They are spectacular, dramatic, and often deadly.

Tornadoes, sometimes called twisters, are tight whirlwinds with wind speeds of up to 250 mph.

They leave a trail of destruction in their wake

They arise out of massive thunderstorms where a layer of cool dry air lies on top of a layer of warm moist air.

In unstable conditions the moist air can rise into the cold air, travelling vertically at up to 100 mph.

Eventually the air cools and falls to earth and the rising air is forced into a small funnel, where it spins as it rises.

Some areas of the US see 1,000 tornadoes a year

Experts still do not know exactly what makes a thunderstorm spawn a tornado. They are also difficult to predict because they are so small and short-lived.

Tornadoes are not uncommon in Britain. It is estimated there were about 50 last year. Most are very small and cause very little damage.

If global warming continues, there could be more.

Forecasting tornadoes is difficult

Professor Derek Elsom of the Tornado Research Organsiation, said: "If global warming suggests that we may get increased thunderstorms or perhaps more severe thunderstorms then the suggestion may be that we get an increase in tornadoes."

But Britain has a long way to go before it matches America, where vulnerable areas suffer 1,000 tornadoes a year.

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