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Monday, 11 November, 2002, 20:14 GMT
Tornadoes: Small but terrifying
Miami tornado
Spin cycle: a tornado rips through Miami
Though much smaller and shorter in duration than their hurricane cousins, tornadoes can be some of the most terrifying meteorological phenomena, wreaking havoc that has no equal in nature.

They are spawned within storm clouds, where hot upcurrents meet cold downdraughts.

The opposed air currents at this boundary create a fast-moving vortex, or funnel of wind. At the core of these vortexes winds can whirl at up to 480 kilometres per hour (300mph).

Tornado in Minnesota, US
Tornadoes are formed in storm clouds
The damage caused by tornadoes is due to this ferocity of the winds at their centre, which can destroy buildings and pick up debris - including human beings and animals.

Houses can seem to explode when a tornado passes overhead.

This is due to what experts call "hydraulic ramming". It occurs when tornado winds catch on a small opening into the house, suddenly jamming the house with air - like blowing up a balloon.

The resultant "overpressure" can cause the house to blow outwards, appearing to explode.

Hurricanes, typhoons and cyclones are the same phenomena, but differ in name according to which geographic region they occur in.

Tornadoes, however, can form all over the world apart from around the polar regions, according to Dr David Reynolds at the UK-based Tornado and Storm Research Organisation.

Tornado in Oklahoma, US
Tornado winds can flatten areas in seconds
They are much smaller than hurricanes, ranging from just 10 metres to - in extreme cases - over a kilometre in diameter. They are usually about 100 metres across.

Tornadoes usually last only a few minutes or an hour at most - though in this time they can rampage over 200km. Hurricanes, in contrast, can last days.

"Twisters" can sometimes form at the forward right flank of hurricanes, as long as thundery conditions are also present.

Waterspouts are simply tornadoes that form over water - usually warm shallows - rather than land.

See also:

11 Nov 02 | Americas
18 Sep 00 | Science/Nature
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