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| Sunday, 27 November 2005 |
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Tropical cyclones and the Bay of Bengal by Rashomi Silva
Situated in the tropics of the Northern hemisphere, Sri Lanka is blessed with abundant sunlight and ample rain. But there are times when we often wish we are less favoured with both, the heat and the rain. Rainy months and incessant downpours for more than 24 hours experienced less than a week ago, make many wish, rain and storm would go away forever, making us forget about the months and months we waited anxiously for rain, and wished that the sun would go away and it would rain ceaselessly. Fortunately, mere wishing would not make either come true. The fact is that Sri Lanka, situated as we are, get a wealth of solar radiation, the main source of energy in the atmosphere. The tropical and sub-tropical greens absorb the energy in radiation and transmit them to energy, to work and to sustain life. Beyond sub tropics there is a dearth of sun's rays, and hence a dearth of energy, but nature has invented a means to convey the surplus energy to other parts of the planet, including the poles. Scientists call it general circulation, in more general terms it is the wind or the breeze. General circulation will go from equator to the poles, balancing the energy imbalance in the atmosphere. But often this general circulation, while is not enough. Despite the general circulation there is still a surplus of energy in the tropics, and a deficiency of energy beyond the sub-tropics. The surplus energy gives rise to a phenomena known as the Tropical Cyclone (TC), one of the most devastating and powerful meteorological system, that is essential to balance the energy disequilibrium. What is a TC? Tropical cyclones are small cyclonic whirls, with an inactive centre marked with a low pressure area. This centre called the eye of the cyclone has very strong winds circulating around it in counter-clockwise (Clockwise in the Southern hemisphere) direction. Compared to the centre, its periphery is marked with a high pressure zone. The difference of pressure between the centre of the periphery makes surrounding winds come to the centre at a fatal speed. Instead of blowing directly inward, due to the earth's attraction, wind blows in whirls, making it all the more forceful and destructive. TCs whilst moving very slowly, rotate with a devastating speed, collect humid and water vapour, when passing through oceans, bring along with them torrential rains, with disastrous impacts, on the land it crosses. Sri Lanka, due to its geological position near the equator in the Northern hemisphere, and due to its proximity to the Bay of Bengal, which is one of the largest cyclogenesis areas of earth, of ten experiences the effects and impacts of TCs. Most cyclones originating in the Northern part of the Bay of Bengal move Westwards and North Westwards, and pass through the Indian Ocean to the East of Sri Lanka, and enter the Indian subcontinent through Orissa, and some others re-curve and turn East, North of Bengal and enter Bangladesh, near, Chitagon. Fortunately, Sri Lanka is not directly in the path of cyclones. But almost all TCs generate in the Bay of Bengal and pass very close to Sri Lanka, close enough to feel their presence. Some of them deviate from the generally predicted path and cross Sri Lankan territory, some very powerful ones even enter the Arabian sea, across our island.
Major cyclones that crossed Sri Lanka in recent times. Trincomalee-Mannar cyclone: December 22-23, 1964. First appeared as a low pressure area in the South China sea, East of Malaya, on December 12. Moving westwards four days later, the low pressure area had deepened into a depression. It moved Westwards at about 5-10 knots.It developed into a tropical cyclone five days later and tore through Trincomalee, and moved over Mannar two days later, and died out in the Arabian seas on the eve of Christmas. Batticaloa cyclone: November 23, 1978 Was formed in the Bay of Bengal on November 17, moved North-Westward and crossed the East coast of Sri Lanka at Batticaloa on the evening of the 23rd. It's reported wind speed exceeded 100 knots, The diameter of the eye was said to be over 42Km. Pottuvil cyclone: December 12, 1992. Was formed in the Bay of Bengal on November 11. Moved North-Westward and crossed Sri Lanka from the Eastern shore of Pottuvil in the afternoon. The cyclone had moved with a speed of 10Knots at the time of landfall. (Courtesy: Meteorology Department, Sri Lanka) Some facts about tropical cyclones Most commonly experienced during monsoonal and inter-monsoonal periods. In Sri Lanka the period is between October to December. Months in which cyclones reached Sri Lanka: January, March, October, November, December. Globally 80 to 100 TCs are formed over tropical oceans every year. Average life period: three to five days. What is a Tropical Depression? When a tropical low pressure system has a wind speed is between 22 and 33 Knots. What is a Tropical storm? Wind speed between 34-63 knots. What is tropical cyclone? When the wind speed is more than 64 knots. Some local names Indian Ocean: Tropical cyclone. China sea: Typhoon Atlantic waters: Hurricane Australia: Willy-Willy Mexico: Cordonazo Off Philippines: Baguios |
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