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Last Updated: Thursday, 7 December 2006, 19:43 GMT
How could a tornado hit London?
Destruction caused by tornado
The tornado measured up to two out of six on a damage scale
It blasted through windows, crashed through walls, ripped up trees and left six people in need of medical attention.

There was no doubting its power as the tornado tore through north-west London leaving a trail of destruction.

But just how powerful was it? And can the UK expect to see many more as the UK's seasons continue to blur?

"This was a proper tornado," says BBC meteorologist Alex Deakin.

"It's difficult to say the scale without confirmed footage but the damage is certainly consistent with a tornado.

"Eye witnesses suggested it being as much as 20 metres wide and there's no reason to doubt them."

Every tornado is measured on the Fujita damage scale, from zero to six. This one, Mr Deakin suggests, could be a one or two.

Many go unrecorded

It may not sound a lot but in the UK this makes it one of the most powerful of the year.

Every year there are as many as 40 tornadoes across the UK, and this latest appears to be at the top end in terms of strength, according to Mr Deakin.

You cannot predict tornadoes but you can predict when the atmosphere is right for them
Alex Deakin

Most land in the countryside where the scale and damage goes unrecorded. But when they hit an urban area, it is big news.

In July last year, Birmingham came face to face with a tornado measuring three or four on the scale and experienced 130mph winds and devastating damage to homes and businesses.

Some described it as the UK's worst in 25 years.

In relation, it appears the streets in Kensal Rise have become acquainted with its weaker cousin.

"At the moment, we don't know all that much about it," says Mr Deakin,

"We can't measure its wind speed and we'll have to gauge how powerful it was by the damage."

They are also particularly difficult to predict.

"You cannot predict tornadoes but you can predict when the atmosphere is right for them," says Mr Deakin.

Visible cone

During thunderstorms with gusty winds an upward movement can become very rapid and wind flowing in other directions can cause it to rotate.

A visible cone drops out of the cloud to the ground. Where it touches the ground, destruction will usually follow.

Mr Deakin says there was little basis to predict that Kensal Rise would have been visited by such an unwelcome guest.

"It's not really to do with geography, it's to do with where the thunder storms occur and is nothing to do with the lay of the land in London."

He also said the likelihood of thunderstorms at this time of year rather than the recent mild climate would have given the clue as to its timing.

"The weather has not been particularly unseasonal.

"It has been mild throughout autumn and the extra warmth can be a trigger, causing updrafts. But thunder storms are not that unusual at this time of year."

For Mr Deakin and his colleagues, the tornado has made for a busy, but thrilling day.

"Days like today, that's why I come to work. We don't want to see people hurt or damage to houses but this is the sort of weather we love."

See the aftermath of the tornado


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