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Saturday, 4 May, 2002, 13:34 GMT 14:34 UK
Derek's Weatherwatch -Tornado troubles
BBC Wales weatherman Derek Brockway
BBC Wales weatherman Derek Brockway comments on a tornado which blew up in south east Wales this week.

On Thursday 2 May 2002, there was some interesting weather across Wales.

While many parts enjoyed sunshine, a few showers developed, especially in the south east, where there was hail and thunder.

But strangest of all, a tornado was reported in Caldicot in Monmouthshire.

Tornado in America
Tornadoes in the US can be more destructive
It did not do any significant damage but touched the ground, lifted rubbish from a skip into the air and covered cars in dust.

Tornadoes are more common than you might think in the UK.

On average we can expect 33 every year, but in 1982 there were 152.

In fact there are more tornadoes per square mile in Britain than in the USA - but thankfully they are not as destructive.

Is is not correct to describe tornadoes as freak, because they are a natural weather phenomenon.

In the UK, most tornadoes form over the Midlands and southern England, but some do occur occasionally in the north and over Wales, while they are rare in Northern Ireland and Scotland.

Rotating column

Tuesday's tornado in south-east Wales was caused by a combination of events

There was an unstable north-westerly air flow so as temperatures rose, cumulus and cumulo nimbus clouds formed quickly over the land with showers soon breaking out.

Sometimes these tall clouds produce thunderstorms and tornadoes.


Tornadoes should not be confused with hurricanes, which only form in the tropics

Derek Brockway
A tornado is defined as a violently-rotating column of air which spins around and upwards, similar to the action of a cork being pulled from a wine bottle.

Tornadoes in America are more destructive because the contrast in temperature is more dramatic as warm, moist from the Gulf of Mexico mixes with cool, dry air from the Rockies.

A tornado will usually last for a few minutes, and move across the ground for between one to three miles, producing windspeeds typically between 72 and 113 miles per hour.

But tornadoes should not be confused with hurricanes, which only form in the tropics where the sea is very warm - at least 27 Celsius.

Meanwhile, tornadoes which form over the sea are called waterspouts.

See Derek Brockway on Wales Today at 1330GMT and 1830GMT weekdays on BBC One Wales and hear him on BBC Radio Wales.

Click here to watch the Wales Today webcast, or here to listen to BBC Radio Wales live online.

If you have a weather topic you want Derek to explain, send an email to newsonline.wales@bbc.co.uk

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See also:

29 Apr 02 | Americas
In pictures: Tornadoes hit US
22 Mar 02 | Northern Ireland
'Mini tornado' hits Tyrone
27 Jan 02 | UK
Gales whipping up over UK
26 Jan 02 | England
Tornado rips into houses
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