THE ENVIRONMENT
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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, and with the completion of the High Island reservoir scheme, desalination processes on a large scale will become necessary.
Hong Kong lies in the frost-free 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 31 per cent of the area being used for agriculture. Fish ponds are also an important form of rural land use. The upland areas are mostly grass-covered and in several places, as in the Castle Peak area, severely eroded. Afforestation has been developed since 1945 but the area covered is still relatively small. The most important function of the uplands is for water catchment areas. To some extent this is now conflicting with the needs of the crowded urban areas for recreational space, and problems of rural conservation in this and other respects are becoming pressing.
Climate and the Year's Weather
Climate
Although Hong Kong lies within the tropics it enjoys seasonal weather conditions, which is unusual for tropical countries. The winter monsoon blows from the north or north-east and normally begins during September. It prevails from October until mid-March but can persist until May. Early winter is the most pleasant time of the year when the weather is generally dry and sunny. After the New Year there is often more cloud and although rainfall remains slight it is often fairly persistent. Coastal fog and drizzle occur from time to time in early spring-during breaks in the monsoon —when warm south-easterly winds may temporarily displace the cool north-easterlies.
The summer monsoon blows from the south or south-west and can occur from mid-April until September, but it is not as persistent as the north-east monsoon of winter. Summer is the rainy season and is almost continuously hot and humid. The annual rainfall measured at the Royal Observatory has varied between 901.1 mm in 1963 and 3040.7 mm in 1889 but the mean value is 2168.8 mm.
The mean daily temperature ranges from about 15°C in February to about 28°C in July and the average for the year is 22°C. February is normally the coldest month and July the hottest. The absolute minimum and maximum temperatures ever recorded at the Royal Observatory were 0.0°C and 36.1°C respectively. However, greater extremes may occur in the New Territories where ice occasionally forms on high ground. Afternoon temperatures are usually about 5°C higher than those during the coldest part of the night. The mean relative humidity exceeds 80 per cent from mid-February until early September. November is the least humid month with a mean relative humidity of 69 per cent, but the lowest reading of 10 per cent was recorded in January. The average daily duration of bright sunshine ranges from three hours in March to over seven hours in mid-July and late October.
Gales caused by tropical cyclones may be expected in any of the months from May to November but they are most likely from July to September. The passage of