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Tuesday, July 6, 1999 Published at 13:47 GMT 14:47 UK


'Carrots' over the UK

These are not "freak" events

"It was like a giant carrot coming down the road," was how one excited person described a tornado over the West Midlands this week.

Richard Bilton reports from the West Midlands on a night of storms
Just occasionally, the UK's mild, wet weather can have a ferocious bite. These images above were recorded across Central England. The picture on the right was taken in the Selly Oak area of Birmingham where the twister damaged 21 houses.

It surprises many people that tornadoes can occur in the UK - they assume the country's famous weather cannot generate the sort of energy contained within these violent storms. In fact, according to the Meteorological Office, there are 30-50 tornadoes in the UK every year.

Philip Eden: "They are usually associated with very big thunderstorms"
It is difficult to be precise about numbers because these short-lived events, inevitably, go unreported as they travel across fields and other unpopulated areas.

1982 was a particularly prolific year for twisters with more than 150 spotted around the country. Rare they may be, but they are definitely not freak events.

High frequency

"Freak means contrary to nature," says BBC weather expert Philip Eden, "and these are perfectly normal events."

Weather forecast
Indeed, it was the American meteorologist Dr Ted Fujita who discovered in 1973 that the UK actually has the highest frequency of reported tornadoes per unit area in the world.

The fastest winds on Earth are found inside a tornado. In one recent twister in Oklahoma City in the US, scientists even recorded a wind speed of 509km/h (318 mph). British twisters rarely get above 160 km/h. Even so, they can still do plenty of damage. One tornado in South Wales in 1914 killed six people.

Don Moody saw a tornado coming up his drive
A tornado is a violent, rotating column of air which is usually associated with a thunderstorm. This is not always the case and they do not have to be accompanied by thunder either.

The conditions have to be just right: a combination of wind speed and direction changes create a horizontal spinning effect near the Earth. This horizontal wind is pushed upwards, until it is vertical and funnel like.

As it comes into contact with the Earth, a tornado is born, creating the carrot shape with which we are all familiar.

Tilting clouds

"We've had the classic conditions for small tornadoes," says Mr Eden.

"If the winds at low levels are pretty non-existent with a bit more wind at upper levels, the cumulonimbus cloud actually tilts slightly. And if you've got that tilt in the big thundercloud, you are more likely to get the circulation that eventually creates this vortex - this funnel cloud - which, if it reaches the ground, becomes a tornado."

[ image: The conditons need to be just right]
The conditons need to be just right
There is always a tendency for people to blame global warming when events such as tornadoes occur in the UK. However, it is impossible to make this connection.

One-off events are just part of the normal chaos we call weather and there is no real evidence that violent storms like the ones witnessed in the UK over the past 24 hours are becoming more frequent - certainly not at a level which is statistically significant.

It is true, on the other hand, that the models that scientists have created for climate change do predict a speeding up of the hydrological cycle - the movement of water around the world.

This could fuel bigger, more violent storms. But even if we accept that global warming is taking place, it is still impossible to say precisely how weather patterns will change in the different regions of the world.

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