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INTRODUCTION: In 1966, three rainstorms resulted in the loss of 86 lives and in June 1972, 148 people died in landslips caused by heavy rain. To minimise the loss of life and damage to property due to these natural disasters, the Hong Kong Observatory started issuing thunderstorm and heavy rain warnings in 1967. In 1983, the warning system was re-organised to provide for separate warnings of thunderstorms, floods and landslips. Rainstorm warnings were introduced in 1992. The system was revised in 1998, and the Observatory now provides warnings of thunderstorms, rainstorms and landslips in Hong Kong, and issues special announcements regarding flooding in the northern New Territories.

THUNDERSTORMS: Thunderstorms, which may or may not be accompanied by heavy rain, are localised phenomena. It is possible for intense thunderstorms to pass very close without any effect and thunderstorms that appear to be approaching often die out before they arrive. Also, a thunderstorm affecting one part of Hong Kong may not affect other parts.
Lightning can damage electrical installations, start fires and cause death by electrocution. Violent gusts often occur in squall lines associated with thunderstorms. Boats are known to have capsized in squalls. Windshear associated with squall lines is also a potential hazard to aircraft during landing and take-off.
Under certain favourable conditions, thunderstorms may give rise to intense columnar vortices in the shape of funnel clouds reaching the ground or the sea. These vortices are known as tornadoes or waterspouts depending on their occurrences over land or sea respectively. They have very strong winds and a very low central air pressure. When a tornado passes directly overhead the wind and pressure effects result in great forces which could shatter weak structures. Nine tornadoes are known to have occurred in Hong Kong during the period 1982 to 2015. Waterspouts are rather more common and were reported in local waters on 34 days during the period 1959 to 2015. There have been reports of waterspouts capsizing small boats in nearby waters and damaging coastal facilities in Hong Kong as they made landfall.
Hail sometimes forms in well-developed thunderstorm clouds. Hailstones are hard pellets of ice which are usually a few millimetres in diameter. Larger hailstones have alternate rings of clear ice and frost. Large hailstones can damage crops, particular young fruits and vegetables, and can break windows, glass houses and car windscreens. During the period 1967 to 2015 there were 39 days in which hail was reported in Hong Kong.

HEAVY RAIN: Although heavy rain is not uncommon at any time of the year in Hong Kong, it occurs most often during the summer months. Indeed, close to 80 per cent of the annual rainfall occurs between May and September. During the period 1884 to 2015, the highest hourly rainfall recorded at the Hong Kong Observatory is 145.5 millimetres which occurred during the rainstorm on 7 June 2008. Rainstorms often bring about serious road flooding and traffic disruptions.
Flooding occurs when the rainfall rate is so large that natural or artificial drainage is insufficient to drain away the fast accumulating water on the ground. Floods are usually fairly transient in the urban area but may last up to a few hours in those rural areas with large catchment and gentle slopes, such as the plains in the northern New Territories.
Hong Kong is mostly of hilly terrain. During heavy and prolonged rain, man-made and natural slopes may fail and therefore pose considerable threat to people living or in the vicinity of these slopes.
Due considerations are given to both the rain intensity and duration when landslip warnings, rainstorms warning signals and special announcements on flooding are issued.

METEOROLOGICAL WATCH: In order to issue timely warnings to the public, the weather forecaster in the Observatorys Central Forecasting Office keeps a continuous watch on the weather. Besides weather reports from manned stations and automatic weather stations in Hong Kong and nearby areas, the forecaster is aided by satellite cloud pictures, weather radar images, upper-air information from wind profilers, raingauge readings, and information on lightning locations.

THE WARNINGS: Warnings of thunderstorm, rainstorm and landslip supplement routine weather forecasts by drawing attention to the severe weather. Special announcements of flooding alert people in the northern New Territories to floods caused by heavy rain in the area. The warnings and announcements are intended to prompt the public to take precautionary measures and to assist engineers, contractors and others who are likely to suffer losses due to the natural disasters. They also alert the relevant government departments and organisations to take appropriate actions, such as opening of temporary shelters, search and rescue operations, closure of individual schools and relief work. They are issued irrespective of whether tropical cyclone signals are displayed.
Like all forecasts, the warnings and announcements represent the forecasters assessment of the most likely development in the weather based on the latest information available at the time. There will unavoidably be false alarms as well as occasions when thunderstorms or heavy rain develop suddenly and affect some parts of Hong Kong before a warning or announcement could be issued.
Once issued, the warnings and announcements are sent to the Information Services Department for dissemination to government departments and organisations concerned, and to radio and television stations for immediate broadcast. They are also issued to information service providers for dissemination to those who subscribe to their special alerting services.

