipitations including freezing rain and snow. As time progressed and the snow storm continued to unfold, a calamity was in its making. Why was the weather cold and cloudy in Hong Kong during the same period? The formation of the snow storm of southern China bears some common features with the cold and cloudy weather in Hong Kong starting mid-January 2008. The upper air temperature and wind direction over Hong Kong on 29 January 2008 would provide the clues. An inversion appeared below 800m (Fig.3) over Hong Kong, where the temperature increased with height. Hence, it was warm and moist near the top of the inversion but cold below the inversion. Figure 4 shows the winds below 800m over Hong Kong. Warm and moist southerly winds appeared aloft while cold northerly winds appeared at low altitudes. In fact, the inversion was more or less a permanent feature during the spell of cold and cloudy weather in Hong Kong.Why was there no freezing rain in Hong Kong? There was no freezing rain in Hong Kong because the ground temperature was higher than the freezing point (Fig.5). Let’s take the atmospheric temperature profile over Hong Kong on 29 January 2008 8a.m. (Fig.3) as an illustration. The atmospheric temperatures below 4000m were well above the freezing point.The longest cold weather warning of Hong Kong Although there was no freezing rain in Hong Kong, the weather was cold with some rain. The cold weather warning was in force continuously for 594.5 hours and spanned 26 days during the period from 24 January 2008 to 18 February 2008.  This is the longest warning since the cold weather warning service was launched in 1999. During the period in which the cold weather warning was in force, the minimum daily temperatures of Hong Kong were mostly below 12 degrees Celsius (Fig.5). 

Hot and Cold Weather

December 2009

https://www.hko.gov.hk/en/education/weather/hot-and-cold-weather/00049-the-2008-south-china-snow-storm-and-the-cold-weather-of-hong-kong-a-retrospect.html

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Beware of Health Effects of Extremely Hot Weather

In summer, everyone should pay more attention to heat related alerts or warnings from the Observatory and take all necessary protective measures against the heat and the sun. Those who are more vulnerable to heat stroke should pay special attention to prevent discomforts caused by the heat.

The impact of very hot weather on health cannot be ignored. Are we ready to deal with it? Let us first explain the heat-related alerts or warnings issued by the Observatory (Figure 1):Hot Weather Special AdvisoryWhen the weather becomes hot, especially when temperature is high, humidity is high and winds are weak, the public should take appropriate precautions to reduce the risk of heat stroke. When such weather conditions are expected, the Observatory will issue the "Hot Weather Special Advisory" to remind members of the public to prepare for hot weather.Very Hot Weather WarningWhen Hong Kong is expected to be affected by very hot weather, the Observatory issues the Very Hot Weather Warning (VHWW) to alert members of the public to beware of heat stroke and take precautionary measures. Under prolonged heat condition or weather generally becomes extremely hot, the Observatory will also issue the following Special Weather Tips as necessary when the VHWW is in effect:The above messages will be disseminated through the Observatory's website, the "MyObservatory" mobile application, TV and radio broadcasts. App users can also opt for receiving push notifications. People who are more vulnerable to heat stroke (Figure 2) should stay particularly vigilant or need more care. They should drink more water and take all necessary protective measures to prevent discomforts caused by very hot weather, and pay due attention to health conditions. Under extremely hot weather, vigorous physical activities should be avoided. Other measures against the heat and measures for sun protection should not be ignored. For more information on protection against the heat, please visit the following websites (with links):In addition to temperature, other factors such as moisture, winds and solar radiation can affect heat stress and the risk of heat stroke as well. Therefore, the Observatory established the "Hong Kong Heat Index" (HKHI) which comprehensively considers temperature, humidity, airflow and solar radiation. According to the joint study by the Observatory and a local university, the public should take appropriate precautions when the HKHI at King’s Park is around 30 or above to avoid health effects caused by hot weather.The HKHI is not temperature but an index that takes into account the aforementioned factors. Under the same temperature conditions, the HKHI will be lower with lower humidity, weaker solar radiation, or stronger winds. For example, on 14 September 2022, it was sunny and very dry in Hong Kong with a maximum temperature of 35.5 degrees Celsius recorded at the Observatory. The highest HKHI was only 27.5 as the daily average relative humidity was just 44% at King’s Park. On the other hand, the HKHI will be higher under conditions of higher humidity, scorching sun, and lighter winds. For example, Hong Kong had a higher relative humidity under the influence of a southwesterly airstream on 12 August 2019. The maximum temperature recorded by the Observatory was 34.0 degrees, not reaching the extremely hot level. However, the maximum HKHI at King’s Park reached 32.4 and the average relative humidity at King’s Park that day was 82%. The HKHI is composed of multiple elements and is affected by changes in multiple factors. Among them, the value of solar radiation varies due to cloud cover or showers with higher fluctuations. There are still limitations in accurately predicting the index by computer forecasting models.In summer, everyone should pay more attention to heat related alerts or warnings from the Observatory and take all necessary protective measures against the heat and the sun. Those who are more vulnerable to heat stroke should pay special attention to prevent discomforts caused by the heat.

Hot and Cold Weather

December 2023

https://www.hko.gov.hk/en/education/weather/hot-and-cold-weather/00706-Beware-of-Health-Effects-of-Extremely-Hot-Weather.html

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A Chat of Very Hot Weather

High temperature is a natural weather phenomenon, whereas heat wave is a prolonged period of excessively hot weather.There is no universal definition of heat wave. Since there is a great difference in the climate between various regions of the world, the weather services in different countries will formulate heat advisories or warnings with respect to their own local climate.

When we talk about weather hazards, what come first to our mind are tropical cyclones, heavy rain, thunderstorms etc.  However, sunny weather may also evolve into heat waves, posing threat to the public.High temperature is a natural weather phenomenon, whereas heat wave is a prolonged period of excessively hot weather.  It may have negative effects to human beings, animals and plants when they experience difficulties in adaptation.  A notable example is the heat wave across Asia and Europe in 2007.  In June that year, temperatures in India and Pakistan soared to near 50 deg C, resulting in over 200 deaths in these two countries.  The heat wave then propagated to southeastern Europe, where the maximum temperatures generally rose above 40 deg C.  The extreme temperature caused a total of several tens of deaths in Cyprus, Austria, Romania and other nearby countries.There is no universal definition of heat wave.  Since there is a great difference in the climate between various regions of the world, the weather services in different countries will formulate heat advisories or warnings with respect to their own local climate.  Take Hong Kong as an example, the Hong Kong Observatory will consider the temperature, relative humidity, wind speed etc. in issuing the Very Hot Weather Warning.Hong Kong is located along the South China coast, and belongs to the subtropical climate.  High temperature weather occurs rather frequently in the summer.  From 2007 to 2009, there was a total of 76 days with heat stroke cases.  From the graph below, we can see that when the maximum temperature exceeds 30 deg C, the number of days with heat stroke cases increases sharply.There are two predominant types of weather pattern associated with high temperature in southern China, namely: (1) Subtropical ridge; (2) Tropical cyclone.(1)    Subtropical ridgeThe Pacific Ridge is a perennial stable weather system.  It becomes more prominent during the summer and occasionally extends westwards to cover southeastern China.  As it is dominated by subsiding air, the region under its control is characterized by calm winds, cloud-free and hot weather.  The following example occurred in late July 2007.  From the weather map below we can see the south China coastal areas were dominated by the westward extension of the ridge of high pressure from the Pacific.  From 24 July onwards, there were 12 consecutive days of which the maximum temperature reached 33 deg C or above.(2) Tropical cycloneWhen the center of a tropical cyclone is still at a great distance from Hong Kong, its associated inclement weather may not yet affect our territory, but the subsidence air at its outer circulation often brings very hot weather to the coast of Guangdong.  One example is that on 13 September 2008, when Typhoon Sinlaku was located over the seas east of Taiwan.  Affected by the subsidence air at Sinlaku’s outer circulation, the weather was very hot in Hing Kong and the maximum temperature reached above 34 deg C.

Hot and Cold Weather

June 2011

https://www.hko.gov.hk/en/education/weather/hot-and-cold-weather/00046-a-chat-of-very-hot-weather.html

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What is "Heatwave"?

This article explains what heatwave is and how heatwave brings abnormal hot weather.

Under global warming, extreme weather events, such as extreme high temperatures, have become more frequently. From time to time, we learned from the media that a region was hit by "heatwave" with record-breaking temperatures. 　So, what is "heatwave"? How does "heatwave" bring abnormal hot weather?"Heat" in the first part of the word "heatwave" refers to temperatures higher than the climatological normal of an area. When those anomalous high temperatures sustain for a certain period of time, the situation will be named as "Heatwave". As the characteristics of climate vary place by place, there is no universal definition of "heatwave" in terms of the degrees of temperature above the climatological normal and the duration.Some may have mistaken heatwave as a weather system, being the "cause" of persistent and abnormal hot weather. Indeed, heatwave is a phenomenon, and it does not "hit" or "depart" from a place like a weather system. Its spread and persistence depend on the weather systems that give rise to the heatwave. In other words, heatwave is the "effect" instead of the "cause" of persistent and abnormal hot weather. For example, heatwaves over mid to high-latitude regions are usually caused by blocking[1]. Under blocking pattern, the atmospheric circulation becomes relatively stagnant. Subsiding air of the ridge of high pressure inhibits cooling. Together with hot air mass being transported from low latitudes to mid and high latitudes, such pattern is conducive to sustained hot weather in summer.The heatwave that started affecting the northwestern part of North America in late June 2021 was in fact a result of blocking. Figure 1 shows temperatures over some places in the northwestern part of the United States and the southwestern part of Canada were more than 10 degrees above the climatological normal. Lytton, a village located in the southern part of British Columbia, Canada, even recorded a record-breaking temperature of 49.6 ℃, which was also the highest ever observed in Canada[2].Prolonged exposure to high temperatures will cause heat exhaustion, as well as deterioration of health conditions of some patients with chronic diseases[3], leading to fatalities. Over 10,000 deaths were attributed to the heatwave over Russia in July 2010[4]. Besides, increase of electricity demand over cities and possibility of crop failure are also some socio-economic impacts of heatwaves.Researches revealed that heatwaves are very likely to occur with higher frequency and longer duration under global warming[5]. We must take every effective measure now to save energy and reduce emissions, in an effort to slowing down global warming, and to adapt to climate change and combat against extreme weather. Otherwise, mankind will face the unbearable consequences.

Hot and Cold Weather

October 2021

https://www.hko.gov.hk/en/education/weather/hot-and-cold-weather/00661-What-is-Heatwave.html

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Forecasting of Very Hot Weather

Hong Kong’s climate is sub-tropical, very hot weather in summer is usually associated with the subtropical ridge or the subsiding air at the periphery of a tropical cyclone at considerable distance from Hong Kong[1][2]. For the evolution of such relatively large-scale weather systems in the coming few days, computer weather prediction models are often able to provide reliable forecasts. Yet, it is still a challenge to forecast accurately the daily maximum temperatures in Hong Kong.

In summer, the weather in Hong Kong is generally hot with an average daily maximum temperature of around 31 degrees Celsius in July and August. Temperatures can even be higher on days with fine weather. During the period of 2010 to 2019, there were generally more than 20 “very hot days” (with maximum temperature reaching 33.0 degrees Celsius or above) in a year recorded at the Hong Kong Observatory (HKO) headquarters, and the number of “very hot days” even exceeded 40 in 2020. Since very hot weather can increase the risk of heatstroke and may worsen chronic health conditions, high-temperature weather is one of the focus areas in weather forecasting in summer.Hong Kong’s climate is sub-tropical, very hot weather in summer is usually associated with the subtropical ridge or the subsiding air at the periphery of a tropical cyclone at considerable distance from Hong Kong[1][2]. For the evolution of such relatively large-scale weather systems in the coming few days, computer weather prediction models are often able to provide reliable forecasts. Yet, it is still a challenge to forecast accurately the daily maximum temperatures in Hong Kong.Figure 1 shows the variation of air temperatures at the HKO headquarters predicted by a few major computer weather prediction models on 15 July 2020 for the following ten days (purple, green and red lines). The actual recorded temperatures were also included and depicted as a black line in the figure for easy comparison with the model forecasts. It can be seen that the temperature predictions by computer models could be at least two or three degrees lower than the observations. One of the reasons is that the limited resolution of computer models cannot fully represent and simulate the effects of urbanization on air temperatures. In view of this, the Observatory adjusts the latest predictions of models based on their recent biases, and then uses a weighted average method according to the recent performance of each model to develop a temperature forecast guidance product for forecasters’ reference[3]. As compared with the predictions from individual computer models in Figure 1, in general this temperature forecast guidance (blue line) was found to be much closer to the actual observed values. Furthermore, forecasters will also make reference to the probability forecasts[4] and extreme weather forecast products[5] based on ensemble prediction systems to assess the likelihood of very hot weather in the coming few days. Apart from the evolution of large-scale weather systems, forecasters also need to consider other factors affecting air temperatures in forecasting very hot weather, and one of which is showers. In summer, when affected by the previously mentioned weather systems, the weather will be generally fine over southern China. However, if the atmospheric conditions are unstable, showers sometimes develop over the coastal areas and slightly mitigate the high temperatures. Generally speaking, under southerly or southwesterly winds, showers often occur in the morning, while in light wind conditions, showers triggered by high temperatures often develop after midday. Yet there is considerable randomness as to when and where the showers actually develop on each day. As a result, the temperature variation in different parts of Hong Kong may not be the same.Figure 2 shows the daily rainfall distribution in Hong Kong and the maximum air temperatures from 15 to 23 July 2020. During the period, the local weather was mainly fine under the dominance of the subtropical ridge, but isolated showers also affected the territory. On days when the urban areas were affected by showers (such as 16 and 20 July), the maximum air temperatures recorded at the Observatory were slightly lower compared to the other days. As for 21 July, although there were showers over the northern part of the New Territories, the urban areas were not affected by showers and the temperature at the HKO headquarters climbed to around 35 degrees Celsius. With current technology, computer models are unable to capture the detailed temporal and spatial evolution of these showers and their impacts on temperatures at individual locations. As such, on some days there were still discrepancies between the actual temperatures and the model forecasts even after adjustments were made based on past biases (Figure 1).Hong Kong is located at the south China coast, and winds are another factor affecting the maximum temperatures at various parts of the territory. As temperature rises more slowly over the sea than over land, sea breeze brought by such temperature difference helps slow down the rate of increase in temperatures over land. This explains why daytime air temperatures over the New Territories are typically higher than those in the urban areas closer to the coast (Figure 2). However, when winds are weak, temperatures over land cannot be effectively moderated by sea breeze, often resulting in a larger temperature difference between the New Territories and the urban areas. Figure 3 shows the daytime wind conditions and the maximum air temperatures on 24 June and 24 July 2020. Although there was prolonged sunshine on both days, there were slightly stronger southwesterlies on 24 June. Temperatures over the New Territories were only a couple of degrees higher than those in the urban areas. In contrast, on 24 July when winds were weaker, temperatures reached 37 degrees Celsius over the northern part of the New Territories, which were higher than those in the urban areas by three degrees or more.In conclusion, even though computer weather prediction models can broadly capture the temperature trend in the coming few days, forecasting very hot weather, especially the spatial variations of temperatures in different regions, requires forecasters’ skilful analysis of various factors affecting the maximum temperatures. Under the influence of global warming and urbanization effect, the occurrence of very hot weather is expected to become more and more frequent in the future. On hot summer days, please pay attention to the information issued by the Observatory regarding high temperatures, in particular the Hot Weather Special Advisory and the Very Hot Weather Warning [6], and take corresponding measures against potential health impacts.

