Thunderstorm Warning

Thunderstorms

Thunderstorms commonly occur in spring and summer months in Hong Kong. On average, thunderstorms are reported at the Hong Kong Observatory about 5 days per month during April to September.

Most thunderstorms form in cumulonimbus cloud. A cumulonimbus cloud typically covers an area of a few kilometres in diameter with a vertical extent of ten kilometres or more, like a tower hanging in the sky. The typical life span of a cumulonimbus cloud is a couple of hours.

In the vicinity of active weather systems such as low-pressure troughs, cumulonimbus clouds may form one after another. Such thunderstorms are more widespread and persistent. Occasionally, cumulonimbus clouds merge to form severe thunderstorms called supercells.

On a summer day that sets off with fine weather, insolation may trigger the growth of cumulonimbus clouds and the formation of thunderstorms. Such thunderstorms are often localized and short lived. The weather may even be sunny not far away from the storms. Sometimes, the cumulonimbus clouds will drift with the wind and affect other areas. The areas of formation of the clouds may also vary with time.

Thunderstorms are characterized by flashes of lightning and claps of thunder. In an unstable and moist atmosphere, water droplets and ice pellets in the cloud will become electrically charged in convective motion. When the electric field arising from the piling up of charges reaches a certain breakdown value, lightning discharges take place between clouds or between cloud and the earth's surface. The explosive expansion of the surrounding air produces the rolling sound of thunder. As a rule of thumb, if the sound of thunder reaches an observer three seconds after a lightning flash, the thunderstorm is about one kilometre away from the observer. The distance of thunderstorms for different time delay of the thunder sound can be estimated similarly.

Heavy downpour, lightning and squalls often occur with thunderstorms. Outbreaks of heavy rain can be dangerous to people engaged in outdoor activities. Lightning can damage electrical installations, cause fire and kill by electrocution. Apart from direct strokes, people can also get hurt by electric shock when using or in contact with conductors or electrical appliances which come under the influence of these strokes. Squalls or violent gusts often occur with thunderstorms. They bring about abrupt increases in wind speed and drastic changes in wind direction. Boats are known to have capsized in thunderstorm related squalls in local and nearby waters. Wind shear associated with squall lines is also a potential hazard to aircraft during landing and take-off.

Under certain favourable conditions, thunderstorms are accompanied by tornadoes (over land) and waterspouts (over sea). They are intense columnar vortices in the shape of funnel clouds with very strong winds and very low pressure near the centre. During the passage of tornadoes or waterspouts, the direct impact of very strong winds and the difference in pressure between the interior and outside of a building can shatter weak structures, and even blow away trees and vehicles. There have been reports of small boats capsizing in nearby waters on encountering waterspouts. Although much less frequent, tornadoes or waterspouts reaching the coast have caused damage to buildings in Hong Kong.

Hailstones are hard pellets of ice usually of only a few millimetres in diameter although larger stones occasionally occur. They are formed in well-developed cumulonimbus clouds associated with thunderstorms, mainly in spring. Large hailstones can damage crops, and break windows, glass houses and windscreens of cars.

Tornadoes, waterspouts and hailstorms are infrequent in Hong Kong, occurring once every one or two years on average. Violent gusts refer to wind gusts with instantaneous wind speed reaching 88 km/h or higher (may also be understood as wind force instantaneously reaching storm force or higher categories) and are likely to occur in Hong Kong every year at varying frequency. For example, it was relatively more frequent in 2019 with a total of 8 occurrences but less frequent with only once in the year before.

The Thunderstorm Warning

Thunderstorm warnings issued by the Hong Kong Observatory are intended to give short-term (within one to a few hours) notice of the likelihood of thunderstorms affecting any part of Hong Kong. Once issued, the warning is broadcast over radio and television, and announced on the Observatory's website, the “MyObservatory” mobile app and the Dial-a-Weather system (1878200).

Thunderstorm warnings are issued irrespective of whether thunderstorms are widespread or isolated. For isolated thunderstorms, the warning issued by the Observatory will indicate the regions that will be affected during the warning period to alert members of the public to take appropriate precautions. If thunderstorms are expected to persist or affect other parts of the territory, the warning will be extended. When thunderstorms are widespread or the areas being affected vary, it will be mentioned in the thunderstorm warning that Hong Kong will be affected by thunderstorms without specific reference to individual regions.

