Tornadoes: Not just a U.S. phenomenon

Tornados in the UK are not normally as violent as those in the U.S.

The recipe for a tornado is a good thunderstorm, with cold dry air above warm moist air, and winds running counter to each other.

The strong air currents within a storm cloud can create a high speed vortex (spiral) or funnel of winds.

CNN weather anchor Guillermo Arduino says: "When the end of the vortex touches the surface it generates incomparable destruction. Near the centre of a tornado, winds may spiral around at more than 480 km/h."

In Europe, however, a typical tornado is 100 ft or so across, lasts less than 10 minutes and has wind speeds of less than 110 mph.

But in parts of the United States, where conditions are much more dramatic, they can last more than an hour, span up to one-and-a-half miles wide.

On average 80 people a year die in the US as a result of tornadoes. One of the worst disasters was on March 18, 1925, when 689 people died in Indiana, Illinois and Missouri.

Forecasters say that in Britain conditions for tornadoes are ripe only on about 11 days a year, and the vortices tended to be no more than a few metres in size and normally last less than half an hour.

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Britain has between 30 and 40 tornadoes every year -- mostly in the southern part of the country -- but they tend to be small and do little damage.

The strongest tornado ever recorded in Britain hit Plymouth in 1810. It was reported to have reached 213 mph -- a T8 rating.

In May 1950, a tornado left a 100-mile trail of damage from Great Missenden, Buckinghamshire, to Blakeney in Norfolk, and in 1971 astonished witnesses saw a twister move a 90-ton railway engine 50 metres along the track.

In January 1998 a 100 mph twister caused an estimated £100,000 damage when it hit the Sussex seaside town of Selsey.

Tornado and Storm Research Organisation

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