Tornadoes occur when a column of warm air gets caught below a layer of cold air, above which a strong cold wind is blowing. The warm air rises, forming a funnel through its cold surroundings; it is replaced by more warm air at ground level, which is in turn sucked up as the air at the top of the funnel is blown away. Give the whole apparatus a small twist, and it will whirl away with ever-increasing angular momentum. Parts of the United States have just the right climatic conditions favouring the production of tornadoes, where warm, moist air from the Gulf of Mexico or the Caribbean meets cool air coming down from Canada. The direction of these air masses even provides the twist needed to start it all off.
Now here are some of the things that you may not have known about tornadoes and related weather disasters:
The largest number of deaths from a single tornado in America is usually given as the 698 killed in 1925 by a tornado that stretched for 219 miles. This was probably, however, not a single tornado but a family of them coming close on one another, similar to the ones that have just hit Florida.
In the 1980s, 521 Americans were killed by tornadoes, but 726 were killed by lightning.
In Ohio, six per cent of all deaths by lightning happen as the victims are playing golf.
The country that experiences the most tornadoes per unit area is Britain.
More Americans are killed by tornadoes in April than in any other month.
You cannot (though they did in the film) assess the rating of a tornado until after it is over and the damage is assessed.
A house (despite Dorothy's experience in The Wizard of Oz) cannot be lifted bodily by a tornado. It will collapse first.
A car would not sucked straight up into the funnel of a tornado; it would rise and move through the air sideways too.
Caravan parks seem to attract more than their fair share of tornadoes only because mobile homes blow over more easily than houses. The same tornado that might cause havoc among caravans could pass through a built- up area without anyone bothering to record it.
Nobody has been killed by a tornado in California.
Tornadoes turn anti-clockwise in the northern hemisphere - or to put it another way, cyclones turn cyclonically.
The tornadoes in the film Twister were made by random-movement and vortex- creation computer programs. No wind was involved.
A large truck would not be picked up by a tornado. It would be blown over and twisted out of shape. Even a car would first capsize before flying through the air.
The average age of Americans killed by tornadoes is 45, the average American killed by lightning is only 30.
More than twice as many men as women are killed by extreme weather in America.
Although Florida has more tornadoes per square mile than any other American state, until this week they were mostly small ones.
Get your hats, coats, gloves, scarves and perhaps even snow-shoes out. The Met Office predicts that the real winter will finally arrive with icy arctic winds at the weekend. Well, it's good news for the Scottish ski slopes anyway.