Michael McCarthy: How winds get in a spin and become a destructive force of nature

Tornadoes are violently rotating columns of air that usually form in the unstable atmosphere of a thunderstorm, when winds of opposite directions meet and start a column of air spinning.

Put crudely, the process is rather like rolling a cigarette. Fingers push one way round the cylinder of paper and tobacco, thumbs push the other, and it revolves.

Tornado formation is this process, scaled up millions of times to produce extremely high wind speeds which can pose a real threat to property and life on the ground (in the most extreme cases they can hit hundreds of miles an hour).

The most propitious conditions for tornado formation are when a mass of warm air from one direction meets a mass of cold air coming from another, and this is what happens in the world's most tornado-prone area, "Tornado Alley" in the US. This is a belt of the American Midwest, particularly Oklahoma, Kansas, Missouri and northern Texas, where every spring, cold, dry air coming down from the Rocky Mountains meets warm moist air coming up from the Gulf of Mexico to produce the spectacular twisters that can devastate communities.

But Britain also sits on a warm air/cool air boundary, at the meeting point of the cold airflow from the Arctic, and the warm airflow from the tropics. The result, says Dr Terence Meaden of Britain's Tornado and Storm research Organisation (Torro) is that Britain has the highest incidence of tornadoes per unit of land area in the world.

That is not to say the UK has more twisters than the US; in some years thousands are reported in America. But we do have more per square kilometre, says Dr Meaden.

His organisation has records of UK tornado frequency and strength since 1960, and these shows a long-running average between then and 2000 of about 33 a year in Britain. (The annual figures can be as low as 15, or as high as 160, as in the exceptional year of 1981.)

But in the past five years, Torro has recorded a big increase in reported British tornadoes, up to between 60 and 70. Does this mean UK tornadoes are suddenly on the increase, and is this perhaps a sign of climate change? Probably not, says Dr Meaden. He thinks it is more a factor of increased publicity and interest, added to much more available recording equipment such as cameras in mobile phones.

In fact, he says, it is not possible to differentiate between a real increase in UK tornadoes in recent years and an increase in the effort in observing them.

A warmer world might be expected to produce more tornadoes, because there will be more heat energy and water vapour in the atmosphere, but there is not the direct link as there is with hurricanes, whose formation is crucially dependent on sea-surface temperature.