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BANGLADESH: Tornado season begins in earnest

17 Oct 2007 11:13:37 GMT
Source: IRIN

DHAKA, 17 October 2007 ([!CDATA[ <a href="http://www.IRINnews.org">IRIN</a>]]) - A spate of deadly tornadoes has struck parts of Bangladesh following this year's annual monsoon season.

On 15 October three tornadoes tore through the southern districts of Barisal, Gopalganj and Bhola, killing seven people and destroying over 500 houses. According to the United News of Bangladesh news service, at least 3,000 people were left homeless.

Five days earlier, a tornado wreaked havoc on the village of Barek Tila in Tahirpur sub-district of the country's northern Sunamganj District, injuring at least 25 people and damaging 10 houses.

Later on the same day, about 500 homes and 200 hectares of sugarcane were damaged by yet another tornado in Deawanganj sub-district in north-central Jamalpur District.

Tornadoes are a regular occurrence in this disaster-prone nation of 150 million plus inhabitants; with experts warning of more on the way.

"More local severe storms are likely to hit Bangladesh as the cold weather approaching from the northeastern Himalayas meets the existing hot and humid weather system for the next six to eight weeks," Dr Samarendra Karmakar, director of the country's meteorology department, told IRIN in Dhaka, the Bangladeshi capital.

"Not all local severe thunder storms are tornadoes," Karmakar clarified. "In tornadoes the wind whirls around a central air funnel that resembles an elephant's trunk. They take a zigzag route and cause severe damage within minutes. Other storms are not as severe," he said.

According to experts, a tornado is a violently rotating column of air extending from a thunderstorm to the ground, with the most violent tornadoes capable of tremendous destruction, with wind speeds of some 400km per hour (kph) or more.

Two tornado seasons

In Bangladesh, due to the orbital movement of the earth the wind changes direction twice a year - once when the Tropic of Cancer starts moving towards the sun in March-May, and once when it starts moving away from the sun in October-November. The transitional periods are usually referred to as pre-monsoon (March-May) and post-monsoon (October-November).

These two transitional periods are characterised by local severe thunder storms - including severe local storms, tornadoes and cyclones.

The frequency of devastating pre-monsoon local storms usually reaches its peak in April, while a few occur in May and the least in March.

Post-monsoon storms are usually weaker and they are fewer in number. They are smaller in size, and hence cause less devastation.

Tornado types

Based on their intensities and power of destruction, tornadoes can be categorised into three groups - weak, strong and violent.

Nearly 70 percent of all tornadoes are weak tornadoes, resulting in less than five percent of tornado deaths. Their lifetime is 1-10 minutes with wind speeds of less than about 180 kph.

Strong tornadoes comprise 28 percent of all tornadoes and result in nearly 25 percent of all tornado deaths. They may last 20 minutes or longer, with wind speeds of 180-350 kph.

Only two percent of all tornadoes are classified as violent in nature, but result in around 70 percent of all deaths. They can last over 60 minutes and involve winds of over 350 kph.

Generally, tornado-producing thunderstorms form during the afternoon hours.

Bangladesh's experience

Bangladesh experienced what has been described as the deadliest tornado in recorded history on the evening of 26 April 1989.

Some 1,300 people were killed, 12,000 injured and 80,000 left homeless when a twister ripped through the Daulatpur-Saturia region of central Tangail District.

The tornado had a path of 16km, reportedly destroying all structures in an area of 6sqkm.

Other major tornadoes of recent times include the Demra tornado of 14 April 1969 with 923 fatalities; the Manikganj tornado of 17 April 1973 with 681 fatalities; the Madaripur-Shibchar tornado of 1 April 1977 with 500 fatalities; and the Jamalpur-Tangail tornado of 13 May 1996 with 750 fatalities.

Generally, about 25-30 local severe thunderstorms and tornadoes of various intensities lash the country during each of the two tornado seasons.

"Tornadoes are a common natural hazard. The annual economic loss caused by tornadoes and severe thunder storms is perhaps second only to that of the annual floods. They kill and maim people, destroy houses, factories, schools, livestock and plantations. They form so randomly and quickly and hit so fast that they can't be forecast using existing machineries and communications systems", said Arjumand Habib, deputy director of Bangladesh's department of meteorology.

"Severe thunderstorms and tornadoes can be monitored if state-of-the-art forecasting and monitoring devices are put in place. With modern forecasting and monitoring machineries it is possible to warn people to take precautionary measures against such natural disasters," Habib said.

sa/ds/cb

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Last updated:Wed Oct 17 11:16:38 2007