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Last Updated: Friday, 8 September 2006, 14:44 GMT 15:44 UK
How a tornado is formed
Tornadoes are among the most violent storms on Earth, with the potential to cause very serious damage.

Although no two tornadoes are the same, they need certain conditions to form - particularly intense or unseasonable heat.

Conditions likely to cause tornado

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

Moist air heating and rising

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

Moist air exploding upwards

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

Storm developing

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

Movement of air becoming rapid

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

Cone or funnel dropping from cloud

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.

Tornado continues

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

Damage on the Fujita scale

  • F0 (0-73mph) - Light damage: Some damage to chimneys. Branches broken from trees and some trees blown over.

  • F1 (73-112mph) - Moderate damage: Moving cars blown off roads, mobile homes overturned, or pushed off their foundations.

  • F2 (113-157mph) - Considerable damage: Mobile homes demolished, large trees snapped or uprooted, cars lifted off the ground.

  • F3 (158-206mph) - Severe damage: Trains overturned, most trees uprooted, heavy cars thrown, walls of homes destroyed.

  • F4 (207-260mph) - Devastating damage: Well constructed buildings destroyed, large objects thrown.

  • F5 (261-318mph) - Incredible damage: Cars thrown more than 100 metres, strong buildings swept away.

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