What caused the Bicester Twister?

Swirling vortex causes alarm in Oxfordshire but UK is no stranger to the phenomenon. Michael McCarthy reports

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The Independent Online

It was certainly a twister. Indeed, it was a twister near Bicester. But it may not quite have been a proper tornado.

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A massive storm with a dark hanging cloud-funnel which affected Bicester, Eynsham, Kidlington and other parts of Oxfordshire earlier this week seems to have been a supercell, an unusual, revolving thunderstorm from which tornadoes are born – but it did not quite touch the ground.

The storm started in Wiltshire, and moved across Oxfordshire and on to Buckinghamshire, alarming people who came into contact with it along its path.

“It was very wet, we were just driving on the A34 and looked up and realised one part of the sky was moving in one direction and another in the opposite direction,” said Richard who drove through the storm with his wife and son. “I thought, 'that looks like a tornado!' We pretty much drove through it, we were right underneath it.

“As we drove into it the trees were blowing left to right, and as we got through it they were blowing the other way.”

Brendan Jones, forecaster at MeteoGroup, said it was likely the storm was a ‘supercell’ thunderstorm -  in which, unlike in normal storms, the air in is spinning or rotating, he said.

Supercells are common in the US but rare in Britain. The UK had seen plenty of reports of tornadoes and funnel clouds, which do not touch the ground, but not supercell storms, said Mr Jones, adding: “This one was fairly special.”

Supercells are big thunderstorms characterised by a rotating updraft which can be very severe in their effects; they are most common in the Great Plains region of the USA, but they can be found occasionally in many parts of the world. They can produce large, damaging hailstones, cloud-to-ground lightning, heavy rain, severe winds, and in extreme cases, tornadoes.

There have already been several reports of tornadoes in Britain this year.  Two weeks ago suspected tornadoes struck Rugby in Warwickshire and Halstead in Essex, 100 miles away, on the same day. Although no-one was hurt, witnesses reported alarming experiences.

Rugby residents said a tornado had ‘ripped a path’ through properties toppling a chimney stack and knocking down fences. “There was a roaring and we thought something was coming through the front window,” said Beryl Clarke, of Wentworth Road. “We rushed to the dining room and everything was flying up in the air, the greenhouse was going up, the glass was going everywhere. The shed came over the fence from next door, it was like something out of a film. Next door's got a trampoline outside their dining room and I don't know where it's come from. Everybody's got everyone else's stuff.”

Near Halstead, farmer Alan Barrow said he was lifted off his feet and thrown to the ground by the tornado which blew apart his farm buildings and a chicken coop, killing 20 chickens,  at Brook St Farm in White Ash Green.

Britain experiences more tornadoes than many people might realise, with between 30 and 50 reported every year, although the vast majority are very weak.  There are suggestions that in fact, Britain suffers from more twisters per square mile than any country on earth, although other reports hand this distinction to Belgium, and put Britain in second place. Britain’s deadliest ever tornado appears to be the one which hit Edwardsville in the South Wales valleys in October 1913, killing six people.

The world’s major centre of tornado activity is the USA, which averages nearly 1300 large tornadoes a year, more than any other country. But last year, 1,897 twisters were reported, making it the record year.

Tornadoes form when warm, humid air collides with colder air to form a swirling vortex that extends down from the clouds and sometimes reaches the ground where they can cause extensive damage.

Warmer air rises rapidly above colder air, forming an updraft that begins to move in a swirling motion which reaches along the length of the column to cause the classic spinning, or twisting of a tornado.

Just as water leaves a bath more efficiently by spinning down the plughole, so warm, humid air rises more effectively through a storm cloud by twisting in the vortex of a tornado.