Hot and Cold Weather

September 2020

https://www.hko.gov.hk/en/education/weather/forecasting/00553-forecasting-of-very-hot-weather.html

en

A Brief History of “Typhoon Gun”

Have you ever heard of “typhoon gun” to warn public for bad weather associated with tropical cyclone ? This article will let you have a brief history of using typhoon gun and explosives as tropical cyclone warning in Hong Kong.

While you are probably familiar with the current numbered tropical cyclone warning signal system (1-3-8-9-10) in Hong Kong, have you ever heard of “typhoon gun”?Soon after the establishment of the Hong Kong Observatory in 1883, a tropical cyclone warning system was implemented as one of the main tasks of the Observatory. The signal system consisted of the non-local and local warning signals at the time. The non-local signals were to provide mariners and shipmasters with the position of tropical cyclone, its direction and speed of movement; whereas the local signals provided warning of the threat of winds associated with tropical cyclones to the public.Firing typhoon gun[Note] was the first local tropical cyclone warning signal adopted by the Observatory after its establishment. At that time, the typhoon gun was placed at the Police Barracks in Tsim Sha Tsui, facing the Victoria Harbour. When a tropical cyclone was approaching and was expected to bring gales to the territory, the typhoon gun would be fired once to warn the public. When winds with hurricane force were expected locally, the typhoon gun would be fired twice. When the wind direction was likely to change suddenly, the typhoon gun would be fired again to remind the public. According to available records, the typhoon gun was fired for the first time on 21 August 1884, but no gales were recorded at the Observatory and Gap Rock (a small island about 40 kilometers southwest of Hong Kong). It is worth noting that the typhoon gun was also used as the mail gun in announcing the arrival of postal services from London at that time, thus easily causing confusion to the public. In 1886, the Director of the Observatory, the Acting Harbour Master and the Postmaster General recommended that the use of mail gun should be discontinued.After the disastrous typhoon that battered Hong Kong in September 1906, a review committee that was set up at that time made suggestions on the tropical cyclone warning signal system. Since 1907, the typhoon gun was replaced by explosives which produced louder sounds. When winds with hurricane force were expected locally, three explosives would be fired at the Police Barracks and the Harbour Master’s Office at intervals of ten seconds. Moreover, a Black Cross was also hoisted above the non-local signals. This is the predecessor of the hurricane signal later (that is, the No. 10 typhoon signal in 1931/the No. 7 typhoon signal from 1917 to 1930). After the introduction of the local numbered tropical cyclone warning signal in 1917, the measure of firing explosives was maintained till 1937. The last time that explosives were fired locally was in September 1937.The records of the use of typhoon gun and explosives over a century ago were mainly documented in historical reports and newspapers. In recent years, the Hong Kong Observatory recovered and compiled relevant historical records from 1884 to 1937 and showed them in the webpage.

Tropical Cyclone Warning

May 2023

https://www.hko.gov.hk/en/education/weather/weather-warnings/00693-A-Brief-History-of-Typhoon-Gun.html

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The Tropical Cyclone Warning System in Hong Kong

The Hong Kong Observatory conducted a comprehensive review of the tropical cyclone warning system in 2006. During the review process, the Observatory reached out to different sectors of the community to consult their views. A public opinion survey was also carried out to gauge the public needs and expectations with respect to the warning system.

The Hong Kong Observatory conducted a comprehensive review of the tropical cyclone warning system in 2006. During the review process, the Observatory reached out to different sectors of the community to consult their views. A public opinion survey was also carried out to gauge the public needs and expectations with respect to the warning system.Starting from the tropical cyclone season in 2007, the Observatory implemented a series of measures with respect to the tropical cyclone warning system. In particular, the reference for the issue of No.3 and No.8 signals was expanded from the Victoria Harbour to a network of eight reference anemometers covering the whole of Hong Kong.In 2013, the Observatory replaced Wetland Park by Lau Fau Shan as one of the eight reference anemometers in the tropical cyclone warning system.With a view to improving the public understanding of the tropical cyclone warning system and encouraging a better use of the weather information provided by the Observatory, some salient points are given below in the format of questions and answers.What are the differences between the present tropical cyclone warning system and the old system just before 2007?The present tropical cyclone warning system has been in use since 2007.Comparing with the old tropical cyclone warning system before 2007, the present system has changed in the following aspects:(a) expand the reference for the issue of No.3 and No.8 signals from the Victoria Harbour to a network of eight reference anemometers near sea level covering the whole of Hong Kong (see Fig.1 and Note below);(b) issue No.3 or No.8 signal (as the case may be) when half or more anemometers in the reference network register or are expected to register strong winds or gale/storm force winds respectively and the wind condition is forecast to persist;(c) enhance dissemination of regional wind information. In tropical cyclone bulletins for the media, specifically indicate the locations where the winds are significantly higher than the general wind conditions in Hong Kong; and(d) issue an advisory in tropical cyclone bulletins that the traveling public should check with airlines before departing for the airport when weather conditions likely to cause significant disruptions to flight operations are expected.[Note: Lau Fau Shan station has replaced the nearby Wetland Park station as a reference anemometer station starting from 2013. The anemometer at Chek Lap Kok has moved from the Centre Runway to the North Runway since 2024.]In the present system, why only 8 anemometer stations but not all are chosen as the reference stations?When devising the network of reference anemometer stations, consideration has been given to the terrain of Hong Kong being naturally separated by mountain ranges. The eight anemometers have been selected on account of their good exposure and geographical distribution. Together, they provide a broad picture of the wind condition in Hong Kong.What is the reason for replacing the reference station at Wetland Park by Lau Fau Shan?Wetland Park station has been working normally since its launch in November 2005. The station is located inside Wetland Park near Tin Shui Wai in the northwestern part of the New Territories. It is exposed to natural wetlands to its east, while some high-rise buildings are located to its far west (about 500 to 700 metres away). Generally speaking, the station is fairly well exposed. However, the density of buildings near the station has been increasing in recent years. Statistics over the years indicated that the monthly mean wind speed of Wetland Park station was on a gradual decreasing trend in the past few years. This revealed that the wind speed at Wetland Park would be decreasing under the influence of its changing surrounding environment including the construction of new buildings.In addition, there are planned construction projects about 300 metres to the west of the station. Buildings of 3 to 11 storeys, with the tallest one of about 50 metres high, will be constructed. This is expected to further reduce the exposure to the west of Wetland Park station. To ensure that the tropical cyclone reference anemometer network can continue to reflect the general wind condition over Hong Kong, it is necessary to review the reference station in the northwestern part of the New Territories.The Lau Fau Shan station is located next to the Wetland Park station. There has been no significant change in the mean wind speed at Lau Fau Shan in the past few years. The station is considered to be a suitable replacement of Wetland Park station.Why the criterion for the issue of No.9 or No.10 signal is different from that for No.3 or No.8 signal?As for No.9 and No.10 signals, they are normally meant to warn of direct or near direct hit of a typhoon. As there is enormous spatial variation of wind direction and speed near the eye of a typhoon, there is little point in making reference to winds at a single location in the consideration of signals. Thus, the HKO has all along made reference to the wind condition of the whole territory in deciding on the issue of No.9 and No.10 signals.Can the tropical cyclone warning system reflect winds in different regions of Hong Kong?Due to local topography and the built environment, wind condition in different parts of Hong Kong can vary appreciably. A numbered tropical cyclone signal can provide a general warning for the public, but it has an inherent limitation in communicating varying wind speeds at different locations. In the present system, regional wind information will be disseminated to assist people in making decisions for their arrangements.What is the impact on wind speed when the rainband of a tropical cyclone moves across Hong Kong?When the rainband of a tropical cyclone is close to or moves across Hong Kong, it is usually accompanied with squalls. Wind speed in places of Hong Kong may abruptly increase . After the passage of the rainband, the high wind speed brought about by intense gusts usually drops in the order of ten minutes. The situation is similar to that when a squall line moves across Hong Kong, bringing short-lived severe and potentially damaging squalls.How does the Observatory enhance the dissemination of regional wind information?Regional wind information are readily available on the Observatory website, MyObservatory and from the Dial-a-weather system. The information is updated every 10 minutes. The Observatory will liaise with the media to broadcast such information on an hourly basis when wind conditions at certain locations are significantly above the level indicated by the territory-wide warning signal under the present system.Are aircraft operations affected by tropical cyclone warning signals?The tropical cyclone warning system is designed for use by the public. Aircraft operations do not depend on tropical cyclone signals. In respect of wind, aircraft operations at the airport are mainly affected by crosswinds (i.e. winds blowing across the aircraft from the side) as well as windshear and turbulence, rather than wind speed per se. Even if the No. 8 signal is in force, if the wind is blowing along the runway, it may not affect aircraft operation. On the other hand, if the wind is blowing across the runway, it may exceed the operating limit of aircraft, thus affecting aircraft operation. The traveling public should check the latest flight information and the transportation conditions to and fro the airport before departing for the airport.What should people do if they are in gales-affected region after HKO has issued the No.3 signal?If the employees have difficulties in reporting for duty during severe weather situations, they should inform their supervisors or employers as soon as practicable. The employers should give prime consideration to employees' safety both in the workplace and during their journeys to and from work. They should also adopt a flexible approach in dealing with cases of absence or late for work due to inclement weather condition. On the student side, parents should pay attention to the announcements on school arrangements made by the Education Bureau, and use their discretion in deciding whether or not to send their children to school based on the condition in their regions. In addition, the Education Bureau and the Labour Department will help reinforce the relevant message to schools, parents and employers.Is it still needed to have arrangement of work suspension with the present building structures and mass transportation network?The Observatory issues tropical cyclone warning based on the wind condition in Hong Kong to alert people of the general wind condition over the whole territory. It also provides regional weather information to assist members of the public and various trades and industries to take appropriate precautionary measures and make proper work arrangements. At present, various trades, industries and organisations will make reference to the guidelines provided by the Labour Department and decide their work arrangements having regard to the nature and requirements of their operations when tropical cyclone warning is in force.Is it possible for the Observatory to enhance the communication with the Education Bureau in inclement weather situations?The Observatory has been keeping close liaison with the Education Bureau (EDB) during severe weather situations. The Observatory will provide EDB with the necessary weather information in support of their decision-making on class suspension. In addition, the Observatory has also implemented a dedicated webpage for schools in its website to provide regional weather information for reference.

Tropical Cyclone Warning

https://www.hko.gov.hk/en/education/weather/weather-warnings/00054-the-tropical-cyclone-warning-system-in-hong-kong.html

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Interesting Statistics on Rainstorm Warning Signals

According to the statistics from 1992 to 2010, the average numbers of "Amber", "Red" and "Black" rainstorm warning signals issued each year are 21.3, 4.6 and 1.2 respecively. June is the month with the highest number of rainstorm warning signals issued. The average numbers of "Amber", "Red" and "Black" rainstorm warning signals issued in June are 5.3, 1.5 and 0.3 respectively. No rainstorm warning signals has ever been issued in January, February and December.

The rain season in Hong Kong normally lies between April and September. Troughs of low pressure would affect the south China coastal areas in May and June and bring heavy downpours to Hong Kong. These rainstorms will cause traffic disruption, floodings, landslips and even casualties. In 1992, the Observatory first operated a colour rainstorm warning system for different intensity of rainstorms to alert the public. The rainstorm warning signals were classified as "Green", "Amber", "Red" and "Black" colours. In 1998, the rainstorm warning system was revised to adopt a 3-level warnings classified as "Amber", "Red" and "Black" rainstorm warning signals. According to the warning signal definitions, "Amber" rainstorm warning signal represents heavy rain has fallen or is expected to fall generally over Hong Kong exceeding 30 millimetres an hour; "Red" rainstorm warning signal represents heavy rain has fallen or is expected to fall generally over Hong Kong exceeding 50 millimetres an hour and "Black" rainstorm warning signal represents heavy rain has fallen or is expected to fall generally over Hong Kong exceeding 70 millimetres an hour.According to the statistics from 1992 to 2010, the average numbers of "Amber", "Red" and "Black" rainstorm warning signals issued each year are 21.3, 4.6 and 1.2 respectively. June is the month with the highest number of rainstorm warning signals issued. The average numbers of "Amber", "Red" and "Black" rainstorm warning signals issued in June are 5.3, 1.5 and 0.3 respectively. No rainstorm warning signals has ever been issued in January, February and December. The earliest issuance for "Amber", "Red" and "Black" rainstorm warning signals in a year were 5 March in 2009, 3 April in 2000 and 19 April in 2008 respectively while the latest issuance were 21 November in 2006, 18 October in 2002 and 2 September in 2001 respectively. The highest number of "Black" rainstorm warning signals issued in a year was 3 times, happened in 2000 and 2006. The shortest break between two "Black" rainstorm warning signals was recorded on 23 and 24 August 1999 in which the separation of these two rainstorm warning signals was less than 17 hours. The longest durations for "Amber", "Red" and "Black" rainstorm warning signals being in effect were 17.4, 13.2 and 5.8 hours respectively.There is a tendency for "Red" and "Black" rainstorm warning signals to occur at late night or in the early morning. In fact, the effective time for "Red" rainstorm warning signal during the period from midnight to noon is about 1.5 times of those from noon to midnight and that for "Black" rainstorm warning signal is even about 2.5 times higher. One of the possible reasons is that the low-level advection of warm moist air from the South China Sea towards the coastal region tends to occur at late night or early morning during the summer time which reduces the atmospheric stability and favours the formation of heavy rain. Furthermore, the rainstorms affecting Hong Kong are usually caused by troughs of low pressure, tropical cyclones, southwest monsoon and cold fronts. By analysing the rainstorm cases of "Red" and "Black" rainstorm warning signals, most of them were attributed to troughs of low pressure, about 59%. About 24% of the cases were attributed to tropical cyclones. Southwest monsoon and cold fronts contributed 6% and 4% respectively.In recent years, the heaviest rainstorm in Hong Kong was the one on 7 June 2008. Under the influence of an active trough of low pressure, a rain band swept across Hong Kong from west to east in that morning. Due to the rapid development of the rain band, the Observatory first issued "Amber" rainstorm warning signal at 5:15 am, followed by "Red" rainstorm warning signal in 40 minutes later and finally "Black" rainstorm warning signal at 6:40 am. The "Black" rainstorm warning signal last for 4 hours and 20 minutes till 11:00 am, making it the 4th longest duration of "Black" rainstorm warning signal being in force. During the period of the "Black" rainstorm warning signal, the total rainfall recorded at the Observatory for one hour between 8 and 9 am was 145.5 millimetres, which was the highest record of one hour rainfall since 1884.

Rainstorm Warnings

June 2011

https://www.hko.gov.hk/en/education/weather/rain/00043-interesting-statistics-on-rainstorm-warning-signals.html

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Monsoon Lows and Tropical Cyclones

Although a monsoon low and a well-developed tropical cyclone both have rotational circulations, there are obvious differences in structure.Whether the former will develop into the latter depends on the atmospheric environment, such as vertical variations in winds, transport of moisture, sea surface temperature, and high altitude divergence.

