THE ENVIRONMENT
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and sedimentary rocks with younger intrusions of granitic rocks mostly of the Juras- sic 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.
Because of 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 five or 71⁄2 centimetres. 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, bringing about the necessity 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. 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. Fish ponds also are an important form of rural land use.
Climate
Hong Kong lies within the tropics but, unlike other tropical places, it experiences distinct seasonal changes in weather. The winter months are characterised by frequent outbreaks of cold and often dry air originating from the Asian continental anticyclone. It is not uncommon during January, February and March for temperatures to drop below 10 degrees Celsius although the mean temperatures from December to March are around 15 to 18°C. During outbreaks of cold air, winds often become strong from the north or east. The prevailing wind during winter and also for most of the year is moderate easterly.
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In summer, the weather is tropical: hot and humid with occasional showers or thunderstorms. Winds are generally moderate in strength and rather variable in direction although the south-west monsoon is the prevailing wind affecting Southeast Asia. Afternoon temperatures frequently exceed 32°C between June and September with mean temperatures around 27 to 29°C.
Tropical cyclones are most common from July to September. In an average year, about five can be expected to cause strong winds and about one gale-force winds or higher in Hong Kong. Tropical cyclones occur in the Pacific and the South China Sea throughout the year, although none has ever caused gales in Hong Kong during the five months between December and April. When a tropical cyclone is 700 to 1,000 kilometres from Hong Kong, the weather is usually fine and very hot. As it moves closer, winds increase and rain becomes heavy and widespread. The severe weather associated with a tropical cyclone usually affects Hong Kong for one to three days.