ENG-1976 — Page 276

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THE ENVIRONMENT

volcanic and sedimentary rocks with younger intrusions of granitic rocks mostly of the Jurassic period. The oldest sedimentary rocks found in Hong Kong are those of the Tolo Harbour Formation. This formation is exposed at Ma Shi Chau and contains fossils dated as most probably Permian in age.

However, its stratigraphic relationships are somewhat uncertain. The formation of minerals associated with the intrusion of the granitic rocks has been of limited economic benefit to Hong Kong. Lead, zinc, tungsten, beryl and graphite have been mined intermittently, but only in small quantities.

Due to the hilly terrain, agricultural land is restricted. The most important area is the flat alluvium around Yuen Long in the Deep Bay area. Outside the alluvial areas soil cover is usually thin, sometimes no more than two or three inches. In general the natural residual soils are acidic and of low fertility, needing the addition of lime, potash and superphosphates. However, given intensive labour input, water supply rather than soil condition tends to be the controlling factor in farming. The predominantly crystalline character of the rock formations unfortunately makes them unsuitable for underground storage and this makes it necessary to concentrate on the collection of surface run-off for all water supplies. The highly variable rainfall of the area has led to periodic water shortages. Most of Hong Kong's surface water supply has now been captured through the construction of catchments and reservoirs. After completion of the High Island reservoir scheme, desalination processes on a large scale may become necessary.

Hong Kong lies in the double-cropping rice zone of East Asia, but more profitable vegetable crops have increasingly displaced rice during the past 25 years and it is now grown on only about 30 per cent of the area being used for agriculture. Fish ponds are also an important form of rural land use.

Climate

Although Hong Kong is just within the tropics it is unlike many other tropical places in that it has distinct seasonal changes of weather. During the winter, Hong Kong is subject to frequent outbreaks of cold and often dry air from the very persistent Asian continental anticyclone. It is not uncommon during January, February and March to have temperatures dip below 10°C. The winter monsoon is persistent and blows mainly from the east.

In summer, winds are much less persistent, being lighter and more variable. Weather is hot and humid with rather frequent showers or thunderstorms. In the spring, temperatures fluctuate widely from day to day and the weather is characteris- tically cloudy, becoming increasingly wet and humid with periods of coastal fog and drizzle. The autumn and early winter months are usually sunny and dry.

June is the wettest month of the year with rain on two days out of every three and an average of 458.6 mm of rainfall. December is the driest month with an average of only 26.8 mm of rain and only one wet day in every six. The mean annual rainfall is 2 248.1 mm, with about 80 per cent falling between May and September. Climato- logical values of the weather elements are given in Appendix 39.