ENG-1997 — Page 475

THE ENVIRONMENT

March and April can be very pleasant with occasional spells of high humidity. Fog and drizzle can sometimes disrupt air traffic and ferry services because of reduced visibility.

May to August are hot and humid with occasional showers and thunderstorms, particularly during the mornings. Afternoon temperatures often exceed 31 degrees, but at night temperatures generally remain around 26 degrees.

Hong Kong is most likely to be affected by typhoons in September, although tropical cyclones of varying strength are not unusual at any time between May and November. On average, about 31 tropical cyclones form over the western North Pacific and the China Seas every year, and about half of them reach typhoon strength with maximum winds of 118 kilometres per hour or more.

When a tropical cyclone is about 700 to 1 000 kilometres south-east of Hong Kong, the weather is usually fine and exceptionally hot, but isolated thunderstorms sometimes occur in the evenings. If the cyclone's centre comes closer to Hong Kong, winds will increase and rain can become heavy and widespread. The heavy rain may last for a few days, and subsequent landslips and flooding sometimes cause considerably more damage than the winds.

The mean annual rainfall ranges from around 1 300 millimetres at Waglan Island to more than 3 000 millimetres around Tai Mo Shan. About 80 per cent of the rain falls between May and September. August is the wettest month with an average rainfall at the Hong Kong Observatory of 391.4 millimetres. The driest month is January with 23.4 millimetres and it rains on only about six days in the month.

Severe weather phenomena that can affect Hong Kong include tropical cyclones, strong winter and summer monsoon winds, monsoon troughs and thunderstorms with associated squalls that are most frequent from April to September. Waterspouts and hailstorms occur infrequently, while snow and tornadoes are rare.

Climatological data are given in Appendix 53.

The Year's Weather

It was the wettest year since records began in 1884. Rainfall at the Hong Kong Observatory amounted to 3 343.0 millimetres, 51 per cent above normal and exceeding the previous record set in 1982. Over 70 per cent of the rain fell in June, July and August. The extreme rainfall coincided with an exceptionally strong El Niño event in the equatorial Pacific. The first tropical cyclone to affect Hong Kong did not come until the last day of July, which was the latest in any year on record. The passage of Typhoon Victor on August 2 necessitated the hoisting of the Increasing Gale or Storm Signal No. 9, which was the highest warning signal since Typhoon Ellen in 1983. The year's total duration of bright sunshine of 1 558.2 hours was the shortest recorded. The annual mean temperature was 23.4 degrees, making 1997 the fourth-warmest year on record.

January was warmer and wetter than normal. The monthly mean temperature of 16.7 degrees and total rainfall of 44.6 millimetres were 0.9 degrees and 21.2 millimetres higher than the respective averages.

The wet weather continued into February. The monthly rainfall totalled 111.7 millimetres which was 63.7 millimetres above normal.

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