DUE to the high profile they are given in the United States, we tend to think of tornadoes as American phenomena, but they can occur virtually anywhere in the world outside the polar regions. Next to the mid west of the US, however, they are most frequent in the plains just east of the Andes in South America, and the third most frequent location is the eastern part of the Indian subcontinent. It was here, in Bangladesh, that a particularly vicious example struck last Monday.
Tornadoes are smaller but more violent than hurricanes - or cyclones, as they are called in those parts - to which Bangladesh is also tragically prone. Cyclones are relatively large, being perhaps a hundred miles or so in diameter, and have a lifetime of a week or more. For these reasons, since it affects a much greater area, the death toll from a cyclone is often very high indeed: one which crossed Bangladesh on November 13th 1970, for example, is reckoned to have killed a million people.
A tornado, by contrast, is a small scale localised phenomenon - so small that it cannot be represented by a system of isobars on a normal weather chart. But it is the most violent of all windstorms. It may be anything from a few yards to half a mile in diameter, it forms and dissipates quickly, having a life span of anything from a few minutes to several hours, and it moves along a relatively short and narrow path with winds raging around it at speeds up to 300 mph.
A feature which adds to the destructive power of a tornado is the very sharp drop in atmospheric pressure at the centre of the wind spiral, falls of up to 200 hectopascals having been recorded. When anything containing air is encompassed by the tornado's funnel, the reduction in the surrounding pressure causes the internal air to burst outwards with explosive violence. The passing whirlwind pulls corks from bottles, bursts the tyres of cars, and even causes houses - quite literally - to explode.
Tornadoes have two other notable characteristics. The first is the familiar corkscrew cloud, undulating downwards to the ground below. And the second is the loud distinctive noise of the whirlwind. People unfortunate enough to have first hand experience of a close encounter have used strange similes to describe the sound: it resembles, they say, "a thousand railway trains", "the roar of a flight of jets", or perhaps most ominous of all, "the loud buzzing of a million bees".