Thunderstorm Warning

The thunderstorm warning is intended to give short-term notice (within one to a few hours) of the likelihood of thunderstorms affecting any part of Hong Kong. It is issued irrespective of whether they are widespread or isolated. The warning will be renewed if thunderstorms are expected to persist beyond the time stated in the original warning or cancelled if thunderstorms are not expected to occur in the remaining part of the original validity period.

Rainstorm Warning Signals

When the Amber Rainstorm Warning Signal is issued, it gives alert about potential heavy rain that may develop into Red or Black Rainstorm Warning Signal situations. There will be flooding in some low-lying and poorly drained areas. The Red signal means that heavy rain could cause serious road flooding and traffic congestion, and may affect schools and public examinations. The Black signal means that there are major disruptions and inclement weather. The public will be given clear advice on the appropriate actions to take.

Special Announcement on Flooding in the northern New Territories

A special announcement of flooding due to heavy rain in the northern New Territories will be issued by the Observatory whenever significant flooding is expected to occur or is already occurring in the low-lying plains of northern New Territories. This announcement is broadcast by radio and television to the public, and will be updated at appropriate intervals until heavy rain ceases.

Announcement on Localised Heavy Rain

When heavy rain is recorded in individual districts of Hong Kong and may bring serious flooding and risks to the districts, the Observatory will issue the Announcement on Localised Heavy Rain. The announcement indicates the affected districts and the respective rainfall recorded to alert the public.

Landslip Warning

A warning of landslips will be issued by the Observatory in consultation with the Geotechnical Engineering Office whenever landslips are considered to be likely as a result of heavy rain which has occurred and is expected to continue in the next few hours. A Landslip Special Announcement is broadcast by radio and television stations to the public. The announcement is updated at appropriate intervals until the likelihood of landslips diminishes.

Table 1 - Monthly Normals of Rainfall Parameters at the Hong Kong
Observatory for the 30 Years 1981-2010 and
Extreme Values for 1884-1939 & 1947-2015

Table 1 - Monthly Normals of Rainfall Parameters at the Hong Kong
Observatory for the 30 Years 1981-2010 and
Extreme Values for 1884-1939 & 1947-2015
Month Total rainfall
(in mm)
Maximum daily rainfall
(in mm)
Maximum hourly rainfall
(in mm)
Number of Days with
0.1 mm or more 25 mm or more 50 mm or more
1 24.7 99.8 21.8 46 5.37 0.23 0.00
2 54.4 94.1 31.9 89 9.07 0.53 0.10
3 82.2 130.0 56.0 101 10.90 0.87 0.37
4 174.7 237.4 92.4 99 12.00 2.23 1.10
5 304.7 520.6 109.9 106 14.67 3.97 1.73
6 456.1 411.3 145.5 111 19.07 5.27 2.60
7 376.5 534.1 115.1 85 17.60 4.60 2.27
8 432.2 334.2 82.1 97 16.93 5.37 2.47
9 327.6 325.5 84.0 78 14.67 3.80 2.00
10 100.9 292.2 71.6 46 7.43 1.20 0.70
11 37.6 149.2 46.6 38 5.47 0.43 0.13
12 26.8 177.3 51.7 40 4.47 0.20 0.07
Year 2398.5 534.1 145.5 935 137.63 28.70 13.53

Table 2 - Monthly Normals of Thunderstorm Activity at the Hong Kong
Observatory for the 30 Years 1981-2010

Table 2 - Monthly Normals of Thunderstorm Activity at the Hong Kong
Observatory for the 30 Years 1981-2010
  Month     No. of days with lightning     No. of days with thunderstorm  
1 0.13 0.13
2 0.90 0.87
3 1.90 1.77
4 4.13 3.50
5 6.77 5.20
6 9.07 7.03
7 9.77 6.60
8 11.23 8.33
9 7.13 4.40
10 0.97 0.53
11 0.27 0.23
12 0.03 -
Year 52.30 38.60