THE ENVIRONMENT
279
deposits of iron, lead, zinc, tungsten, beryl and graphite, they have been mined only in small quantities.
Because Hong Kong lacks large rivers, lakes and underground water supplies, reservoirs have had to be constructed in large valleys such as Tai Lam Chung, in the New Territories, and in coastal inlets such as Plover Cove and High Island where the land has been reclaimed from the sea. The areas surrounding Hong Kong's reservoirs and their water catchment areas have become part of country parks.
The most important agricultural area Hong Kong possesses is the flat alluvium around Yuen Long in the New Territories. These alluvial lowlands have emerged from the sea only within the last 2 000 to 3 000 years, and some coastal areas are still prone to flooding when heavy rainfall coincides with high tides. The natural deposition of sediment is continuing around the Deep Bay area where brackish fish ponds have been successfully established in areas that once were mudflats, mangrove swamp or salt-water rice paddies.
Climate
=
Although Hong Kong lies just inside the tropics it has a remarkably temperate climate for nearly half the year. During November and December there are pleasant breezes, plenty of sunshine and comfortable temperatures, and many regard these as the best months of the year. During January and February there is rather more cloud and occasional cold fronts followed by dry northerly winds which can at times be too cold for comfort. 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 sometimes disrupted by the reduced visibility. May and June are hot and humid with frequent showers and thunderstorms, particularly during the mornings. Afternoon temperatures often exceed 32°C and nights are humid with temperatures generally around 26°C. There is usually a fine spell in July which may last for one or two weeks, or even longer in some years.
On average, 31 tropical cyclones form in the western North Pacific or China Seas every year and about half of them reach typhoon strength (maximum winds of 33 metres per second or more). September is the month during which Hong Kong is most likely to be affected, although gales can occur any time between May and November. When a tropical cyclone is about 700-1 000 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 consequent 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. Climatological information on Hong Kong's weather is given at Appendix 39.
Severe weather phenomena that can affect Hong Kong include tropical cyclones, strong winter monsoon winds, and thunderstorms with associated squalls that are most frequent