Sometimes during the Northern Hemisphere summer, an elongated region of dense clouds several thousand kilometres long (the area marked by yellow dotted line) appears on the satellite images. Meteorologists call this the "Intertropical Convergence Zone", while the section from the South China Sea to the Indian Ocean is also known as the "monsoon trough". Due to the converging winds from opposite directions - southwest monsoon from the south and the northeast trade winds from the north (see blue arrows), convective activity can easily be triggered along the monsoon trough and low pressure areas can develop. The resulting phenomena are referred to as monsoon lows.Although a monsoon low and a well-developed tropical cyclone both have rotational circulations, there are obvious differences in structure (see table). Whether the former will develop into the latter depends on the atmospheric environment, such as vertical variations in winds, transport of moisture, sea surface temperature, and high altitude divergence. The tropical depression forming over the coastal waters of western Guangdong on the morning of 17 August 2016 was an example of a successful evolution from a monsoon low to a tropical cyclone.

Monsoon

April 2017

https://www.hko.gov.hk/en/education/weather/monsoons/00491-monsoon-lows-and-tropical-cyclones.html

en

Will Easterlies Bring a Fine or Gloomy Day?

Following the easterly airstream may be a fine or rainy day. What are the reasons for such a difference in weather?

In autumn and winter, we often hear from weather reports that Hong Kong is under the influence of an 'easterly airstream'. Following the easterly airstream may be a fine or rainy day. What are the reasons for such a difference in weather?As autumn and winter progress, the Sun becomes further away from the Northern Hemisphere. Hence, air temperatures drop over the Asian continent and an area of high pressure is formed. Monsoon is developed as air flows from this high pressure area to the ocean where the pressure is lower. The area of high pressure does not stay over the Asian continent over a long period of time, instead it moves from west to east across the continent. When the centre of the high pressure area is over the inland region to the north of Hong Kong, Hong Kong will be generally affected by a relatively dry northerly airstream. When the centre of the high pressure area moves to eastern China, winds in Hong Kong will generally turn to the east (Figure 1). In this case, the air mass reaching Hong Kong usually takes a track along the coastal areas of southeastern China. It can be dry or moist, depending on the time over the land or the sea. When the area of high pressure moves further east to Japan or Korea, local easterlies will persist. However, the air mass will become more humid as it traverses a longer sea track.A humid air mass favours cloud formation as it contains more water vapour. Let us take 27 November 2014 as an example. It was cloudy with a few rain patches in Hong Kong under moderate to fresh easterlies. The satellite imagery taken in the afternoon showed that a band of clouds was covering the eastern coast of Guangdong (Figure 2). The backward trajectory map showed that the air mass reaching Hong Kong originated from the seas near Japan and the Korean peninsula, and moved over the East China Sea and the Taiwan Strait (Figure 3). The humidity of the air mass increased as it traversed a longer sea track. As such, cloud and rain could be formed easily.If the easterly winds affecting us are drier, it will be more difficult for clouds to form or sustain. Let us take 28 October 2014 as an example. Fresh to strong easterlies prevailed over Hong Kong and the Strong Monsoon Signal was in force for the whole day. However, the weather was generally fine in Hong Kong and over the coast of eastern Guangdong (Figure 4). Again, the backward trajectory map in Figure 5 can give us some clues. The air mass arriving at Hong Kong on that day originated from eastern China and tracked along the coast of southeastern China. Since the air mass came from the land and the sea track was short, its humidity was relatively low. Even if clouds could form, they were thin and would dissipate easily under insolation. Local weather was therefore generally fine during the day.From the above, we can see the characteristics of the easterlies that affect Hong Kong. It may originate from the land or travel along a longer sea track. Therefore, it can be dry or moist, bringing us fine or gloomy weather. It is also worth noting that if the easterlies are strong and moist, turbulence generated by the strong winds helps lift the air near the surface and favours the formation of clouds.

Monsoon

December 2014

https://www.hko.gov.hk/en/education/weather/monsoons/00446-will-easterlies-bring-a-fine-or-gloomy-day.html

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An Abrupt Change of Weather due to the Passage of a Severe Cold Front

The passage of a severe cold front and its associated winter monsoon bringing an abrupt and significant change of the weather in Hong Kong.

The difference in heat absorption between land and sea is the major reason for the formation of typical monsoon during winter and summer.  In addition to the dynamical effect due to terrain, the Tibetan Plateau over western China has a significant effect on the monsoon.  An area of high pressure ass℃iated with cold air normally forms over the Tibetan Plateau, being of one of the sources of cold air in winter.  Cold air will basically follow an anticyclonic outflow to spread southwards over China.  The winter monsoon in the Northern Hemisphere generally prevails from December to February.  There will still be outbreaks of northeast monsoon in March, or even up to May or June, but the strength and frequency decreases as season progresses.Below is an example on the passage of a severe cold front and its ass℃iated winter monsoon bringing an abrupt and significant change of the weather in Hong Kong on 13 March 2009.  The surface weather chart in figure 1 delineates a cold front over southern China on the morning of that day.  The cold front was expected to reach Hong Kong in the afternoon on the same day.  Before the arrival of the cold front, the weather in Hong Kong was generally calm with light breeze, mainly from the west and temperatures rose to 25℃ in parts of Hong Kong in the early afternoon.  With the passage of the cold front after 3:10 p.m., winds at the Hong Kong International Airport (HKIA) turned from westerly of about 5 knots (Figure 2) to north-northwesterly of over 25 knots (Figure 3) within 5 minutes.  Gale force winds were recorded in the western parts of the New Territories and the Lantau Island.  Temperatures at the HKIA dropped from about 24℃ at 3:15 p.m. to about 17℃ at 4:00 p.m. (i.e. a drop of 7℃ in 45 minutes) and further down to about 14℃ before midnight (Figure 4).  There was a sharp rise of air pressure from 1014 hPa at 3 p.m. to 1024 hPa at 9 p.m. (i.e. an increase of about 10 hPa in 6 hours) (Figure 5).  The onset of an intense winter monsoon will bring a significant change of weather within a short period of time.  Forecasters will provide not only the expected timing of the arrival of the cold front, but also the effect on the weather elements before and after the passage of the cold front to the public.  The public should take note of the weather forecast especially when there are significant weather changes.  The Hong Kong Observatory will issue the Cold Weather Warning to alert members of the public to the cold weather and its ass℃iated adverse health effect.

Monsoon

December 2010

https://www.hko.gov.hk/en/education/weather/hot-and-cold-weather/00048-an-abrupt-change-of-weather-due-to-the-passage-of-a-severe-cold-front.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. 

Monsoon

October 2010

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

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Why the winter monsoon in Hong Kong is generally called the northeast monsoon ?

The prevailing winds in winter monsoon will generally be blowing from the sector between the north and the east over southern China, hence the term northeast monsoon.

The primary causation of winds is due to the variations of temperature and pressure. Land and sea breezes are familiar weather phenomenon and these breezes begin to blow as a result of the diurnal temperature and pressure variations between land and sea. Monsoon winds have periodic alternation of direction and may be thought of as a very large-scale land and sea breezes, but having a seasonal characteristic: winter and summer, instead of diurnal. The primary cause of the winter monsoon is the differential warming and cooling over land and ocean. In winter, air over the land cools faster with lower temperature than the air over the ocean. The temperature differences give rise to pressure differences which in turn generate air flowing from land to sea. With the arrival of winter surge, air temperatures over the mainland China are lower than the air temperatures over the South China Sea and give rise to pressure differences. The cold airmass from the high pressure area over the mainland China flows southwards towards the relatively low pressure area over the South China Sea due to the pressure gradient force. Winds will be diverted slightly due to the effect of the Earths rotation which is known as the Coriolis effect.  The Coriolis force always acts perpendicularly to the wind direction,  steering the air to the right, and finally the air spirals clockwise around the high-pressure system in the northern hemisphere.  In the southern hemisphere, the Coriolis force acts to the left and winds spiral anti-clockwisely around the high-pressure system.   Apart from the Coriolis force, winds near the Earths surface will be retarded by the effect of friction which is always opposite to the direction of air motion.  Under the combined effect of the pressure gradient force, the Coriolis force and friction, the prevailing winds in winter monsoon will generally be blowing from the sector between the north and the east over southern China, hence the term northeast monsoon. (see Figure 1).  

Monsoon

December 2010

https://www.hko.gov.hk/en/education/weather/monsoons/00068-why-the-winter-monsoon-in-hong-kong-is-generally-called-the-northeast-monsoon.html

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The northeast monsoon and its effects to the weather of southern China in autumn

In normal circumstances, the northeast monsoon starts to affect southern China in autumn.Tropical cyclones over the South China Sea around the autumn equinox frequently makes landfall along the coastal areas of Guangdong and the Hainan province. Typhoon Damrey, Tropical Storm Francisco and Typhoon Ketsana are a few examples... the effect of the northeast monsoon to the movement of Megi over the South China Sea presented a challenge to weather forecasters.

The northeast monsoon in China is basically attributed to the cold air and high pressure area residing over the Asian Continent in the latter part of a year.  The air that flows from high to low pressure regions is called the northeast monsoon.  In normal circumstances, the northeast monsoon starts to affect southern China in autumn.In Hong Kong, autumn usually begins from mid-September to late October.  Since autumn this year (2010), several surges of the northeast monsoon and their replenishments had affected Hong Kong.  Local temperatures dropped from over 30℃ in summer to around 20℃.  Hot weather was replaced by mild conditions with winds turning from southwesterlies/southeasterlies to northeasterlies/northerlies.  One would find that cumulonimbus clouds and the associated showers became rare, while stratocumulus clouds would occasionally bring rain to the territory.  All these changes are related to the northeast monsoon.  From early to mid-September, the subtropical ridge1 shifts southwards from 29-30 to 24-25 degree North (latitude) in concert with the weakening of the summer monsoon, while the cold high pressure system establishes itself over northern China.  Northerly winds at the 850 hecto-pascal (hpa) level (around 1500 metres above ground) spread from Changjiang region southeastwards, signifying the establishment of the northeast monsoon.  By the end of September (around the Autumnal Equinox), cold air from the north could ride over the Nanling mountain and intrude into Guangdong.  In early October, the subtropical ridge further retreats to the south, dry and cold air from the north could then frequently reach the coastal areas of southern China.  The daily mean temperature during this period goes below 23℃ with dry and windy weather.Apart from bringing about changes in winds and temperatures, the northeast monsoon can also affect tropical cyclones over the South China Sea.  If the northeast monsoon happens to meet the warm and moist air associated with tropical cyclones, windy and rainy weather could follow and is often called "Wet Cold Dew Wind" (WCDW) weather in Mainland China.  One such example is the heavy rain episode at Hainan in early October this year.  The heavy rain severely affected the harvest of late crop and vegetable.  The interval between two WCDW occasions is usually 6 to 8 days, being shorter in northern Guangdong and longer in the south.  During a WCDW occasion, the average daily rainfall is in general in the order of 30mm but it could be over 100mm in the close proximity of a tropical cyclone.  The northeast monsoon helps to intensify the rain-bands associated with the tropical cyclone and significantly raises the probability of daily rainfall over 100mm in the Hainan province and the coastal areas of Guangdong.  Similar weather affected Hong Kong between 10 and 11 October 2010.  At that time, the combined effect of a low depression area and the northeast monsoon brought over 50mm of rainfall to Shatin in Hong Kong (Fig.1).  The Strong Monsoon Warning Signal was in force during that period.  Although a low pressure area instead of a tropical cyclone is involved in this occasion, it is a good example of the interaction of the northeast monsoon with warm and moist air associated with the low pressure area, bringing an intense rainband to southern China.Apart from enhancing the rainbands of tropical cyclones, the northeast monsoon can also affect their movement.  Tropical cyclones over the South China Sea around the autumn equinox frequently makes landfall along the coastal areas of Guangdong and the Hainan province.  Typhoon Damrey (25/9/2005), Tropical Storm Francisco (24/9/2007) (Figures 2 and 3), and Typhoon Ketsana (28/9/2009) are a few examples.Tropical cyclone usually moves along the southern edge of the subtropical ridge in October and November, crossing the Bashi Strait or Balintang Channel or the northern part of Philippines and entering the South China Sea.  They would then be steered to the southwest by the strengthening northeast monsoon to affect the coastal areas of Guangdong.  When the tropical cyclone encounters the northeast monsoon, it usually moves on a westward track and makes landfall to the west of the Pearl River Delta region or coastal areas of Hainan.  However, there are exceptions and Severe Typhoon Megi in 2010 was an example.  Movement of tropical cyclones in summer is usually dominated by the steering flow at the 500hpa level (around 6000 metres above ground) while the role of the 850hpa level flow becomes more prominent in autumn.  Because of the different air flow at different altitudes, the track of a tropical cyclone is not straight forward.  Between 20 and 22 October 2010, the effect of the northeast monsoon to the movement of Megi over the South China Sea presented a challenge to weather forecasters (Figures 4 and 5).  It eventually took a northerly track and landed on the coast of Fujian, bringing severe damages to nearby areas.  Fortunately, there was  no major impact to Hong Kong. 

Monsoon

December 2010

https://www.hko.gov.hk/en/education/weather/monsoons/00071-the-northeast-monsoon-and-its-effects-to-the-weather-of-southern-china-in-autumn.html

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Winter monsoon - from the perspective of strong monsoon signal

In normal circumstances, the northeast monsoon starts to affect southern China in autumn.

Winter monsoon normally blows from the north or from the east. The Strong Monsoon Signal (SMS) will be issued when winds associated with the monsoon are over or are expected to exceed 40 kilometres per hour anywhere near sea level in Hong Kong. Depending on the time of the season, and the juxtaposition of various weather systems, surges of cold air will arrive in Hong Kong as northerlies, northeasterlies or easterlies. The SMS will alert the public of the direction most exposed to high winds.  Below, we shall see how the wind direction given in the SMSs can shed light on some seasonal properties of the winter monsoon.Table 1 reveals that in winter, the occasions of cold air arriving in Hong Kong from the north are about twice of that from the east; and that this ratio reverses towards the end of the season. There, in fact, is a reason for this.In winter, cold air accumulating over Siberia spreads outward, and the cold air mass moves to the east while it extends towards the south. Figure 1 and figure 2 show a typical surface weather map for a northerly surge, and an easterly surge, respectively on 27 February 2008 and 24 February 2008. The Observatory issued SMS on both days. With significant cold air outbreaks, surges extend southward rapidly and the high pressure area associated with the air mass will be situated to the north or northwest of Hong Kong, bringing northerly winds to the territory. When the outbreaks are weaker, or being affected by other weather systems, the cold air mass would have normally moved over to eastern China. The cold air will then reach the south China coastal areas passing through the Taiwan Strait, bringing easterlies to Hong Kong. Strong northerlies in winter will bring dry air and significant drop in temperatures due to large thermal difference between the coastal and inland areas. Whereas for easterlies, part of the cold air mass is over the seas and modified by the warmer sea waters, temperature drop is generally less and the air is less dry. However, because of less surface friction over waters, and the establishment of an easterly ridge over the coastal areas of southeast China due to terrain effect, winds from the east could easily reach strong force, necessitating the issuance of SMS even under weaker cold surges.Figure 3 shows the distribution of the northeast monsoon when other seasons are also considered. Cold air may reach the south China coastal areas commencing in autumn, dominated by easterly surges. During the peak season of the northeast monsoon, the majority will be northerly surges. In spring, easterlies take over again and the number of the northeast monsoon in Hong Kong drops quickly.Even though the above analysis is based fully on the statistics of Strong Monsoon Signals and the northeast monsoon cases are not exhaustive, they are indicative guidance on the understanding of the process of strong cold air outbreaks over the South China coastal areas.

