THE ENVIRONMENT
When a tropical cyclone is about 700 to 1 000 kilometres south east of Hong Kong, the weather is usually fine and exceptionally hot, but isolated thunderstorms sometimes occur in the evenings. If its centre comes closer to Hong Kong, winds will increase and rain can become heavy and widespread. Heavy rain from tropical cyclones may last for a few days and subsequent 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 only about five days a month.
Severe weather phenomena that can affect Hong Kong include tropical cyclones, strong winter monsoon winds, and thunderstorms with associated squalls that are most frequent from April to September. Waterspouts and hailstorms occur infrequently, while snow and tornadoes are rare. Climatological data are given in Appendix 45.
Topography and Geology
The topography of Hong Kong is characterised by steep granitic and volcanic mountains. Much of the footslope terrain is blanketed by debris flow deposits and other forms of colluvium_transported by erosion and mass movement from the hillsides. Some 40 per cent of the landmass is volcanic in origin, about 20 per cent is granitic, 15 per cent is colluvial and almost 10 per cent is alluvial in nature. The highest peak is Tai Mo Shan (957 metres) located in the central New Territories, and there are four peaks which exceed 750 metres, all on Lantau Island. Victoria Peak, the highest on Hong Kong Island and best known as a major tourist attraction, is ranked 18th in the territory with an elevation of 554 metres.
The territory lies on the edge of the ancient Sinian landmass formed more than 600 million years ago, and which now extends from Shandong in northern China to the Gulf of Hainan. The geological strata forming the landmass were deposited beneath the sea but were subsequently folded and faulted in mountain-building earth movements. A second period of mountain building, beginning about 160 million years ago, was accompanied by explosive volcanic activity that resulted in the deposition of thick layers of ash and lava culminating in the intrusion of large granitic bodies between 147 and 136 million years ago. The mountains formed from these deposits were subsequently eroded, with the material washed from the slopes into the valleys, plains and shallow seas.
From the beginning of the Quaternary period, about two million years ago, the lower-lying areas of the region were exposed as sea levels fell due to water being trapped in the expanding polar ice caps. Warming of the climate and melting of the polar ice then caused a rise in sea level and the consequent inundation of low-lying areas. About 100 000 years ago, Hong Kong was surrounded by broad plains crossed by rivers depositing sands and gravel. The plains were gradually inundated as sea levels rose further about 15 000 years ago, and ended when the sea reached more or less its present level about 6 000 years ago.
The erosion of the hills accelerated following destruction of the natural vegetation during the widespread colonisation of the area in the Song Dynasty (960-1279). Hong Kong's rocks are generally deeply weathered and much of the terrain is prone to landslips. More than 20 per cent of the terrain in the territory shows evidence of instability.
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