ENG-1981 — Page 305

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THE ENVIRONMENT

Climate Although Hong Kong lies just inside the tropics it has a remarkably temperate climate for nearly half the year. The best months are usually November and December when there are pleasant breezes, plenty of sunshine and comfortable temperatures. During January and February there is rather more cloud and occasional cold fronts followed by cold dry northerly winds which can at times be too cold for comfort. It is not un- common for temperatures to drop below 10°C and ice is occasionally reported in the New Territories.

March and April can also be very pleasant except for occasional spells of fog, drizzle and high humidity which can be particularly troublesome on high ground exposed to the southeast.

May and June are hot and humid with frequent showers and thunderstorms. These are most frequent in the mornings and afternoon temperatures often exceed 32°C. At night temperatures generally remain around 26°C with high humidities.

There is usually a fine spell in early July which may last for one or two weeks.

An average of 13 tropical cyclones of various intensities enter the South China Sea each year mostly between July and October and about five of them become typhoons – that is with winds exceeding 33 m/s near the centre. Fully developed tropical cyclones generally have a relatively calm eye averaging 50 km in diameter surrounded by a wall cloud about 20 km thick in which the strongest winds rotate anti-clockwise and the heaviest rain occurs. Outside the wall cloud there are bands of cloud and rain spiralling in some 500–1 000 km towards the centre. Slow moving tropical cyclones generally produce the most rainfall especially if they approach Hong Kong from the south.

Tropical cyclones occur in the Pacific and South China Sea throughout the year but none has ever caused gales in Hong Kong during the period December to April. When a tropical cyclone is about 700–1 000 km southeast of Hong Kong the weather is usually fine and exceptionally hot. 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 last from one to three days and consequent landslips sometimes cause more damage than the winds.

The mean annual rainfall is 2 225 mm of which about 80 per cent falls between May and September. The wettest month is June when rain occurs about two days out of three and the average monthly rainfall is 431.8 mm. The driest month is December when the monthly average is only 25.3 millimetres and when rain usually falls on only about five days in the month. October is the sunniest month when an average of 58 per cent of possible hours of sunshine is recorded. Climatological information on Hong Kong's weather is given in Appendix 39.

The severe weather phenomena that can affect Hong Kong include tropical cyclones between May and November, strong winds from the winter monsoon between October and March, frost and ice on hills and inland in the New Territories between December and February, and thunderstorms that are most frequent from April to September. Water- spouts, hailstorms and snow are rare. Although the lowest temperature recorded at the Royal Observatory in Tsim Sha Tsui is 0°C, sub-zero temperatures are recorded at times at higher elevations and in the New Territories.

The Year's Weather

1981 was the eleventh driest year on record. Rainfall at the Royal Observatory was only 1 659.5 mm compared to a normal of 2 224.7 mm with the deficit mostly occurring in June