Monsoon

December 2010

https://www.hko.gov.hk/en/education/weather/monsoons/00070-winter-monsoon-from-the-perspective-of-strong-monsoon-signal.html

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Combined Effect of Tropical Cyclone and Northeast Monsoon

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

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

Monsoon

March 2010

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

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... winds freshening from the east later (with a few rain patches)" in Weather Forecasts

The leading edge of an easterly surge is often marked by a distinct, narrow band of clouds. Its passage will normally lead to a sudden increase in cloudiness or even rain.

During the winter and spring seasons, if the cold air from the north takes an eastern route i.e. along the coast of southeastern China, as it heads south, the monsoon usually reaches Hong Kong in the form of an "easterly surge". When the cold air impinges upon a warmer and more moist airmass, the latter will be lifted as a result of its lower density, cooling and condensing its water vapour content in the process (Figure. 1). Under suitable meteorological conditions, the leading edge of an "easterly surge" is often marked by a distinct, narrow band of clouds. Its passage will normally lead to a sudden increase in cloudiness or even rain, hence its movement is closely monitored by weather forecasters.The evolution of local weather during an "easterly surge" on 26 January 2014 happened to be well captured by the high-resolution polar-orbiting satellite. Figure. 2 shows the arrival of a cloud band, associated with a fresh easterly airstream, over the eastern waters of Hong Kong around 2 p.m. that day. Meanwhile, wind speed recorded at Waglan Island saw a rapid rise from 2 - 3 ms-1 to about 10 ms-1.The cloud band associated with this "easterly surge" covered a relatively small area, being only around ten kilometres wide or less in the vicinity of Hong Kong. This is smaller than the horizontal resolution of many global and regional numerical weather prediction (NWP) models. Figure 3 shows a high resolution NWP model successfully simulated the cloud band associated with the "easterly surge" in this case. Hence, to accurately capture the weather evolution due to such fine-scale systems, high resolution modelling system would be a very useful tool.

Monsoon

June 2014

https://www.hko.gov.hk/en/education/weather/weather-phenomena/00461--winds-freshening-from-the-east-later-with-a-few-rain-patches-in-weather-forecasts.html

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Fog Explained

What is fog? What is the difference between fog and mist? What causes the fog commonly seen in springtime? What is the relationship between atmospheric stability and fog? What are the effects of fog on our daily life?

Fog is made up of a large amount of tiny suspended water droplets near the ground. In Hong Kong, fog is common in springtime and may greatly affect shipping and aviation.What is fog?Under light wind, stable and humid conditions, if the air near the ground cools down sufficiently, water vapour in the air may condense into tiny water droplets. These droplets reduce the visibility near ground level. The phenomenon is called fog. In Hong Kong, fog is common in springtime between February and April.What is the difference between fog and mist?According to the World Meteorological Organization, fog is said to have occurred when the visibility is reduced to less than 1 km by water droplets suspended in the air. For mist, there is no uniform definition at present. Different places in the world use slightly different definitions. In Hong Kong, mist is said to have formed when the visibility falls between 1 to 5 km.What causes the fog commonly seen in springtime?In springtime, Hong Kong and neighbour areas are affected by alternate cold and warm air. As cold air from the north recedes, warm and humid air comes in from the sea. During this time, as the water near the coast is still rather cold, the warm and humid air may be cooled sufficiently by the underlying cold water. This results in condensation of water vapour into droplets and hence formation of fog.What is the relationship between atmospheric stability and fog?When the air near the ground is colder than the air aloft, the denser air near the ground will not rise and will remain in its original position. The atmosphere is said to be stable. Stable atmosphere is favorable for fog formation. The atmosphere is usually more stable in spring, and is relatively unstable in summer. In the same way, under stable and dry conditions, dust particles suspended in the air are not easily dispersed, resulting in haze.What are the effects of fog on our daily life?Fog has a great effect on traffic. In Hong Kong, dense fog has led to marine crashes and aircraft delays, resulting in injuries and economic losses. On the highway, traffic accidents have also resulted because of dense fog.

Visibility

https://www.hko.gov.hk/en/education/weather/visibility/00090-fog-explained.html

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More on Fog

Advection fog, mixing fog and radiation fog are commonly observed fog in Hong Kong.

In "Fog Explained", the formation of fog is described.  Here we shall further illustrate the mechanism using commonly observed fog in Hong Kong as examplesAdvection FogWhen cold surge or northerly cold air gradually weakens, the approaching airstream which has traversed the ocean to the east or southeast of Hong Kong is relatively warm and humid.  When it moves over the underlying cooler water or land surface along the coast of Guangdong, the air temperature near the surface will gradually drop, resulting in condensation of water vapour into small water droplets with diameters of about a ten-thousandth of a centimeter.  They are carried (or advected, in meteorological jargon) by the east to southeasterly wind towards Hong Kong (Fig.1) and the coastal areas of Guangdong as a layer of fog (hence called advection fog) which is also commonly known as "sea fog".Mixing FogIn spring, Hong Kong may be affected by airstreams from different origins at the same time (Fig.2). These airstreams are nearly saturated but with slightly different temperatures.  If the winds are not strong, these competing airstreams will mix at the interface, forming water droplets by condensation and resulting in another common kind of fog called mixing fog.A typical case can be seen on 16 February 2011. That morning, a humid easterly airstream brought some mists and light rain to Hong Kong.  The visibility was about 2000-5000 m. Around noon, a weak northerly airstream started to affect the western part of territory (Fig.3). With the penetration of the cooler northerlies, the temperature dropped immediately (Fig.4).  Since the air became saturated and the water vapour condensed due to mixing of air coming from the north and the east, mixing fog formed. The fog caused rapid drop in visibility over the waters in western part of Hong Kong and near the Hong Kong International Airport to about 1000 m within a couple of hours (Fig.5). However, the eastern territory of Hong Kong was only slightly affected. Radiation FogIn valleys and low-lying regions, during nighttime of winter and spring with cloudless sky and calm wind, the layer of air adjacent to the surface is cooled as the temperature of Earth surface drops more rapidly due to radiation cooling effect.  The air near the Earth surface becomes cooler than the air immediately above it, causing the atmosphere to become more stable.  If the air is sufficiently moist and cools to its dew point, condensation occurs and radiation fog forms (Fig.6).  Before dawn, the surface temperature reaches its minimum and results in thick radiation fog.  Upon sunrise, the Earth surface is gradually heated up, and the fog will begin to evaporate.  As the temperature continues to rise, the atmosphere will become less stable and radiation fog dissipates.

Visibility

March 2011

https://www.hko.gov.hk/en/education/weather/visibility/00087-more-on-fog.html

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Use of Fog - a source of water supply

Fog could serve as an efficient water supply. Twenty years ago, in a village called Caleta Chungungo neighbouring Antofagasta, villagers were celebrating for having a new source of water supply. The celebration caught media attention around the world.

In general, fog brings us inconvenience to our daily life. However, have you ever thought of using fog as an alternative water source? In fact, in some arid but foggy places, fog could serve as an efficient water supply.Antofagasta is located in the Atacama Desert which is situated at the northern part of the capital of Chile, San Diego. Although the Atacama Desert is located near the coast (Fig.1), the coastal waters are under the influence of the South Pacific subtropical ridge with prevailing dry descending offshore winds. When the cold Peruvian (Humboldt) Current (a cold ocean current) crosses the coastal waters, only low level clouds or fog but not rain will form because of the subsidence. Besides, the Andes mountains block the moist air from the Atlantic Ocean, making the desert renowned for being extremely dry. It is known that the annual average rainfall is just around 1mm[1]. Twenty years ago, in a village called Caleta Chungungo neighbouring Antofagasta, villagers were celebrating for having a new source of water supply. The celebration caught media attention around the world. The water supply did not come from water pipes of neighboring cities, but from a network of fog fence (or fog collector)mounted on Chile El Tofo mountain. The network could provide a daily average of 15,000 litre of water[2].There is around 0.05 mg of water in 1m3 fog. The diameter of water droplet in fog is in general around 1 to 40 µm (1 mm = 1,000 µm) , much smaller than that in rain (500 to 5,000 µm). Owing to its tiny dimension, the water droplets in fog blow with the winds. The fog fence collects water by making use of this feature. The design of fog fence is basically a double-layer fine plastic mesh, standing vertically among the fog with the mesh facing the prevailing winds. When fog moves across a fog fence, water droplets in the fog would deposit on the mesh and the double-layer structure would cause the droplets to coalesce, and run through the collecting pipe located at the bottom of the fence. Fog fence is considered to be simple, energy saving and cheap; so many developing countries actively investigate the feasibility of using fog fence to provide water. However, the effectiveness of using fog fence would depend on local geography and climate; besides, water could be contaminated by dust or bacteria in the course of water collection. All these bring challenges to the use of fog fence as an alternative water source.

Visibility

March 2011

https://www.hko.gov.hk/en/education/weather/visibility/00088-use-of-fog-a-source-of-water-supply.html

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Upslope fog

Fog could serve as an efficient water supply.An example of upslope fog occurs at Mount Emei of China. Owing to the high altitude of Mount Emei, monsoon is blocked by the mountain and rich moisture rises along the mountain slope, forming fog by condensation. Mount Emei is shrouded in fog almost all the year. The average number of fog days is more than 300 days in a year.

Fog can be generally divided into advection fog, radiation fog, mixing fog and upslope fog, etc, depending on its formation mechanism.  The first three types of fog have been introduced in "Fog Explained" and "More on Fog" in detail.  Here we talk about a fog which forms throughout the year – an upslope fog.  In the ancient times, it was named by Chinese as "shanlan" which means fog or mist in the mountain.When winds blow towards a mountain, air will be lifted up along the mountain slope.  As the air temperature decreases with height, water vapour in the air will condense into small water droplets, forming fog over the mountain (Figure 1).  An upslope fog usually begins from the foot or middle of the mountain and covers a wider area.  It becomes sea of clouds when it accumulates over valleys (Figure 2).  Orographic cloud or cap cloud is more likely to form near the top of a peak.An example of upslope fog occurs at Mount Emei of China.  Owing to the high altitude of Mount Emei, monsoon is blocked by the mountain and rich moisture rises along the mountain slope, forming fog by condensation.  Mount Emei is shrouded in fog almost all the year.  The average number of fog days is more than 300 days in a year.On a sunny day, upslope fog will form over the mountains without any warnings.  This is because high grounds tend to heat up rapidly by strong sunshine during the day, resulting in an expansion of air and lower surface pressure over the region.  Air from foothill or regions with higher surface pressure will rise along the slope, forming upslope fog.  We will see clouds above us from the foothill and encounter fog going up the mountain slope.  Further up to the top of the peak and looking down, we will find a beautiful sea of clouds below.

Visibility

June 2011

https://www.hko.gov.hk/en/education/weather/visibility/00084-upslope-fog.html

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Regional variations in visibility over Hong Kong

Visibility is defined in meteorology as the maximum distance that an object can be identified by un-aided eyes. It drops when light rays are absorbed or scattered by tiny water droplets, dust or other particulates suspended in the air. Hong Kong, though small in size, may occasionally see large regional variations in visibility due to localized geographic and atmospheric conditions. This can be illustrated by the following two cases.

Visibility is defined in meteorology as the maximum distance that an object can be identified by un-aided eyes.  It drops when light rays are absorbed or scattered by tiny water droplets, dust or other particulates suspended in the air.  Hong Kong, though small in size, may occasionally see large regional variations in visibility due to localized geographic and atmospheric conditions.  This can be illustrated by the following two cases.On 17 February 2005, southerly wind prevailed over the coastal areas of Guangdong.  When this moist air encountered the relatively cool underlying sea surface, the moisture condensed into tiny droplets, giving rise to fog.  In the early morning, the atmosphere in the vicinity of Waglan Island was saturated with a relative humidity of 100%.  Under the influence of the sea fog, the visibility over the region fell to around 300 m.  In contrast to the foggy situation near the island, the visibility at Chek Lap Kok stayed above 10 km with relative humidity varying between 75% and 80%, an unfavorable condition for fog formation.  The regional contrast was even greater at 8:00 am when Waglan Island and Chek Lap Kok saw visibility of 100m and 15km respectively (figure 1).Another case occurred on 1 January 2007 when the dry northeast monsoon brought fine weather to southern China.  In the afternoon, moderate east to northeasterly winds generally prevailed over Hong Kong; the visibility at the Observatory Headquarters was up to 30 km.  Over the western part of the territory, near Chek Lap Kok, winds were light northwesterlies.  The convergence of the localized northwesterlies and the prevailing easterlies, plus the stable atmospheric conditions associated with the inversion layer at 1 km aloft, provided a condition in which the suspended particulates could not disperse readily.  Consequently, the visibility at Chek Lap Kok was reduced to 3100 m (figure 2). Real time visibility readings at various districts can be obtained from the Observatory's web page at https://www.hko.gov.hk/en/wxinfo/ts/index_vis.htm

Visibility

March 2011

https://www.hko.gov.hk/en/education/weather/visibility/00086-regional-variations-in-visibility-over-hong-kong.html

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Influence of Tropical Cyclones on Visibility in Hong Kong

Reduction in visibility is mainly caused by the absorption and scattering of visible light by suspended particulates. In summer, although the visibility in Hong Kong is usually better than in other seasons, reduction in visibility often occurs when a tropical cyclone is situated in the vicinity of Taiwan. According to a study by the Observatory, the causes of reduced visibility are associated with the change in the source of air mass, the convergence of horizontal and vertical airstreams as well as the low wind speed.

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

Visibility

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

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.

Visibility

June 2012

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

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Visibility observations by Light Detection And Ranging System (LIDAR)

The LIDAR systems at the Hong Kong International Airport is possible to provide picture of visibility distribution inside and around the airport.

The LIDAR systems at the Hong Kong International Airport are mainly used for measuring winds and detecting windshear that may be encountered by the aircraft.  At the same time, they provide the backscattered power data due to suspended particulates and water vapour in the air.  By combining the backscattered data and the measurements of visibility sensors on the airfield, it is possible to produce visibility maps that provide a two dimensional (2D) picture of visibility distribution inside and around the airport.The figures below show an example on the observations of fog and mist on 8 February 2005. Figure 1 is the photo taken at the Airport Meteorological Office at about 8 a.m., showing a shallow layer of fog patches just outside the airport.  The same phenomenon is displayed by the visibility map produced at that time (Fig.2).  The LIDAR-based visibility maps are generated in real time for weather forecasters and observers to monitor visibility condition around the airport.

Visibility

March 2011

https://www.hko.gov.hk/en/education/weather/visibility/00085-visibility-observations-by-light-detection-and-ranging-system-lidar.html

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Haze as a result of Northeast Monsoon

What actually is haze? How is haze formed?Environmental Protection Department's "Study of Air Quality in the Pearl River Delta Region" in 2002 showed that the accumulation of suspended particulates is related to the air pollution. In addition to suspended particulates, fine dust in the natural environment can also be brought in by the northeast monsoon.

