How Tornadoes Can Happen - Even In Genteel Sussex

If you thought tornadoes were just an American experience, as in the film Twister, think again. About 50 hit Britain each year, but most don't hit populated areas, and are smaller than yesterday's storm.

Tornadoes have their origins in thunderstorms. In the United States, meteorologists have learnt to watch for rain patterns with an "S" or "6" shape - indicative of a funnel cloud about to form, or already rotating. This usually happens along a storm front when large masses of cold dry air, fast-moving frigid dry air, and low-lying warm wet air collide.

The masses begin rotating, with the warm air trapped beneath, trying to rise, and the cold air trying to fall. If the twisting warm air punches a hole up through the cold air, the scene is set for a funnel cloud.

Generally, half of funnel clouds dissipate, but the others worsen, as the winds speed the funnel up. It tightens and extends down towards the ground. When it touches, the tornado is born.

Some have wind speeds of more than 250mph, and move across the ground at more than 50 mph. Their severity is measured against the 7-point Fujita scale, ranging from F0 to F6, the "inconceivable tornado" with winds of more than 370mph, which isn't expected to happen on Earth. Even F2s and F3s are killer tornados.Andy Yeatman, spokesman for the Meteorological Office, said: "[The Selsey] tornado was considerably more destructive than those we normally witness."

The funnel core is the most vicious killer for anyone unlucky enough to be sucked into it: people are sandblasted to death by tiny debris.