Royal - HKU Libraries - The University of Hong Kong

Official estimates put typhoon damage in Hong Kong,the Philippines, Korea and Japan in 1981 at $11,992.5million. The loss of life is not accurately known, but isbelieved to be more than 2 000 a year on average inSoutheast Asia alone.A mature typhoon is malevolent, capricious and extremelydangerous. It is almost unbelievably powerful. Oneestimate is that an average tropical cyclone generates some20 million megawatts of mechanical power - enough, ifconverted into electricity, to power more than 6 000 citiesthe size of Hong Kong.The formation, development and progression of a tropicalcyclone is a complex process. Modern technology, includingsatellite surveillance, allows man to watch and track a typhoon,thus partly reducing its potential to cause harm, but the detailsof cyclone formation are still not fully understood. However,it is known that three basic conditions must exist:• The sea surface temperature must be higher than 26°C;• Air at low levels must converge inwards over a largearea, picking up moisture from the sea. This moisture latercondenses to form rain, releasing enormous amounts oflatent heat, and this heat is the main energy which powers atyphoon.• Air flow at high levels must be divergent, to sustainthree-dimensional circulation.A tropical cyclone is a huge rotating mass of warm,humid air, with the lowest sea level pressure near the centre.A typhoon has a relatively calm centre, or eye, averagingsome 50 kilometres in diameter. This is surrounded by wallsof cloud up to 20 kilometres thick.The strongest winds rotate anti-clockwise (in the northernhemisphere; in the southern hemisphere the rotation isclockwise). Outside this wall of cloud are the spiral rainbands which give the typhoon its characteristic appearanceon a radar screen.In tropical latitudes, typhoons move at relatively slowspeeds, but their pace, like that of a plodding cargo ship, can90