First Observation of Hail at the Hong Kong Observatory Headquarters
First Observation of Hail at the Hong Kong Observatory Headquarters (9 April 2001)
A warm and humid airstream brought fog to Hong Kong on Monday (9 April, 2001) and visibility was below 1000 metres inside the harbour in the morning. Later, an easterly airstream also arrived at the Pearl River Estuary. The convergence of these two airstreams caused the air to lift and an area of active thunderstorms developed rapidly. Strong uplift and downdraught caused the ice pellets in the thunderstorms to grow. Hailstones originated from large ice pellets which fell to the ground before melting completely.
Between 5:30 and 6:00 p.m., the Hong Kong Observatory received hailstone reports from 9 members of the public in Tsim Sha Tsui, Hung hom, Mongkok, Yaumati, King's Park, Yau Tong and Wan Chai. Hailstones were also observed at the Hong Kong Observatory headquarters, the largest of which was estimated to be about 2.5 cm in diameter. This is the first time that hailstones were reported at the Observatory headquarters since records began in 1884. Figure 1 shows the path of the thunderstorms that produced the hailstones in Hong Kong. Figure 2 shows hailstones observed at the Observatory headquarters.
Hailstones were reported on 31 days between 1967 and 2000 in Hong Kong (Table 1). Most of these occurred in spring (March & April). The previous hail report was on 22 February 1998.
| Month |
Jan |
Feb |
Mar |
Apr |
May |
Jun |
Jul |
Aug |
Sep |
Oct |
Nov |
Dec |
Total |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Days with hail report |
0 |
5 |
9 |
12 |
2 |
0 |
2 |
0 |
1 |
0 |
0 |
0 |
31 |
Table 1: Number of days with hail reports received by the Observatory between 1967 and 2000
Thunderstorms in the afternoon of 9 April also brought heavy rain to Hong Kong. 50 millimetres of rainfall were recorded over various areas while rainfall in excess of 80 millimetres were recorded in parts of the New Territories. Figure 3 shows the rainfall distribution in Hong Kong on 9 April up to 10 p.m. The following signals were issued by the Hong Kong Observatory:
| Thunderstorm warning | 3:05 p.m. to 8:00 p.m. |
| Amber rainstorm warning | 5:35 p.m. to 7:05 p.m. |
| Special announcement of flooding in northern New Territories | 5:50 p.m. to 7:05 p.m. |
The weather on Tuesday (10 April) will be mainly cloudy with rather low visibility. Temperatures will range between 22 and 24 degrees. Winds will be moderate easterlies, fresh offshore. A cold front is expected to reach the South China coast on Tuesday night, and winds will then freshen from the north with some thunderstorms. It will become cooler on Wednesday (11 April).
For the latest weather forecast, please refer to the following webpages:
http://www.weather.gov.hk/wxinfo/currwx/flw.htm (local weather forecast) &
http://www.weather.gov.hk/wxinfo/currwx/f5d.htm (5 day forecast).