In recent year, the number of haze hours has increased in Hong Kong and there is a growing public concern for the reduced visibility. So, what actually is haze? How is haze formed?According to definition by the World Meteorological Organization, haze occurs when the visibility is reduced to below 5,000 metres by suspended particulates in an atmosphere with a relative humidity of 80% or below. Environmental Protection Department's "Study of Air Quality in the Pearl River Delta Region" in 2002 showed that the accumulation of suspended particulates is related to the air pollution. In addition to suspended particulates, fine dust in the natural environment can also be brought in by the northeast monsoon.The following is a case of haze on 15 October 2005. A weak northeast monsoon reached the South China Coastal areas in the morning. Under the influence of light northerly winds, haze was reported with the visibility at Chek Lap Kok falling below 4,000 metres.If you would like to know more about the relationship between haze and visibility, please refer to "Analysis of Reduced Visibility due to Haze in Hong Kong", Y.S. Li and C.H. Yung, Hong Kong Observatory, 2005 (in Chinese only).

Visibility

https://www.hko.gov.hk/en/education/weather/visibility/00091-haze-as-a-result-of-northeast-monsoon.html

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Duststorms over northern China and the East China Sea (21 March 2002)

Satellite pictures indicate that, at 1 p.m. 21 March Thursday (Hong Kong Time), the front end of the duststorms is near 30 degrees north over the East China Sea, which is more than 1000 kilometres away from Hong Kong.

The duststorms that affected northern China in the past couple of days have spread into the East China Sea. Satellite pictures indicate that, at 1 p.m. 21 March Thursday (Hong Kong Time), the front end of the duststorms is near 30 degrees north over the East China Sea, which is more than 1000 kilometres away from Hong Kong.Spokesman of the Hong Kong Observatory indicates that the prevailing southerly winds over southern China do not favour further spread of the duststorms to southern China. From the night of 22 March (Friday) to 23 March (Saturday), a cold front will reach southern China and winds over the lower atmosphere will change to northerlies then. As rain near the cold front is expected to precipitate out dust in the air, the chance of the duststorms to affect Hong Kong is rather low.Updated information (22 March 2002)In contrast with the image the day before (21 March 2002, Thursday), extensive rain and clouds cover the East China Sea on 22 March (Friday). The rain will have the effect of washing out the dust.

Visibility

https://www.hko.gov.hk/en/education/weather/visibility/00093-duststorms-over-northern-china-and-the-east-china-sea-21-march-2002.html

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Duststorms over northeastern China (8 April 2002)

The satellite picture received at about 1 p.m., 8 April 2002 (Monday) shows widespread dust affecting northeastern part of China, Bo Hai, the Korean Peninsula and the sea to its east. The dust is far away (more than 1,500 kilometres) from Hong Kong and is moving in a general eastward direction.

The satellite picture received at about 1 p.m., 8 April 2002 (Monday) shows widespread dust affecting northeastern part of China, Bo Hai, the Korean Peninsula and the sea to its east. The dust is far away (more than 1,500 kilometres) from Hong Kong and is moving in a general eastward direction. Meantime, an extensive rainband is covering the East China Sea and southeastern part of China. This rainband will have the effect of washing out any dust moving south. The chance of the duststorms affecting Hong Kong is low.Updated information (9 April 2002)The satellite picture received at about 1 p.m. today, 9 April 2002 (Tuesday) indicates that widespread dust affecting the northeastern part of China has generally moved eastward away from the region. The rainband sweeping past Japan and the East China Sea will have the effect of washing out any dust moving south. The chance of the duststorms affecting Hong Kong remains low.

Visibility

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Haboobs

Haboob is produced by intense convective cells (microbursts) over dry regions, usually near dessert areas where dust and sand can be easily lifted up by downdraughts from convective cells.

Every year during the spring season, the northern part of China is affected by dust-storms, resulting in serious air pollution, poor visibility and thereby air-traffic delays. However, over the other parts of the Earth, such as West Africa, Arabian Peninsula and Arizona, USA are threatened by another kind of powerful dust storm, namely the Haboob[1].Haboob is produced by intense convective cells (microbursts) over dry regions, usually near dessert areas where dust and sand can be easily lifted up by downdraughts from convective cells. When the cold air aloft is brought to the ground by the downdroughts, it often spreads out and develops a gust front at the leading edge (Figure 1). The gust front will then build up a sand wall by dragging up dust and debris into the air. The passage of Haboob may cause a rapid change of the weather conditions; such as thundery showers, reduction of visibility to less than a few metres, sudden drop of temperature and abrupt increase of the wind speed.The Sahara in Africa, the worlds largest desert , is also a preferred region for the occurrence of large size Haboobs. During the early summer, dry northeasterly airstream flows across the Sahara desert; meanwhile, the moist summer monsoon migrates into West Africa. When these two airstreams meet, it triggers intense convective activities which may produce Haboobs. According to the reference[2], Haboobs could be over 1000 kilometres in scale and sand particles could be lifted to over 1000 metres. The areal coverage of Haboobs is huge and they are indeed a rare natural phenomenon in the world.

Visibility

March 2013

https://www.hko.gov.hk/en/education/weather/visibility/00094-haboobs.html

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3D Cloud Observation

The height and amount of clouds are two important elements of weather observation. In particular, when the aircraft lands, it is necessary to know at what altitude the runway can be seen beneath the clouds for carrying out relevant checks and corrections.

The height and amount of clouds are two important elements of weather observation. In particular, when the aircraft lands, it is necessary to know at what altitude the runway can be seen beneath the clouds for carrying out relevant checks and corrections. Also, when the Government Flying Service (GFS) performs search and rescue operations, the height and amount of cloud will also be considered in determining the appropriate rescue plan.At the Central Forecasting Office in Tsim Sha Tsui, the Weather Observer of the Observatory relies mainly on nearby landmarks, such as the heights of buildings or mountains, to estimate the height of clouds in their vicinity. Estimation can best be made when the cloud is above the landmark. Taking Figure 1 as an example, with reference to the heights of the peaks on Lantau Island and the location of cloud shadows, it is estimated that the cloud base height was about 2,700 feet. When observing clouds, we should also note that objects at the same height would appear lower when they are further away from the observer, and higher when they are closer to the observer, as the visual effect shown in Figure 2.The ceilometer measures the heights of cloud base above the ground. It transmits laser pulses vertically from the ground into the atmosphere and then measures the time taken by the pulses to return upon reflection by small water droplets in the clouds. Without the help of the ceilometer, besides using naked eyes, i.e. visual cloud observation, we can also use other information as the supplement:1. Satellite imagery – We can use infrared and true colour satellite imagery together to roughly estimate the cloud amount near Hong Kong and the distribution of high, medium and low clouds. Since infrared light cannot penetrate through cloud layers, the infrared radiation emitted from a cloud that can be measured by satellite is from the cloud top. Its wavelength will depend on the cloud top temperature, hence the altitude of cloud top can be estimated. Taking Figure 3 as an example, the vicinity of Hong Kong was covered by medium and high clouds. Top of high clouds are cooler than that of low clouds and appears in brighter white colour in infrared images, but the colour displayed in the true colour image will be lighter. Meanwhile, it can be seen clearly from the true colour image in Figure 3 that there were some small low clouds to the south of Hong Kong. This is because true colour satellite images have higher resolution and can display more detailed cloud structure. However, low clouds are harder to see in infrared images.2. All Sky Imager – We can use all sky images to easily grasp the approximate cloud amount as well as the distribution of high, medium and low clouds without looking up at the sky. Low clouds have a darker cloud base, whilst medium to high clouds are in brighter white colour. Taking Figure 4 as an example, the cloud amount of low clouds is about 3/8, and that of high clouds is about 1/8.3. Weather Photos – We can use weather photos from higher observation points to estimate cloud base heights. Taking Figure 5 as an example, looking to the southwest from the Tai Mo Shan weather station, it can be estimated that the cloud base height is about 3,000 feet, slightly lower than the height of the weather station (3,150 feet). We can even use the directions and elevation angles of different observation points to mutually verify the distance and height of the same cloud.4. Real-time flight information – If you see an aircraft flying under the cloud or passing through the clouds, you can immediately use those webpages or mobile apps which provide real-time flight information to check the flight altitude of the aircraft at that time to help analyse the approximate cloud base height. In the example in Figure 6, the aircraft was flying at an altitude of 1,350 feet (Figure 7). From this, it can be estimated that the cloud base height at the location indicated by the dashed line was about 1,500 feet.

Basic

July 2022

https://www.hko.gov.hk/en/education/weather/clouds/00680-3D-Cloud-Observation.html

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Thermal Vision for Sky Observation

This article explore the use of thermal infrared on weather observation.

Human eyes can only see a narrow frequency spectrum, known as visible light, of the electromagnetic wave. The thermal infrared (IR) is another electromagnetic wave outside the visible light spectrum that is of natural origin that we frequently come across. How would the environment look like if we see them in the thermal IR spectrum?Thermal IR camera pictures the temperature of the object. The thermal IR camera sees the thermal IR emitted by the object itself without any external light source. The hotter the object is, the brighter it appears in the thermal IR camera. While such function may not be useful for observation during day time, it would be useful at night when the target object is not well illuminated. Some examples of thermal IR images are shown in figures 1-3. For comparison, the visible images taken at the same time by another camera nearby is also shown. While during day time, the thermal IR image lacks certain details shown in the visible image (figure 1), it is able to see more at night (figures 2 and 3). For example, in figure 2, the Sunset Peak can be seen in the thermal image but not in the visible image. It is also difficult to tell if it is cloudy from the visible images shown in figure 3 alone while the thermal images can easily distinguish between fine and cloudy sky.The Observatory is further exploring the use of the thermal IR cameras in weather observations.

Basic

January 2022

https://www.hko.gov.hk/en/education/weather/clouds/00665-Thermal-Vision-for-sky-observation.html

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The Classification and Nomenclature of Clouds

The World Meteorological Organization (WMO) has drawn up schemes of cloud classification for distinguishing and grouping clouds according to their appearance, altitudes, and, where possible, to their process of formation.

What are clouds?Clouds consist of an aggregation of minute particles of water droplets or ice crystals suspended in the air and usually not touching the ground. The mass of small water droplets or ice crystals suspended in the atmosphere is visible during daytime, or under moonlight or city light at night. Clouds may also include larger particles of liquid water and ice, as well as particles of fumes, smoke or dust.Who are the pioneers in classifying clouds?The work of the first published classification of clouds was done by the French naturalist Jean Lamarck in 1802. Lamarck defined and named a certain number of interesting cloud forms. In 1803, the English scientist Luke Howard published a more completed cloud classification covering nearly all possible cases. Subsequently, a number of meteorologists enriched the content of cloud classification in further details by adding new species or varieties.What are the characteristics of clouds?The skies are changing eternally. Clouds are continuously evolving and can appear in numerous or infinite variety of forms including cumuliform, stratiform and cirriform. Clouds can be identified and categorized by their dimension, appearance, shape, structure and texture, luminance and transparency, colour, precipitation, cloud base height, altitude and vertical extent. Different types of clouds are related to weather. The World Meteorological Organization (WMO) has drawn up schemes of cloud classification for distinguishing and grouping clouds according to their appearance, altitudes, and, where possible, to their process of formation. Classification of Clouds by characteristic formsThe characteristic forms of clouds are classified in terms of genera (plural of genus). There are 10 main groups of genera or principal cloud types, which are mutually exclusive (see Table 1).Clouds categorization according to their altitudeBy convention, the part of the atmosphere in which clouds are usually present is vertically divided into 3 etages which are high, middle and low. So, clouds are further categorized according to their height above the ground (etages) into high, middle and low level clouds. The vertical limits of etages vary with latitude, which increases from polar regions to tropical regions. The approximate cloud base altitude and vertical limits of etages are listed in the following table (see Table 2). * If the height of some clouds in one etage extends into another etage, the etages could overlap. Examples are the Altostratus, Nimbostratus, Cumulus and Cumulonimbus. In particular, the cloud bases of Cumulus and Cumulonimbus are usually in the low etage but often extend upward into the middle and high etages.

Basic

June 2010

https://www.hko.gov.hk/en/education/weather/clouds/00099-the-classification-and-nomenclature-of-clouds-1.html

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The Forms and Features of High Clouds

High clouds are classified into cirrus, cirrocumulus and cirrostratus.High clouds are often translucent. One of the main features is their resemblance of thin slices of white crystal-like glass that the cloud is mostly clear.

High clouds are classified into cirrus, cirrocumulus and cirrostratus. High clouds are often translucent (Fig.1, Cirrus). One of the main features is their resemblance of thin slices of white crystal-like glass that the cloud is mostly clear. In fact, high clouds are mainly aggregations of ice crystals, suspending at high altitudes in the sky. The degree of translucence depends on the quantity of ice crystals, denseness and thickness of the cloud.While high clouds move along with the high altitude airstreams, their appearance is changeable. Sometimes, it is like flowers floating gracefully in the sky (Fig.2, Cirrus). Sometimes, the edge of high clouds is like waving silk ribbons or feathers exhibiting elegant and exquisite texture of filamentous lines (Fig.3, Cirrus). Sometimes, it is like thousands of pearls filling the sky (Fig.4 and Fig.5, Cirrocumulus). Sometimes, it looks as if the sky is wearing a semitransparent veil, whitish, soft,  smooth but blurred (Fig.6 and Fig.7, Cirrostratus).  When the sun sets, the high clouds are rosy, gorgeous and pleasant in the dusk.  High clouds often help to stage a picturesque, intriguing and romantic setting (Fig.8 and Fig.9, Cirrus).Under suitable conditions, the refraction of sunlight through the hexagonal ice crystals of high clouds can produce atmospheric optical phenomena such as halo or sun dog (parhelion). For example, cirrostratus covering all or part of the sky can sometimes produce halo (Fig.6 and Fig.7). Also, dense cirrus patches can sometimes produce sun dog (Fig. 10).The following photos illustrate the forms and features of high clouds:

High Clouds

September 2010

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The Forms and Features of Medium Clouds

Medium clouds are categorized into altocumulus and altostratus.

The cloud base of medium clouds is 2 kilometers or more above the ground. Medium clouds consist of tiny water droplets, super-cooled water droplets and ice crystals. The lower portion of medium clouds is generally water droplets, whereas the upper portion is super-cooled water droplets and ice crystals. In the tropics, the top of medium clouds can reach 8 kilometers.Medium clouds are categorized into altocumulus and altostratus. Alto is derived from the Latin word altus, meaning "high", and cumulus means "heap" or "pile" in Latin.Altocumulus appear as fragmented clouds and are white or greyish, or greyish white in color (Figure 1). The clouds often take the form of lumps, sometimes appearing as pieces of thin cracked tiles (Figure 2), or cracked bricks (Figures 3 and 5) in groups. Altocumulus could also appear in the form of successive rows of long waves (Figure 4). Altocumulus clouds sometimes come with shading (Figure 5) as a result of scattered diffused sunlight. For regularly arranged lumps of altocumulus, they usually have an apparent width in visual angle of between one and five degrees. Altostratus is greyish or faint bluish. It appears in the form of layer clouds or sheets of clouds, covering all or part of the sky. Thin (but not too thin) altostratus is like ground glass. When altostratus covers the sun, sunlight may pass through.  But since much of the exiting sunlight is diffused, the sun could only appear vaguely (Figure 6) with a fairly blurred outline, though the suns position could still be determined.  An objects shadow cast on ground would not have a clear outline. Unlike cirrostratus, altostratus does not give rise to halo because altostratus contains water droplets. 

