olelog

Over 30 people have been killed this month and hundreds more injured as ‘norwesters’, wreaked havoc across Bangladesh.

Nor'wester thunderstorms, locally known as Kal-baishakhi, often blow over Bangladesh in April-May from a northwesterly direction. Nor'wester thunderstorm coincides with the setting in of the summer season. From mid-March to April the temperature in Bangladesh rises sharply compared to the preceding months (i.e. winter months). In the middle of April the whole country, especially the northwestern part, records a sharp rise in day temperature. Presence of warm and moist air in the lower layer of the atmosphere is an essential precondition for the development of a nor'wester.

The main reasons behind the nor'wester is the warm and moist air coming from the southeast which rises up to 2 kilometres, mixes with the relatively cold and dry jet streams coming from the northwesterly and westerly directions. The mixing of these two dissimilar air masses causes storms. The warm and moist air rises due to the Chotanagpur Plateau, Himalayan ranges, and Assam Plateau. Thunder and lightning is common with a nor'wester. Nor'westers are more frequent in the late afternoon because of the influence of surface heating in producing convection currents in the atmosphere. In the western region of Bangladesh, nor'westers come in the late afternoon and before evening but in the eastern side it comes generally after evening, moving from a northwesterly to a easterly and southeasterly direction. In this season the morning remains calm. Temperature begins to rise from noon creating a convective current and the storm is formed. The average wind speed of a nor'wester is 40-60 km per hour. But in exceptional circumstances the wind speed may exceed 100 km. The duration of the storm is generally less than an hour but sometimes it may exceed an hour.

Much of the country’s rural population lives in huts made of corrugated iron or mud and straw which are ill-equipped to withstand winds powerful enough to uproot trees and knock down electricity pylons.

Norwesters may also strike later in November. Given their ferocity and destructive capacity `norwesters’ are also referred to as tornadoes. For nearly 60 days during the two storm seasons, locally generated storms have hit various parts of the country almost on a daily basis. So far there is no effective early warning system for these storms. They are so local in nature and take shape so suddenly that modern tracking devices can only locate them when they begin to move, at times with a whirling speed of 200 km.

In Dhaka, trees were uprooted and thatched roofs blown away after a powerful storm struck the capital on 2 May 2008. The storms are so frequent in number and so destructive in nature that the total damage done by them is perhaps only second to the damage caused by the annual floods. In terms of damage to life and property, they do more than the floods.

Norwesters often strike when the country’s ‘boro’ crop - the country’s main rice harvest - is ready for harvest, and jute, a major cash crop for the impoverished nation, is at a critical stage of growth. According to the department of meteorology, 30-50 percent of standing crops are damaged in areas where ‘norwesters’ hit.

The crop loss could be minimised or even avoided if the pattern of cultivation is changed, either by planting the crops two to three weeks earlier than now, or by shortening the harvesting season. Most of those who die, die indoors, crushed under mud walls or hit by flying tin roofs - the construction of disaster-resilient houses could save thousands of lives lost under falling roofs and walls.

http://www.irinnews.org/report.aspx?ReportId=78089
http://banglapedia.search.com.bd/HT/N_0208.htm