The United States gets between 1,000 and 1,200 tornados in an average year.
On 3 April 1974, 148 tornados touched down in 13 American states. It was the worst outbreak in US history, leaving 330 dead and 5,000 injured.
America's worst single twister was the tri-state tornado of 18 March 1925, which cut a 219-mile path of destruction through Missouri, Illinois and Indiana, leaving 695 dead and 2,027 injured.
America's tornado experts do not advise trying to outrun a tornado in a car; twisters can change direction suddenly, and they can outrun you. Their advice is to get out of the car and shelter in a solid building or a ditch.
On average, 33 tornados occur every year in the UK. Last year there were 45; in 1984 there were 47.
Britain's most severe tornado may have been the one that destroyed the church of St Mary le Bow and 600 houses in London on 23 October 1091. Four of the church's 26-foot long rafters were reportedly driven so hard into the ground that only four feet of them was visible.
The other candidate for the UK's top twister occurred on 12 September 1810, when one thought to have registered F4 to F5 on the Fujita scale cut a devastating path from Old Portsmouth to Southsea Common. An eyewitness spoke of the roof of a bank being "rolled up like a piece of canvas and blown from its situation".
The Tay Bridge rail disaster in Scotland, on 28 December 1879 and immortalised in verse by William McGonagall, may have been caused by waterspouts - tornados over water. Two or three of these were seen close to the bridge immediately before the accident and may have weakened it.
Britain has its own tornado research group, called Torro (the Tornado and Storm Research Association), which brings together academic meteorologists and enthusiastic amateur storm chasers. Its website can be accessed at www.torro.org.uk.