Medium Clouds

December 2010

https://www.hko.gov.hk/en/education/weather/clouds/00097-the-classification-and-nomenclature-of-clouds-3-the-forms-and-features-of-medium-clouds.html

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Clouds of Gloomy Weather - Stratus

Stratus is one type of low level clouds. Its cloud base is below 2 kilometers. Stratus consists of tiny water droplets.

Stratus is one type of low level clouds. Its cloud base is below 2 kilometers. Stratus consists of tiny water droplets.Stratus appears grey or greyish white in sheets of layer clouds with fairly uniform cloud base without reaching the ground. Stratus sometimes appears in the form of ragged patches too. The cloud base usually is only around three hundred metres or less above ground. It could often mask the middle parts of mountains or the tops of high buildings (Fig.1).If the ground temperature drops, the humidity in the air will increase causing the clouds to thicken downward. The low stratus could also develop over cold sea surface when the air is moist and the wind is light or calm. When the cloud base reaches the ground or sea surface, the stratus becomes fog (Fig.2). Vice versa, the air humidity drops when the ground is heated by the sunshine. The thick fog will become thinner. Then, it will be lifted up above the ground and become low stratus.Stratus usually appears in rainy or foggy days when the lower atmosphere is very moist and the air near the earth surface contains a lot of water. Since stratus is generally relatively thin, stratus itself alone will not rain. However; when the layer of stratus is thicker or under the conditions that the cloud base is very low and the earth surface is very moist, it may drizzle or may generate light rain. If the clouds split into fragments, the clouds appearing in the form of broken pieces are called stratus fractus or fracto-stratus (Fig.3 and Fig.4).

Low Clouds

March 2011

https://www.hko.gov.hk/en/education/weather/clouds/00096-clouds-of-gloomy-weather-stratus.html

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Cirrus

Cirrus belongs to the high cloud group with cloud base height of 6000 meters or above. Its most distinguishable feature is the delicate filaments or narrow bands with fibrous texture.

Cirrus belongs to the high cloud group with cloud base height of 6000 meters or above.  It is commonly observed in Hong Kong all year round and is particularly noticeable in autumn and winter when the lower and middle atmosphere is dominated by dry and almost cloud free conditions. "Cirrus", in Latin means curl or hair-like.  Its most distinguishable feature is the delicate filaments or narrow bands with fibrous texture which look like feathers or tenuous trail (Figure 1).  Up in the higher portion of the troposphere, the depth of cirrus is quite limited, making it rather translucent.  Cirrus, which forms at the level with air temperature of -20℃ or below, is almost exclusively composed of highly-reflective tiny ice crystals.  It is usually in bright white colour against a background of deep blue sky.  In fact, cirrus is generally whiter than clouds in the lower or middle levels (Figure 2).  When sunlight is shinning from the horizon, cirrus can be tinted orange or faint red (Figure 1).   Cirrus is very high in the sky where little moisture is present; even if it turns into water droplets, they tend to evaporate out before reaching ground.  Generally speaking, cirrus is associated with fine weather at least for a few hours.  On some occasions, it can be seen as a precursor of an approaching thunderstorm, tropical cyclone or cold front.  An example is shown in Figure 2 which was taken on the morning of 20 October 2002.  On that morning, the atmosphere was rather unstable with warm southeasterly winds prevailing over Hong Kong, and cumulus clouds occupied almost half of the lower atmosphere.  Well above the cumulus, it is observed that cirrus originated from a cold front, some 200 km to the north of Hong Kong, was spreading south (Figure 3).  In less than 20 hours, the cold front swept across the coast of Guangdong, bringing thunderstorms and showers to Hong Kong.

High Clouds

December 2009

https://www.hko.gov.hk/en/education/weather/clouds/00101-cirrus.html

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Cloud Street and Vortex Street

The strip-like cloud bands are known as "cloud streets". They are generated when strong northerly winds prevail over the above-mentioned sea areas in cold seasons.

If you have the chance of examining the satellite images carefully, you might occasionally notice some well organized strip-like cloud bands over the Yellow Sea and the sea areas to the south of the Korean Peninsula. In particular, chains of cloud vortices might appear to the south of Cheju Island (Fig.1). The cloud patterns are normally observed in cold seasons, typically a couple of days after the southward intrusion of cold air mass from northern China. So what are the factors contributing to the formation of such cloud patterns?The strip-like cloud bands are known as "cloud streets". They are generated when strong northerly winds prevail over the above-mentioned sea areas in cold seasons. When the cold and dry air mass originating from inland areas blows across the warm and moist sea surface, air near the surface will become warmer and more humid, and vertical convection will be triggered. The formation of clouds follow, enhanced through the turbulence induced by the high winds and aligned as cloud streets along a north-south orientation.Apart from the cloud streets, we can also observe the formation of "vortex streets" over the sea areas downwind of Cheju Island. The formation mechanism is closely related to the topography of Cheju Island, an oval-shaped volcanic island. From east to west, the distance across the island is about 175 km. The Halla Mountain, rising to 1,950 m above sea level, is located at the central part of the island. When cold air flows across Cheju Island from north to south, the airstreams will tend to flow around the island on both sides, generating alternating vortices rotating in both clockwise and counterclockwise directions on the lee side of the island. The vortex is known as "von Karman vortex" in fluid dynamics. Clouds formed in the airstream will be dragged downwind by the flow, creating the vortex streets. Similar vortices can also be observed on the water surface downstream of a rock blocking the flow in a brook.From the synoptic weather pattern, the southward advance of cold air in eastern Asia is usually associated with the development of a high pressure area over northern China. As the high pressure area migrates eastwards across northern China (Fig. 2, 3 and 4), the pressure contours over the Yellow Sea will become meridionally aligned. This will generate a wind blowing roughly parallel to the pressure contours with higher pressure to the right (to the left in southern hemisphere). The more tightly packed the contours (larger pressure gradient), the higher the wind speed will be. Hence, the eastward movement of the high pressure area across northern China will often trigger cold and strong northerly winds over the Yellow Sea and the Korean Peninsula, leading to the formation of cloud streets and vortex streets in the region.A cold front crossed the south China coast on 19 November (Fig.2). The high pressure area over northern China then migrated eastwards in the following couple of days (Fig.3, Fig.4), giving rise to strong and cold northerly winds over the Yellow Sea and the Korean Peninsula, and leading to the formation of cloud streets and vortex streets in the region.

Other · Related Information

December 2011

https://www.hko.gov.hk/en/education/weather/clouds/00095-cloud-street-and-vortex-street.html

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Stratocumulus Undulatus

Stratocumulus Undulatus is most frequently observed and noticeable during autumn and winter. Quite often, it exhibits itself in the form of regularly arranged rolls or patches, with blue sky in between. This distinctive pattern reflects that the formation of stratocumulus undulatus is closely related to the wavy motion in the air.

Following the seasonal march, the northeast monsoon becomes the predominant weather system in Hong Kong during autumn and winter.  Temperatures near the surface drop; moisture contents decrease; and the weather becomes relatively stable with temperature inversion frequently observed in the lower or middle atmosphere.  The most observable change is the quite limited cloud development along the vertical direction.  Cumulus clouds prevailing in the lower atmosphere during summertime would be replaced with stratocumulus in autumn and winter.Stratocumulus, being composed of tiny water droplets, can be grayish or whitish.  It belongs to the low cloud group with cloud base height of 2000 meters or below.  It is usually thin with a depth of a few hundred meters and the horizontal coverage often exceeds several kilometers.  Among the varieties of stratocumulus, Stratocumulus Undulatus is most frequently observed and noticeable during autumn and winter.  Quite often, it exhibits itself in the form of regularly arranged rolls or patches, with blue sky in between (figure 1).  This distinctive pattern reflects that the formation of stratocumulus undulatus is closely related to the wavy motion in the air.In fact, the atmosphere is full of waves with different origins.  Some are generated from the boundary between two air layers with different densities and velocities; some are triggered by the upward movement of air forced by mountain range.  These waves induce up and down motion.  When sufficient moisture is present, cloud will form where the wave rises above the condensation level and dissipate where it falls below the level (figure 2).  Eventually cloud strips and gaps are arranged in an alternating pattern.  Stratocumulus undulatus carries limited moisture content and is usually not rain-bearing.  On some occasions, particularly when cloud strips merge together and develop further, it may indicate that rainy weather is on the way.In the middle atmosphere of about 3000 to 4500 meters, there exists another cloud type that resembles stratocumulus undulatus in the lower atmosphere.  They are similar in terms of pattern and formation mechanism.  To casually distinguish a stratocumulus undulatus from an altocumulus undulatus, stretch you arm and point your hand towards the cloud strips that are well above the horizon.  If the strip exceeds the width of the middle three fingers, then it is most likely the former.

Low Clouds

September 2009

https://www.hko.gov.hk/en/education/weather/clouds/00102-stratocumulus-undulatus.html

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Cumulus - a play in the sky

Cumulus belongs to the low cloud type. Cumulus clouds are very common in summer, forming at about 1000-3000 feet.

You may sometimes be surprised when you look up the sky, especially during summertime. The clouds lead you to boundless imagination. They are lumpy and ever-changing, sometimes like a sheep or a flying horse. These tricks are played out by cumulus clouds.Cumulus belongs to the low cloud type. Cumulus clouds are very common in summer, forming at about 1000-3000 feet (i.e. 300-1000 metres) and in various shapes. They have various sizes and shapes. Their base is usually flat, but they can have towering tops. They are very bright under direct sunlight, but relatively dark elsewhere. When the weather is fine and the sky clear, they are just like cotton balls hanging in the sky, presenting a beautiful picture.In a way, cumulus is also an elementary cloud, because it can change its character and become a different cloud according to the circumstances. When the atmosphere is moist and unstable, the air rises and causes cumulus clouds to further develop. With strong convection, they can develop into cumulonimbus clouds. Cumulonimbus is common in summer, sometimes bringing rain and thunder.Cumulus is also a convective cloud by virtue of the strong upward current inside it. Especially in autumn and winter when the air is not so unstable, cumulus clouds may merge and cover a large part of the sky. They become altocumulus.Cumulus clouds are active and restless in the sky. They play tricks with us, enlightening us with their various shapes. Next time when you look outside the window, see what inspiration they have in store for you.

Low Clouds

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Stratocumulus Cloud in Summertime

Stratocumulus exist in the lower atmosphere of 2000 metres or below, the depth of stratocumulus is quite limited as compared to its horizontal dimensions.

In Hong Kong, stratocumulus cloud is quite common during the cool season.  But, it can also be observed in summertime, particularly when there is a low level temperature inversion associated with the sub-tropical high or a distant tropical cyclone.There are several variants of stratocumulus but they all share the same characters.  Firstly, they exist in the lower atmosphere of 2000 metres or below. Secondly, the depth of stratocumulus is quite limited as compared to its horizontal dimensions.  Stratocumulus, being composed of water droplets, can be grayish or whitish and is quite often in the form of regularly arranged rolls or patches, with blue sky in between (figure 1).On some occasions, stratocumulus may appear as elongated-shape clouds as shown in figure 2 which was taken at the dawn of 20 August 2008.  Under the influence of the sub-tropical high, the weather was fine that morning and the atmosphere fairly stable.  At the time of taking the photograph, the prevailing southerly winds were forced by a mountain range to rise above the lifting condensation level (LCL) and clouds formed.  The clouds were no longer able to develop further upward when they encountered the temperature inversion at 450 metres (see the tephigram in figure 3), and therefore spread out horizontally to form elongated clouds.

Low Clouds

https://www.hko.gov.hk/en/education/weather/clouds/00103-stratocumulus-cloud-in-summertime.html

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Cumulus Congestus Cloud

Cumulus congestus cloud commonly known as towering cumulus, with a thickness of a few kilometers, can block most sunshine from reaching the ground, and may deliver showers to the areas underneath.

The rain season of Hong Kong normally spans from April to September, during which cumulus clouds are most frequently observed.  A cumulus cloud looks like a cotton-ball with noticeable thickness and clearly defined edges, and can be associated with a wide range of weather.  When the cloud is small and isolated, the weather is usually fine and sunny.  If it develops into a “cumulus congestus cloud” with large vertical extent, there is a good chance of showers.Cumulus congestus cloud, commonly known as “towering cumulus”, looks like a great castle and is usually in dazzling white against a background of deep blue sky.  Towering cumulus, with a thickness of a few kilometers, can block most sunshine from reaching the ground, and may deliver showers to the areas underneath.  Towering cumulus clouds are formed when humid air rises to a certain height, where moisture is cooled to form water droplets or ice crystals.  The upward motion can be triggered by strong solar heating, convergence of air near the surface, or forced up-slope flow against hill-sides.Figure 1 shows a towering cumulus cloud taken in the morning of 7 July 2003.  Its cloud base i.e. the height of the lowest visible portion, was estimated at around 600 m above the ground.  One can see that the base of the towering cumulus cloud is rather flat.  Meteorologically speaking, the cloud base corresponds to a height called the “convective condensation level” at which moisture in a convective up-current condenses into water droplets.  In the case under study, the observed cloud base was largely consistent with the convective condensation level (CCL) of about 600 m which was determined based on the surface air temperature of 29℃ and the tephigram (figure 2) recorded on that morning.

Low Clouds

https://www.hko.gov.hk/en/education/weather/clouds/00104-cumulus-congestus-cloud.html

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Cap Cloud

Cap cloud, anchoring at hilltops, is of orographic origin. In Hong Kong, it belongs to low cloud group and its cloud base height is restricted to the lowest 1000m.

If the atmosphere is unstable with plenty of moisture, clouds like cumulus congestus and cumulonimbus may fill the sky; when it is stable and very dry, almost cloud-free conditions can be expected.  The former situation usually occurs in summer while the latter is mainly observed during winter.  What will occur if the atmospheric conditions are in between these two extremes?  Out of the many possibilities, the formation of cap cloud over hilltop is rather common in Hong Kong."Cap cloud", anchoring at hilltops, is of orographic origin. In Hong Kong, it belongs to "low cloud group" and its cloud base height is restricted to the lowest 1000m.  At this altitude, cap cloud is almost surely composed of small water droplets and is usually gray or white in colour.  It is most often in the shape of lentil pod, almond, convex lens, hat or UFO with very smooth feature.  Following the WMO cloud classification, "cap cloud" in Hong Kong could be classified as "stratocumulus lenticularis".Figure 1 shows a cap cloud, covering the Lantau Peak on the morning of 10 November 2009. According to the Observatory's record, the weather was generally fine that morning with cloud amount of no more than 3 oktas i.e. less than three eighths of the sky dome being covered.  However, the prevailing southerly winds encountered the Lantau Peak and was forced to rise along the slope (see figure 2).  Condensation eventually occurred and the Peak was capped by a patch of smooth cloud within which the visibility was rather low.

