Page last updated at 16:29 GMT, Monday, 8 September 2008 17:29 UK

Animated guide: Tornadoes

Graphic showing how the upward movement of warm, moist air into cooler air above creates thunder clouds in the formation of a tornado
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No two tornadoes are the same, but they all need certain conditions to form - in particular intense or unseasonable heat.

As the ground temperature increases, moist air heats and starts to rise.

When the warm, moist air meets cold dry air, it explodes upwards, puncturing the cooler air above. A thunder cloud may begin to build.

A storm quickly develops and there may be rain, thunder and lightning.

Upward movement of air can become very rapid. Winds from different directions cause it to rotate.

A visible cone or funnel drops out of the cloud towards the ground.

The vortex of winds varies in size and shape, and can be hundreds of metres wide.

A tornado can last from several seconds to more than an hour and may travel dozens of miles.

Winds within the tornado may be so fast they cannot be properly measured. Instead, the Fujita damage scale is used to estimate speed.

F0 - Light damage: Some damage to chimneys; branches broken from trees and some trees blown over.

F1 - Moderate damage: Moving cars blown off roads, mobile homes overturned or pushed off their foundations.

F2 - Considerable damage: Mobile homes demolished, large trees snapped or uprooted, cars lifted off ground.

F3 - Severe damage: Trains overturned, most trees uprooted, heavy cars thrown, walls of homes destroyed.

F4 - Devastating damage: Well-constructed buildings destroyed, large objects thrown.

F5 - Incredible damage: Cars thrown more than 100m, strong buildings swept away.

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