ENG-1987 — Page 344

THE ENVIRONMENT

Climate

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Hong Kong's climate is sub-tropical, tending towards temperate for nearly half the year. During November and December there are pleasant breezes, plenty of sunshine and comfortable temperatures. Many people regard these as the best months of the year. January and February have rather more cloud, with occasional cold fronts followed by dry northerly winds. It is not uncommon for temperatures to drop below 10°C in urban areas. The lowest temperature recorded at the Royal Observatory is 0°C, although sub-zero temperatures and ice occur at times on high ground and in the New Territories.

March and April can also be very pleasant except for occasional spells of high humidity. Fog and drizzle can be particularly troublesome on high ground exposed to the southeast, and air traffic and ferry services are occasionally disrupted by reduced visibility.

May to August are hot and humid with frequent showers and thunderstorms, particu- larly during the mornings. Afternoon temperatures often exceed 32°C; at night, tempera- tures generally remain around 26°C with high humidity. There is usually a fine dry spell in July which may last for one to two weeks, or even longer in some years.

September is the month during which Hong Kong is most likely to be affected by tropical cyclones, although gales are not unusual at any time between May and November. On average, about 30 tropical cyclones form in the Western North Pacific or China Seas every year, and about half of them reach typhoon strength (maximum winds of 118 kilometres per hour or more).

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When a tropical cyclone is about 700 to 1 000 kilometres southeast of Hong Kong, the weather is usually fine and exceptionally hot, but isolated thunderstorms sometimes occur in the evenings. If the centre moves closer to Hong Kong, winds increase and rain can become heavy and widespread. Heavy rain from tropical cyclones may last for a few days and subsequent landslips and flooding sometimes cause more damage than the winds.

The mean annual rainfall ranges from around 1 200 millimetres at Waglan Island to more than 3 000 millimetres in the vicinity of Tai Mo Shan. About 80 per cent of the rain falls between May and September. The wettest month is June, when rain occurs about two days out of three and the average monthly rainfall at the Royal Observatory is 431.8 millimetres. The driest month is December, when the monthly average is only 25.3 millimetres and rain falls on only about five days in the month.

Severe weather phenomena that can affect Hong Kong include tropical cyclones, strong winter monsoon winds, 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 44.

The Year's Weather

On the whole, 1987 was unusually warm. The mean minimum temperature of 21.6°C was the highest ever recorded and the mean temperature of 23.4°C was the second highest on record. The mean sea-level pressure of 1 013.8 hectopascals for 1987 was also the highest ever recorded. Although there was an unusually low amount of rainfall from tropical cyclones, the abundant rainfall earlier in the year more than made up for the shortage. The annual rainfall of 2 319.3 mm was about four per cent above the normal amount of 2 224.7 mm.

Relatively few tropical cyclones affected the South China coast during the year. There were only six tropical cyclones in the South China Sea compared with the average of 12 per year. Five tropical cyclones required the hoisting of tropical cyclone warning signals in Hong Kong, but it was the first year since this signal was introduced in 1956 that the Strong Wind Signal No. 3 was not hoisted until late October.