Orographic Clouds

March 2010

https://www.hko.gov.hk/en/education/weather/clouds/00100-cap-cloud.html

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Lenticular Clouds

Since lenticular clouds look like one or more convex lens, they could sometimes be misinterpreted as flying saucers or Unidentified Flying Objects, and so they are commonly known as flying saucer clouds.

I took the first photograph below at 10 a.m., 8 February 2006 near the Observatory's Headquarters in Tsim Sha Tsui, showing clouds over Kowloon which resemble a pair of flying saucers. These clouds are called "lenticular clouds". They form when a layer of relatively stable and moist air-stream is forced to rise by hills, causing up and down wave motions in the air. The rising air in the wave causes the moisture to cool down and condense into water droplets, which then become visible to the eye. Since lenticular clouds look like one or more convex lens, they could sometimes be misinterpreted as flying saucers or Unidentified Flying Objects, and so they are commonly known as "flying saucer clouds".Since the wave motions which result in the "flying saucer clouds" are caused by hills, these clouds would appear more readily in mountainous areas. In Hong Kong, "flying saucer clouds" do not appear frequently but their appearance over Kowloon on 8 February was not the first time. In the morning of 9 January 2004, my colleague C.M. Cheng also photographed another one (see the second photograph below). The weather conditions on these two days were rather similar, with moderate to fresh easterly winds blowing near the surface and over the hills, and a rather stable atmosphere. We may therefore infer that the "flying saucer clouds" were caused by easterly winds blowing across Kowloon Peak and the nearby hills over eastern Kowloon.In the future, if you pay more attention to the wind changes, you may have the chance to get a glimpse of the fascinating "flying saucer clouds"!

Medium Clouds

May 2006

https://www.hko.gov.hk/en/education/weather/clouds/00107-lenticular-clouds.html

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Cavum (Fallstreak Hole)

The ice crystals melt and evaporate while falling, creating a hole with brush-like streaks, hence named fallstreak hole.

A hole within clouds appeared over Hong Kong on 19 January 2015 (Figure 1). This phenomenon is known as "Cavum" (also known as "fallstreak hole") in meteorology. In some high clouds such as cirrocumulus or altocumulus, supercooled water sometimes exists due to a lack of condensation surfaces. As supercooled water is unstable, it may form ice crystals quickly when surrounding air cools down rapidly due to disturbances such as expansion of air caused by a passing aircraft. These ice crystals will grow larger as neighboring droplets freeze, and eventually gain enough weight to fall out of the clouds. The ice crystals melt and evaporate while falling, creating a hole with brush-like streaks, hence named "fallstreak hole".Multi-functional Transport Satellite (MTSAT) of the Japan Meteorological Agency (Figure 2) and the all-sky camera of the Hong Kong Observatory (Figure 3) also captured the images of fallstreak hole over Hong Kong that day.

Supplementary Features and Accessory Clouds

October 2015

https://www.hko.gov.hk/en/education/weather/clouds/00472-fallstreak-hole.html

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Arcus Cloud

Arcus cloud or shelf cloud is a rare type of accessory cloud of cumulonimbus or cumulus congestus, in gray color appearing in horizontal roll or wedge shaped form, just like a shelf supporting a storm.

Have you ever seen this kind of astonishing clouds before? It is called “arcus cloud” or “shelf cloud”. It is a rare type of accessory cloud of cumulonimbus or cumulus congestus, in gray color appearing in horizontal roll or wedge shaped form, just like a shelf supporting a storm. The edging of extensive arcus clouds usually portends an approaching severe storm.Arcus cloud normally develops when cool air from strong downdraft, namely the gust front, associated with mature cumulonimbus cloud sinks rapidly to the ground. This forces the surrounding warm moist air to rise up along the gust front and then cool and condense, forming arcus cloud at the bottom of the cumulonimbus cloud and in the direction of cloud movement. Arcus cloud sometimes only lasts for a short period of time.

Supplementary Features and Accessory Clouds

November 2015

https://www.hko.gov.hk/en/education/weather/clouds/00464-arcus-cloud.html

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Asperitas Appearing in Hong Kong

Asperitas is more chaotic and with less horizontal organization than undulatus.

A spectacular cloud feature appeared in the sky over Hong Kong around noon on 23 April 2015. It is a rather rare cloud type known as "asperitas". Although asperitas had been observed in various forms at other places in the world, the above photo was very likely the first one captured in Hong Kong. In 2009, the Cloud Appreciation Society of the United Kingdom proposed that asperitas should be added as a separate new cloud type. While the formation mechanism of asperitas clouds has not yet been fully known, the World Meteorological Organization (WMO) plans to include it in the forthcoming update of the International Cloud Atlas. This will be the first new cloud type to be officially recognized by WMO since 1951.In general, asperitas has the following basic characteristics: 1. It has well-defined, wave-like structures in the underside of the cloud. It is characterized by localized waves in the cloud base, either smooth or dappled with smaller features, sometimes descending into sharp points, as if viewing a roughened sea surface from below. 2. It is more chaotic and with less horizontal organization than undulatus. 3. Varying levels of illumination and thickness of cloud can lead to dramatic visual effects. 4. Asperitas occurs mostly with stratocumulus or altocumulus.

Supplementary Features and Accessory Clouds

November 2015

https://www.hko.gov.hk/en/education/weather/clouds/00465-asperitas-appearing-in-hong-kong.html

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Pileus Cloud

When there is a stable and moist airstream flowing above the cloud top of cumulus, the airstream condensation and formation of a cap cloud aloft called pileus cloud.

If you are a frequent cloud observer, have you ever noticed that cumulus clouds may sometimes possess atypical "hairstyle" on its top with a smooth and shining appearance? This type of accessory cloud is called "pileus cloud".Persistent strong updrafts usually occur within growing cumulus or cumulonimbus. When there is a stable and moist airstream flowing above the cloud top, the airstream will be pushed upward leading to condensation and formation of a cap cloud aloft called "pileus cloud". However, this atypical "hairstyle" does not last long. The cap will eventually be cut through when the cumulus cloud beneath continues to grow upward.

Supplementary Features and Accessory Clouds

November 2015

https://www.hko.gov.hk/en/education/weather/clouds/00466-pileus-cloud.html

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Iridescent Cloud

Iridescent cloud is a special optical phenomenon when sunlight diffracts through tiny water droplets or ice crystals of fairly uniform size in clouds.

Cloud with rainbow colours is called "iridescent cloud". It is a special optical phenomenon usually observed in altostratus or altocumulus clouds near the Sun when sunlight diffracts through tiny water droplets or ice crystals of fairly uniform size in clouds. When diffraction of light occurs in a larger layer of cloud, corona may appear.As iridescent cloud often appears near the Sun, one should avoid direct sunlight for protecting the eyes. This will also be easier to identify iridescent cloud.

Special Clouds and Phenoomena

November 2015

https://www.hko.gov.hk/en/education/weather/clouds/00467-iridescent-cloud.html

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Circumhorizontal Arc

How does Circumhorizontal Arc form?

An interesting straight "rainbow" feature vaguely appeared in the sky over Hong Kong near noon time on 19 July and 10 August 2017 (Figures 1(a) and (b)). In fact, it was not a rainbow, but an optical phenomenon called "Circumhorizontal Arc".When sunlight or moonlight is refracted or reflected by ice crystals suspended in the atmosphere, a type of optical phenomena generally known as "halo" will be produced. The most commonly observed type of halo phenomenon is the halo surrounding the Sun with an optical radius of 22 degrees (also named as the "22o halo" or "small halo"[1]). Although "Circumhorizontal Arc" resembles a rainbow, it indeed belongs to a type of halo phenomenon. It occurs less frequent than the "22o halo".How does "Circumhorizontal Arc" form? First of all, there exists a layer of cirrus clouds in the upper atmosphere. When hexagonal plate-shaped ice crystals inside the cirrus clouds suspend horizontally in the sky, sunlight entering from the vertical side face of the crystals will be refracted and then emerges from its horizontal bottom face, with different degrees of refraction splitting up white light into different colours (Figure 2a). The most critical factor is that the incident angle of sunlight on the side face of the ice crystals must be greater than 58o (Figure 2b). As such, it may only occur near the noon time. When the angular elevation of the sunlight is about 68o, it reaches its maximum intensity. "Circumhorizontal Arc" is different from and the "22o halo". It is located beneath the Sun and closer to the horizon, extending parallel to the horizon, with red colour on top and violet at the bottom (same colour sequence as that of a "primary rainbow".Whether one can see "Circumhorizontal Arc" also depends on the observer's geographic position. If the observer is located at high latitude of 55o or more (for example, some countries in northern Europe), this phenomenon can never be seen because the elevation angle of the Sun is always lower than 58o. Besides, some people may also easily confuse "Circumhorizontal Arc" with "Iridescent Clouds". Although the two look similar, their causes of formation are different. For details, please refer to the Observatory's cloud ebook "Cloud Appreciation by Dr Tin" under "Iridescent"Wonders in the sky manifest themselves in so many varieties. There is another equally stunning optical phenomenon called "Circumzenithal Arc" (Figure 3). "Circumzenithal Arc" similarly originates from refraction of sunlight by hexagonal plate-shaped ice crystals suspending horizontally inside cirrus clouds (Figure 4a). However, the difference from "Circumhorizontal Arc" is that "Circumzenithal Arc" is located above the Sun, with an arc-shaped surrounding the zenith. It has violet colour on the top (inside) and red at the bottom (outside) of the arc. The colour sequence is opposite to that of the "Circumhorizontal Arc", and it looks like an upside-down rainbow. Besides, "Circumzenithal Arc" occurs only when sunlight enters from the top face of ice crystals and emerges from the side face with angle of incidence less than 32o (Figure 4b). Due to lower incident angle of sunlight, "Circumzenithal Arc" may sometimes occur simultaneously with sundog. If the incident angle of sunlight is greater than 32o, then only the single-coloured (white) "Parhelic Circle "[2] can be produced.As the occurrence of "Circumhorizontal Arc" and "Circumzenithal Arc" depends on a combination of various conditions, they occur less frequently than the "22o halo".

Special Clouds and Phenoomena

August 2017

https://www.hko.gov.hk/en/education/weather/clouds/00499-circumhorizontal-arc.html

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Mountain Wave Clouds

Under stable atmospheric condition, the moist air after passing through the mountain top will continue to propagate like a wave, forming mountain wave clouds.

When moist air flows over mountain, water vapour condenses into water droplets due to orographic uplifting. Under stable atmospheric condition, the air after passing through the mountain top will continue to propagate like a wave, forming mountain wave clouds.

Orographic Clouds

November 2015

https://www.hko.gov.hk/en/education/weather/clouds/00468-mountain-wave-clouds.html

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Waterfall-like Orographic Clouds

Orographic clouds of different species can be categorised into low, mid or high level clouds depending on their cloud base height. They are generally associated with a moist airflow over mountainous areas.

Orographic clouds of different species can be categorised into low, mid or high level clouds depending on their cloud base height. They are generally associated with a moist airflow over mountainous areas. Orographic clouds form when moist air is forced upward by mountain, it cools and condenses into cloud water droplets. They will gradually dissipate on the leeward side of the mountain when temperatures increase in the subsiding air. Depending on the atmospheric conditions including the airflow, humidity profile, atmospheric stability as well as orography and its height, different types of orographic clouds may form, such as cap clouds, lenticular clouds and rotor clouds. In other words, the appearance and movement of clouds can reflect the airflow and atmospheric conditions. Sometimes, orographic clouds seem to be flowing down from the top of the mountain ranges, giving the appearance of a waterfall as in the photo captured on 6 June 2016 (Fig.1). This type of orographic cloud is commonly known as "waterfall cloud".Sea of stratiform clouds formed over the moisture laden valleys surrounded by mountain ranges to the north of the Lion Rock on the morning of 6 June 2016 after the passage of heavy rain associated with a trough of low pressure. Winds were light generally from the northeast (white arrows in Fig.2). Stratus clouds near the valleys capped by an overlying layer of warmer air (the temperature inversion near 200 m as indicated by the red arrow in Fig.3) were forced to climb over the mountain ranges and descend on the leeward side following the prevailing wind (Fig.4). "Waterfall clouds" gradually dissipated on the leeward side of the mountain ranges following the subsiding air. They were short-lived.In Hong Kong, "waterfall clouds" may appear in stratus or stratocumulus clouds under suitable atmospheric conditions near mountain ranges in spring or summer.

Orographic Clouds

July 2016

https://www.hko.gov.hk/en/education/weather/clouds/00484-waterfalllike-orographic-clouds.html

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Gust vs Sustained Wind

According to meteorological definitions, the term gust as in Thunderstorm Warning differs from the sustained wind criteria of Tropical Cyclone Warning Signals.

When the Observatory issues Thunderstorm Warning, members of the public sometimes receive the following message: "Gusts reaching 70 kilometres per hour or above may affect Hong Kong". As wind speed is expected to reach 70 km/h, one may ask whether this situation is similar to the wind speed criteria of the No.8 Gale or Storm Signal (i.e. sustained wind speed of 63 to 117 km/h)?According to meteorological definitions, the term "gust" as mentioned in Thunderstorm Warning differs from the "sustained wind" criteria of Tropical Cyclone (TC) Warning Signals. "Sustained wind" is the wind speed averaged over a certain time period, which is the basis for the Tropical Cyclone Warning Signals. In Hong Kong, the averaging period is 10 minutes. "Gust" refers to the instantaneous wind speed, which is usually higher with larger fluctuations when compared with the sustained wind. As an illustration, let us compare the case of Tropical Storm Wipha in July 2019 and a thunderstorm case in April 2019. On 31 July 2019, TC Signal No.3 and Signal No.8 were once issued during the passage of Tropical Storm Wipha. The 10-minute mean wind speed at Cheung Chau showed an increasing trend during that morning, and maintained at strong to gale force during most of the time in the afternoon and evening (Figure 1a). On the other hand, the gusts associated with thunderstorms can rapidly increase and decrease within a few minutes (i.e. with large fluctuations). On 20 April 2019, an organised band of thundery showers moved southward from inland Guangdong and crossed Hong Kong in the afternoon (Figure 2). The automatic weather station at King's Park, which is located in the urban areas, registered the maximum gust of 117 km/h associated with the passage of the squall line during 2 p.m. to 3 p.m. on that day (Figure 1b), which is the highest record since the operation of the station in 1992 excluding tropical cyclone cases. During the same period (2 p.m. to 3 p.m.), the maximum 10-minute mean wind speed registered at King's Park was only 36 km/h, showing the possible large difference between gust and sustained wind.Generally speaking, high winds brought by tropical cyclones can persist for longer time, and the description of wind strength (such as fresh, strong and etc.) in weather reports refers to sustained wind. On the other hand, violent gusts brought by intense thunderstorms last for shorter duration, but with larger fluctuations associated with the rapid development and dissipation of thunderstorms. Even though the gusts brought by thunderstorms can be rather short-lived, they can be astonishingly destructive due to the rapid soar of wind speeds. Violent gusts can capsize small vessels, and pose danger to people engaging in outdoor activities. The public should take note of the Observatory's information on thunderstorms and the corresponding gust information. When the term "squally thunderstorms" or "violent gust" appear in weather reports, you should be beware of the sudden increase in wind speed or drastic changes in wind direction due to thunderstorms, and take appropriate precautionary measures.One note-worthy point is that violent gusts associated with thunderstorms can occur in TC situations. For example, when Standby Signal No. 1 is in force and the tropical cyclone is still at a distance from Hong Kong, heat-triggered thunderstorms could still bring violent gusts to Hong Kong. Hence, under this situation, the public should also pay attention to the corresponding gust messages in addition to the TC information.