When inclement weather comes with thunderstorms, the Observatory will mention “severe squally thunderstorms” and the associated high-impact weather information in the Thunderstorm Warning with regard to the circumstances, including:
   

• Violent gusts
  
• Hail
• Waterspout
• Tornado

The development, movement and dissipation of thunderstorms can be quite rapid and fairly localized. Thunderstorm warnings supplement the forecast of thunderstorms in routine weather forecasts by drawing people's attention to thunderstorms that are about to or have already taken place, or to warn people of thunderstorms not previously expected. The warning is intended to assist engineers, contractors or other people who are likely to be affected by thunderstorms. It also alerts relevant government departments and organizations to take appropriate actions.

While a thunderstorm warning is in force, managers of outdoor sports facilities, playgrounds or lifeguards at swimming pools should be vigilant about changes of the state of sky and give appropriate instructions to users of the facilities.

Lightning Location Information

In 2005, the Hong Kong Observatory set up a lightning location network in cooperation with the Guangdong Meteorological Bureau and the Macao Meteorological and Geophysical Bureau. The network monitors lightning activities over the Pearl River Estuary round the clock. After lightning has been detected by the network, the locations where lightning occurs are recorded and shown graphically on the Observatory's webpage and “MyObservatory”. A digest of the information is also made available to radio and television stations, and given in the Observatory's Dial-a-Weather system. When the Thunderstorm Warning is in force, the lightning location information enables the public to identify areas affected by the thunderstorms.

For easy identification of the lightning locations, Hong Kong is divided into four regions : (1) New Terrritories West; (2) New Territories East; (3) Hong Kong Island and Kowloon; and (4) Lantau. The delineation of the regions is shown below :

Members of the public can click directly on the map in the “Location-specific Lightning Alert Webpage” to select the location of interest and the alert range according to user needs. The “Regional Information on Heavy Rain and Thunderstorm” webpage of the Observatory also displays dynamical information about Thunderstorm Warning and lightning locations.

Besides, the Observatory also provides one-hour lightning nowcast service. Members of the public can appreciate lightning information in advance by browsing the “Automatic Regional Weather Forecast in Hong Kong & Pearl River Delta Region” webpage or using the “Location-based Rain & Lightning Forecast” function on “MyObservatory”.

As errors in the lightning locations, missed detections and false alarms of lightning are unavoidable, one should always refer to the Thunderstorm Warning issued by the Observatory as well as radar imageries and rainfall distribution available on the Observatory's website when using the lightning location information.

Points to note in thunderstorms

1. Stay indoors. Seek shelter in buildings if you are working outdoor.
2. Do not swim or engage in other water sports. Leave the water and seek shelter.
3. Avoid using telephone or other plugged-in electrical appliances, including computers.
4. Do not touch aerials, water taps, pipes, wire fences and similar metal installations.
5. Do not take shower.
6. Do not handle inflammable materials in open containers.
7. Do not stand on hill tops or near any highly conductive objects. Keep away from trees or masts which are likely to be struck by lightning. Since lightning current is conducted away through the ground, you should not lie down especially when the ground is wet. Instead you should crouch down to minimise the area in contact between you and the ground.
8. Hikers and people engaging in outdoor activities should bring along their radios or mobile phones with “MyObservatory” installed for the latest weather information issued by the Observatory.
9. Be alert to outbreaks of heavy rain. Stay away from river courses or low-lying areas.
10. Drivers using highways and flyovers should be on the alert to violent gusts or intense gusts.
11. In case of violent gusts or intense gusts, people outdoors should seek shelter in buildings immediately or as soon as possible and beware of flying debris and falling objects.
12. People on small boats on the open sea should watch out for the approach of squalls or waterspouts.
13. If you encounter a tornado, seek shelter in a sturdy building. Stay away from windows, crouch to the floor and protect your head with your arms or thick padding. In the outdoors, stay away from trees, cars and other things that can be blown up by the tornado.