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| Tuesday, 4 May 2004 |
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by K. R. Abhayasingha, Deputy Director, Department of Meteorology The Southwest monsoon rains are to come within a few weeks easing off the warmth that made us uncomfortable during the first inter monsoon season in March-April. You may be feeling these days the trend of the change from warm weather to comparatively cooler and pleasant weather. The southwest monsoon may come in the normal traditional way; arrival as a burst under the influence of a low pressure area or a depression or instead the southwest monsoon may establish quietly over our country usually during the third week of May. However, after its arrival is normal for the southwest monsoon to stay with us for nearly four months, June to September, wetting the island with monsoon rains. It is this monsoon that brings most of the rains to the southwest quarter where we have hydro catchment areas which collect and conserve water for domestic use and generation of electric power. Scanning the past data, one can see that most of the reservoirs in the catchment areas are getting filled during the southwest monsoon season. It is the weather that keeps our lives going. But on the other hand it is the most blamed in the world because it does not behave the way we want it to and it has extremes which are disruptive and hazardous. Weather, particularly the rainfall, during the southwest monsoon, the longest climate season in our country, is vitally wanted. But at the same time monsoon weather has its bad and hazardous effects. Unlike the convectional rains that are experienced over many parts of the island during inter-Monsoon, most of the southwest Monsoon rains will be confined to the South West quarter of the island. But under influence of a low-pressure area or a depression formed in the Bay of Bengal, rains may be widespread over many parts of the country while a major portion of the precipitation falls in the southwest quarter. During this Monsoon southwesterly wind streams that usually start from the high -pressure regime in the Madagascar region will blow across Sri Lanka towards North Indian low-pressure areas Southwesterly wind profile up to a vertical height of about 6-8 km will bring a lot of moisture from the Indian ocean to make the lower atmospheric levels well moist to form rain bearing clouds covering the southwest quarter of the island. With a good Monsoon wind flow with speeds exceeding 40 kmph, the moist air is dragged to central hilly region to form not only shallow clouds but also huge clouds called cumulonimbus as a result of the orographic effect over the hills. Under such situations showers or thundershowers are experienced over the hill country bringing fairly heavy rain to hydro catchment areas. Under certain circumstances, prevailing wind sometimes pushes the thunderstorms to the eastern side of the hills making Uva and Eastern Provinces wet with evening thundershowers. The question of most of us these day, when 'this warmness is going to end', will be answered by the coming monsoon weather. Fairly strong Monsoon winds and other weather parameters will change the environmental conditions to a cooler State. This monsoon is the source of a big part of water needed for the paddy sewn both in Yala and Maha seasons. Also the southwest Monsoon rains fill main rivers that provide water for agriculture in many parts of the country. The inter monsoon showers are usually confined to short periods in the afternoon or evening but rainfalls may be fairly heavy and intense. Therefore indirect effects of rain (like floods and landslides) are not much associated with the inter-monsoon showers because the rainwater does not have much time to intrude into the deep soil. On the other hand southwest monsoon rains usually continue to fall over longer time scales, sometimes for more than 20 hours a day with a few short breaks. Therefore a considerable part of the monsoon rainwater gradually percolates into the soil making them damp and muddy, ultimately resulting in hazards. Landslides and falling of huge trees are some of them. Monsoon winds which are gusty at times are strong enough to push down big trees. Falling trees sometimes cause deaths and damage to building. A number of parts in the Kegalle, Ratnapura and Kandy districts are vulnerable to landslides. But with prolong torrential rains during the monsoon land slides can be expected over any highly elevated soft-soil. Monsoon showers accompanied by low-pressure areas, or depressions are usually heavy and widespread resulting floods in low-lying parts of the country and such floods will cause many direct and indirect hazards. Damage to property and life are usually direct effects. Consequently flood will enhance the mosquito spread and growth leading to spread of epidemics like dengue and malaria. Flood will cause a health problem also by polluting the drinking water of the wells. Torrential rains will bring a number of socio-economic problems by obstructing a number of development projects like road and building construction mainly over the hill country. Excess rains will cause considerable damage in the fields of agriculture. It is usual to have most of the monsoon rain during the early part of the season. Many areas in the southwest quarter experience considerable rain during May-July with longer rainy spells and short breaks. The opposite pattern, that is short rain spells with longer breaks, is common during August-September. Natural hazards of different types can be expected during the southwest monsoon season. Floods, landslides and strong winds could occur particularly during the period May-July. Hazards of lightning can also be expected even though it is not frequent as during the inter-monsoon period. Overcast skies and fairly widespread rain could also be experienced in the southwest quarter particularly during the onset of the monsoon. |
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