Wind

November 2019

https://www.hko.gov.hk/en/education/weather/wind-and-pressure/00529-gust-vs-sustained-wind.html

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Beware of Gusts

The word "gusts" often appears in the Thunderstorm Warning, the Tropical Cyclone Warning Bulletin or the Strong Monsoon Signal Special Announcement. It is a meteorological term to describe abrupt surge in wind speed or drastic change in wind direction.

The word "gusts" often appears in the Thunderstorm Warning, the Tropical Cyclone Warning Bulletin or the Strong Monsoon Signal Special Announcement. It is a meteorological term to describe abrupt surge in wind speed or drastic change in wind direction. As high gusts bring safety risks and may cause casualties and economic losses [1], the Observatory reminds the public to stay alert of gusts in warning bulletins. Gust is not the same as sustained wind, and their difference can be quite significant. Figure 1 shows that when the No. 8 Northwest Gale or Storm Signal was in force during the passage of Tropical Cyclone Haima in October 2016, 10-minute sustained winds at 75 km/h, equivalent to gale force winds [2], were once recorded at Lau Fau Shan; while a maximum gust at 102 km/h, equivalent to storm force category, was recorded during the same 10-minute period at the same station – a difference of nearly 30km/h in speed! To understand such a difference, we must first distinguish between sustained wind and gust. Sustained wind is the wind speed averaged over a certain time interval, which is a consecutive period of 10 minutes according to the standard of the World Meteorological Organization. On the other hand, gust refers to the instantaneous wind speed, which is usually higher with larger fluctuations when compared with the sustained wind.Sudden changes in gusts are mostly induced by thunderstorms, squall lines and strong monsoons [3]. With frequent convective activities in summer, the term "squally thunderstorms" often appears in weather reports to remind the public of abrupt increases in wind speed or drastic changes in wind direction due to thunderstorms. Sometimes special orography also enhances the formation of violent gusts in localized region. Despite the short duration of violent gusts, their elusive property can be astonishingly destructive, especially when people are unprepared for the soaring up of wind speeds. Violent gusts can bring about capsizing of small vessels on the sea and blowing down of trees, scaffoldings and even containers, can affect taking off and landing of aircrafts, and more seriously, can cause casualties. The "Shi Hu Feng" 1 occasionally come across by marine workers is one form of violent gusts.It is therefore highly advised that when the Observatory has issued forecast, warning signals or Special Weather Tips on gusts, the public should avoid staying in open areas and secure all loose objects when it is safe to do so. Drivers using highways and flyovers should be alert to intense gusts. Please refer to the Observatory's webpage for information on real-time maximum gusts over the territory [4]. Be aware and stay safe!

Wind

August 2017

https://www.hko.gov.hk/en/education/weather/wind-and-pressure/00492-beware-of-gusts.html

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The Higher the Place, the Stronger the Wind?

There is less friction at higher place which is more exposed. The air flows faster and hence stronger winds.

You may be aware that while hiking at the countryside, winds up the hill are usually found to be stronger than those at lower elevations. If you go kiting, you may find that the kite may easily fall if it is close to the ground, while it will become steady if it is lifted sufficiently high. Why there is difference between the winds at high levels and on the ground?"Winds"' refers to the air flow from a region of higher pressure to a region of lower pressure. The air flow is slowed down by the friction between air and the adjacent objects. Since obstacles near the ground, such as building structures, plants and etc., disrupt air flow, causing larger frictional force, air flow near the ground is hence slower. On the contrary, there is less friction at higher place which is more exposed.  The air flows faster and hence stronger winds (Figure 1).However, winds are not necessarily getting stronger with height. For some weather systems such as low level jet, winds are particularly strong at certain altitudes. Besides, under gusty conditions associated with thunderstorms, winds near the ground can be much higher than those aloft. During the passage of a squall line associated with severe thunderstorms on 30 March 2014, the maximum gust recorded at Tsing Yi exceeded 100 km/h (Figure 2). The Hong Kong Observatory had reminded people to take precautions against violent gusts in the Thunderstorm Warning.In view of the variation of wind speeds with height, wind measuring instrument (i.e., anemometer) is normally installed at 10 metres above the ground and away from nearby obstacles (Figure 3). If you look around carefully, you might be able to find some anemometers around you!

Wind

December 2014

https://www.hko.gov.hk/en/education/weather/wind-and-pressure/00449-the-higher-the-place-the-stronger-the-wind.html

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Introduction to Air Pressure (Part I)

What is air pressure? How to measure air pressure? Why is bad weather usually associated with low pressure and good weather with high pressure? What is the unit of pressure? How do we compare the pressure measured at different locations?

What is air pressure?Air has weight. Air pressure is the weight of the column of air above a horizontal surface of unit area (e.g. one square metre). The column of air extends to the top of the atmosphere. As we go up and reach a higher altitude, the pressure is lower because the column of air is reduced.How to measure air pressure?Mercury barometer and aneroid barometer are commonly used to measure air pressure. The principle of the mercury barometer is illustrated in the following figure. The height of the mercury column AB will vary according to the air pressure. The higher the air pressure, the greater will be the height of the mercury column. By measuring the height of the column AB, the air pressure at the base of the column can be determined.An aneroid barometer consists of a disk-shaped capsule made of a thin metal membrane. The capsule is partially evacuated of air. Changes in atmospheric pressure change the size of the capsule, which in turn moves an ink pointer. In this way, pressure changes are recorded continuously as the pointer moves over a rotating drum.Nowadays, digital barometer is commonly used because it is portable and accurate. An electrical capacitor in it is used to measure the change in air pressure.Why is bad weather usually associated with low pressure and good weather with high pressure?In an area of low pressure, air from neighbouring areas (which are of higher pressure) moves in. It has nowhere to go but up. Upward movement of air causes condensation of water vapour, leading to the development of cloud and rain. Hence, a low pressure area usually associated with bad weather. In contrast, in an area of high pressure, the air at low levels spreads outward and air descends from aloft. Downward motion warms up the air and is favourable for evaporation. Hence, a high pressure area usually has fine and dry weather.What is the unit of pressure?Pascal (Pa) is the international standard unit for pressure. The meteorological community uses hecto-Pascal (hPa) as the unit of pressure. It is the same as 1 millibar, a unit of pressure no longer used nowadays.How do we compare the pressure measured at different locations?To compare pressure readings taken at different locations, it is convenient to convert them to a common level, e.g. the sea level. The conversion takes account of a number of factors which affect the weight of air (e.g. temperature, gravity of the Earth). We shall talk about this in greater detail in Part II.

Pressure

https://www.hko.gov.hk/en/education/weather/wind-and-pressure/00116-introduction-to-air-pressure-part-i.html

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Introduction to Air Pressure (Part II)

How is the air pressure related to temperature and gravity? What is standard atmosphere? What are QFE, QNH and QFF? What is an isobar? What is its use? What are the common pressure patterns?

How is the air pressure related to temperature?We know that air pressure is the weight of the column of air above a horizontal surface of unit area. Everything else being equal, a cold air column, because of its higher air density, is heavier than a warm air column and thus produces a higher air pressure.How is the air pressure related to gravity?We have a weight because the Earth exerts an attractive force on us. We call this "gravity". It tends to vary quite a lot on astronomical bodies. This is why an astronaut can jump higher on the Moon, where the gravity is smaller, than on the Earth. Gravity also varies at different parts of the Earth, though to a much smaller extent. For this reason, everything else being equal, the air pressure (which is the weight of air column above a place) can vary from place to place on the Earth because of the slight changes in gravity.What is standard atmosphere?To enable comparison of air pressure at different places, it is convenient to convert the air pressure to a common level, e.g. the sea level. The meteorological community makes use of the "standard atmosphere" to convert pressure readings at different places to the sea level. The standard atmosphere is a hypothetical profile of atmospheric temperature, pressure and density agreed internationally.What are QFE, QNH and QFF?In aviation, specify terms such as QFE, QNH and QFF are used to describe the air pressure according to different applications. QFE is the pressure at the station (or aerodrome) level. QNH is the pressure at mean sea level, reduced from QFE by applying corrections according to the International Civil Aviation Organisation's standard atmosphere. QFF is the pressure corrected to mean sea level, taking into account the actual temperature conditions.What is an isobar? What is its use?Pressure readings measured at different weather stations (after making corrections according to temperature etc.) at the same time are plotted on a weather chart. Meteorologists then draw lines to show places with the same pressure. These lines are called isobars. Isobars are usually drawn at 2 or 4 hPa intervals. Isobars are useful in revealing the surface pressure pattern. Isobars can also be used to deduce wind conditions. In the northern hemisphere, if the air pressure is higher (lower) on your left, then the wind will generally blow towards you (from your back). Also, everything else being equal, winds tend to blow harder (gentler) where isobars are closer together (further apart). What are the common pressure patterns?The ability to recognize and understand pressure features on the surface is important in weather forecasting. Pressure features commonly observed on a weather chart are described below:-

Pressure

https://www.hko.gov.hk/en/education/weather/wind-and-pressure/00117-introduction-to-air-pressure-part-ii.html

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The Origin of Wind

What is wind? How do we describe wind? What is the Beaufort Scale?

What is wind?We feel winds because the air around us moves. The smoke from a lighted candle gives a good portray of the wind around us.Where does wind come from?When we fan ourselves, we make the air move and it is felt as wind. What makes air move during windy weather? This is because of unequal heating, temperatures at different places are not the same. As the warmer air expands, the density and pressure of air decrease. The surrounding cooler air flows in to fill the void and wind develops. In Hong Kong, windy weather is usually caused by monsoon or tropical cyclone. A monsoon is a seasonal wind flow due to the difference in surface pressure caused by the differential heating of seas and lands. The northeast monsoon generally prevails over the coast of southern China in winter while the southwest monsoon dominates in summer. Tropical cyclone is a high wind speed whirlpool, developed over the warm oceans. It has vertical scale of more than ten kilometres and horizontal dimensions of hundreds of kilometres.Wind cannot be seen or caught. How do we describe wind?Wind is described by its direction and speed. Wind direction is the direction from which the wind blows. It is usually expressed in terms of the point of compass. Air moving from east to west is called easterly wind. Wind speed is the speed of the air flow. It is usually expressed in kilometres per hour or as a force on the Beaufort Scale.What is the Beaufort Scale?The Scale was introduced by Sir Francis Beaufort of the British navy in the early 19th century. It employs the speed of a fully rigged sailing vessel to describe the wind speed. The Beaufort Scale is divided into 13 levels, from the calm wind of force 0 to hurricane wind of force 12. For more information, please refer to here.Why is the wind over sea usually stronger than that over land?The air flow is slowed down by the friction with adjacent objects. The sea surface has smaller friction, hence the wind over sea is usually stronger than that over land.Whenever the weather forecast predicts northerly winds, the winds are westerly in my place. Why?The forecast wind is the prevailing wind in well exposed places over a relatively large area. The direction of local wind may be different from the prevailing wind because of such effects as terrain and temperature differences.

Wind

https://www.hko.gov.hk/en/education/weather/wind-and-pressure/00111-the-origin-of-wind.html

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What is Beaufort Wind Scale?

Beaufort wind scale or Beaufort wind force scale was created by British Admiral Sir Francis Beaufort in 1805.

"Beaufort wind scale" or "Beaufort wind force scale" was created by British Admiral Sir Francis Beaufort in 1805. At that time, ships included fishing boats and warships, where canvas sails were deployed to ride the waves using wind power. Anemometer was not yet available. Wind and waves are inter-related. The stronger the winds, the higher will be the waves.  The wind strength has direct influence on the state of the sea. Beaufort developed the scale based on experience and observations on board a warship (called "44 gun man-of-war"). The scale is in form of a table grading the wind strength from force 0 to force 12 (totally 13 categories). The Beaufort wind scale was originally drawn up to relate the number of canvas sails required to each category of the  wind forces.  The higher the wind force, the less canvas sails would be required.The Beaufort wind scale was revised several times. In 1906, the description was extended from sea state to land observations of objects being blown by winds. In 1926, a set of equivalent wind speeds corresponding to the Beaufort wind force scale was adopted. In 1947, reporting of wind velocity in knots was agreed by the International Meteorological Organization. Description of the sea state and effects on land according to the different Beaufort wind forces (and equivalent velocities) can be accessed here. In 1946, the wind scale was expanded with the addition of wind forces 13 to 17. However, the expanded scale is not widely used.

Wind

https://www.hko.gov.hk/en/education/weather/wind-and-pressure/00112-beaufort-wind-scale.html

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What time in the day is the air temperature the highest? When is the lowest?

Do you think the daily maximum temperature records around noon when the sun reaches the highest position? In fact, the occurrence times of the daily maximum and minimum temperatures in a place may due to a number of factors. This article will give you an answer on the time in a day when the air temperature is the highest/lowest.

When we look at weather forecasts, one of the most noticeable parts will be the forecast of the maximum and minimum temperatures. But what time in a day is usually the air temperature the highest and when is the lowest?On a typical sunny day, when the sun reaches the highest position at around noon, the amount of incoming solar radiation is also the greatest. Should it be the time with the highest temperature in a day?Actually not. Among the received solar radiation, only a small portion is directly absorbed by the atmosphere and the rest is mostly absorbed by the land and sea. After the ground absorbs solar radiation, it then heats up a shallow layer of air above it by conduction, which in turn heats up the air further above by convection and radiation. As the whole process takes time, the daily maximum temperature usually occurs in the afternoon. In other words, the changes in air temperature lag behind the solar radiation variation.In simple terms, if we ignore the complex weather factors, while the heat energy of the atmosphere is continuously radiating out to the outer space, the temperature will keep rising as long as the incoming energy exceeds the outgoing energy. The temperature peaks when the two reach a state of equilibrium, then it starts to fall until sunrise while the outgoing energy exceeds the incoming energy. Thus, the daily minimum temperature usually occurs at around sunrise. When solar radiation returns after sunrise, the temperature will go up again. (Figure 1)The occurrence times of the daily maximum and minimum temperatures in a place may also vary due to other factors such as seasons, the weather of the day, topography and the surrounding environment. In high density urban area, the rate of temperature rise (fall) due to heat absorption (dissipation) is usually slower than that of the rural area, so the daily maximum temperature in the urban area generally appears later than that in the rural area. On average, the temperatures peak at around 3 pm at the Hong Kong Observatory Headquarters in urban Kowloon in summer, about one hour later than at Ta Kwu Ling in the rural northern New Territories (Figure 2). The elevation angle of the sun will change with seasons and geographical locations, which will also affect the occurrence times of the daily maximum and minimum temperatures. Furthermore, certain weather conditions or changes (such as thunderstorm and heavy rain, passage of a cold front, subsiding air ahead of a tropical cyclone) may cause the daily maximum and minimum temperatures to occur at other times in a day.

Sunshine and UV

May 2023

https://www.hko.gov.hk/en/education/weather/sunshine-and-uv/00692-What-time-in-a-day-is-highest-lowest-air-temperature.html

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