Because of their frequency and high profile in the United States, we tend to think of tornadoes as American phenomena, but they can, and do, occur almost anywhere in the world outside the polar regions. Every year there are about 100 to 150 tornadoes in the south of England, and two such events received wide publicity last weekend.
One caused severe damage in the village of Selsey, on the south coast near the Isle of Wight; the other occurred only a few miles away, sweeping in over Bognor Regis from the sea.
Although small-scale and very localised, being perhaps only 10 metres in diameter, a tornado is the most violent of all windstorms - a swirling maelstrom of air with a lifespan of anything from a few minutes to an hour. The essential requirements for its formation are the presence of strong persistent updraughts in the atmosphere, and plenty of moisture near the ground.
It so happens these very same conditions facilitate the development of thunderstorms, hail and heavy showers, which are in turn often associated with deep depressions of the kind that caused the recent storms, so a tornado often coincides with all of these phenomena.
In such conditions, the anomalous buoyancy of the air at lower levels causes it to rise; as it ascends it may be forced to "turn" because of variations in the strength and direction of the wind with height, a phenomenon known to meteorologists as vertical wind shear. Sometimes this turning motion rapidly accelerates to result in a tornado.
Despite the comparatively leisurely rate of travel of the tornado itself, winds of over 200 miles an hour, blowing anticlockwise around the vortex, are not at all uncommon. But much of the damage left in the wake of the whirling monster is not caused by wind at all.
The swirling maelstrom results in a very sharp drop in atmospheric pressure at the centre of the vortex - up to 25 hectopascals. For buildings in the path of the twister, this drop in pressure occurs so suddenly they quite literally explode. The pressure inside remains momentarily unchanged, while the pressure outside drops dramatically; the result is a violent outward explosion.
Moreover, the characteristic feature of a tornado - the whirling spiral of cloud stretching downwards to the ground - is also related to pressure.
The fall in pressure near the core of the vortex causes the air nearby to expand; it cools as it does so, and moisture in the air condenses to form the familiar funnel-cloud.