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| Thursday, 1 January 2004 |
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Be prepared to face the coming weather by K.R. Abhayasingha Year 2003 is over. One who is keen to analyze the weather we experienced during the fading year will not see any significant or unusual incident except the sudden flood in May 2003 which resulted in death of more than 250 people. One who collects data on climate change too will not see any specific change of the weather above or below the normal. It is not an easy task to make decisions on change of the climate in an area based on analysis of data collected during a short period like one year. Not only difficult but also it is not fair if one tries to do so. Data covering a period as long as at least ten year should be studied before arriving in any kind of conclusions of changes of the climate in a particular area. One may conclude that the heavy rainfalls (Deniyaya experienced 730 mm in 24 hours on May 17 fell in May were unusual and also he may argue to account such rainfalls to climate change. But we should not forget that the world record rainfall during a 24-hour period is 1870 mm which was recorded in Reunion in 1952. Can we take 730 mm rain in Deniyaya as unusual when the probability is there for more heavier rainfalls in any place of Sri Lanka during any season even without climate change? Old records of weather experienced in Sri Lanka disclose that there have been many weather situations similar to what we experienced in 2003. Weather with strong winds, tornado, lighting chilly winds, warm air etc. have been repeated in history. The tornado hazard during 0830 - 0900 p.m. on February 20, 1907 is one of many such examples. False It is no wonder to have people spreading false and unscientific information in our country where majority does not have the interest of reading or listening to scientific facts and inventions. This is a common thing in many matters including weather. Some people who have misinterpreted or misunderstood the tornado hazards occurred in Modera area on November 23, 2003 have gone too far to put the blame on to the weather people for not forecasting the event in advance. A number of articles on tornado have been published in media to make the public aware of the vulnerability of tornado and the difficulty faced by the meteorologists in forecasting them even in most developed countries. Also the precautions that should be taken by us in order to minimize the tornado hazards have been discussed in a number of programs. A considerable number of tornadoes in different strength have occurred between the period of Tornado incidents in May 2003 in Bibila and Piliyandala area and the one occurred in Modera area on November 23, 2003. Even though it was not as bad as 2002, lightning caused considerable death and property damage in 2003. Weather pattern during the period of 365 days of a year repeats every year some times with fluctuations and changes. Knowledge in general weather pattern and extreme weather systems or weather events will help us in launching precautionary steps to reduce and mitigate hazards from them. Forecasting capability of some events like location of lightning, tornado and gusty winds in long range scales are poor in the field of meteorology. Conditions are worse in underdeveloped countries. Understanding the weather conditions we are to face in each season is the only motivating power in hazard reduction. I never argue with one who is with the intention of listening to the timely weather forecasts of extreme events. But since this is not feasible, everyone should have some idea about the normal weather conditions going to happen in immediate future days. During the first month, January of the year North-East monsoon conditions prevail over the island. This coincides with the winter season in the high latitudes of the northern hemisphere and therefore this monsoon is named as Winter monsoon by the Indian scientists. North-East monsoon season in Sri Lanka extends up to Mid-March and its name is based on the direction of the prevailing pressure gradient and the low level winds across the country. Direction Like in other climate seasons in the country, weather conditions during northeast monsoon period (December-February) depend on the direction and speed of the wind streams blowing over the region. Wind direction is generally North-East but the origin and the path of the wind streams are not same during the whole season. They may be originating either in high pressure cells over North Indian land mass or in high pressure cells over North-East Asian land mass. Regional pressure distribution provides the steering force necessary for energizing the wind streams. High pressure systems over North Indian and North East Asian land mass are main parameters that produce the steering force needed for pushing the air masses. Depending on the strength of high pressure centres, wind may originate either over North Indian land mass or China-Russia region. The wind streams originating in North India, has to pass a major part over cold, dry land mass and a short path over ocean before reaching Sri Lanka and hence those air streams are cold and poor in moisture concentration. On the other hand, when wind storms start the journey from high pressure region in North East Asia, they pass a long way over pacific and the Bay of Bengal oceans collecting a marvellous mass of moisture in the passage over water bodies. These two different characteristics of wind streams during this season produce different weather conditions over our land area. The dry and cold wind blowing from the Indian land mass will establish a comparatively cold but dry weather over many parts making us feel pleasant and comfortable except for some very cold morning hours. Cloud-free skies during day and night let the days to be with full of sun shine and nights to be pleasant and cold. Such conditions with clear skies will help the earth surface and the adjoining atmospheric layers release their heat by radiating it back to the space as long wave radiation. As a result, the earth surface and the lower atmosphere cool down due to the loss of heat letting latter part of the night and early part of the day be dry and cold. Under these conditions mist or fog is common during early morning hours especially over areas with vegetation or water bodies. Under certain extreme conditions, temperature may fall down to values close to zero degree, which is the freezing temperature of water, over high grounds. Such atmosphere may be cool enough to deposit ground frost on leaves of trees and grass in some hilly parts of the country particularly in Nuwara Eliya District. Such cold weather may be unbearable without warm cloths and some people may feel uncomfortable with dried skin in soft parts of the body like finger tips and lips. On the days when wind streams happen to start the journey from high pressure region in North East Asia and blow over pacific and the Bay of Bengal, they are rich enough of moisture to build up rain bearing clouds and they produce the seasonal rain falls particularly in Northern, North Eastern and Eastern parts of the island. Such wind streams, under certain conditions, help to develop thunderclouds to downpour thundershowers over hilly parts during the afternoon or evening. These thundershowers are pushed to western and sometimes southern regions later by the prevailing easterly winds. Waves It is common thing to develop easterly waves within the wind flow blowing from the east.Under the influence of easterly waves, weather over many parts can be wet for two or three days consecutively with considerable rain over the hills. We should not forget that the North East monsoon rains over the hills and Western parts will usually come in the form of thundershowers since the conditions are favourable for development of Cumulonimbus clouds which are the producers of lightning and tornado. If you have read the above short description with care, you will not wonder if you happen to feel very cold temperatures particularly in the night on some days to come. Also this may be completely different on some days with warm air with showers or thundershowers mostly during the evening/night. That is the normal Northeast monsoon climate. With all these, extreme events like severe lightning activity or tornado hazards cannot be ruled out. (The writer is Deputy Director, Department of Meteorology, Colombo 7.) |
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