The tornado is the enfant terrible of meteorology. Although small-scale and very localised, it is the most violent of all windstorms - a fierce maelstrom of swirling air with a life-span of anything from a few minutes to perhaps an hour. It strikes very suddenly with little warning, and brings the most unbelievable devastation to places in its path.
Tornadoes commonly form within well-developed thunderclouds, often near well-defined cold fronts where the advancing cold air has anomalously overrun and displaced a much warmer and more humid air-mass underneath, causing a very rapid decrease in temperature with height.
In such conditions, the unusual buoyancy of the air at lower levels causes it to rise. As it ascends it may be forced to "turn" because of variations in the strength and direction of the wind with height - a phenomenon known to meteorologists as vertical wind shear; sometimes this turning motion rapidly accelerates, and the result is a tornado.
The Great Plains of the United States provide a favourite breeding ground, mainly because that part of North America has the right combination of moist air moving up from the Gulf of Mexico and cold air coming down from the north-west. The "twisters" form at the boundary between these two air masses, and 20 or more per day are not uncommon in "Tornado Alley", the flat country of the mid-west stretching through the states of Texas, Kansas and Oklahoma.
As it happens, one of the most devastating outbreaks of tornadoes recorded occurred in that region 25 years ago this month. During a 12-hour period on April 3rd and 4th, 1974, 148 of them were observed in a wide corridor stretching from the Great Lakes down to Alabama and Georgia and caused great loss of life and devastation: 315 people were killed, 5,000 persons injured and some 2,500 buildings were laid low.
It is an ill wind, however, that blows no one good. A rare breed of Americans, the "storm-chasers", spend their free time cruising the open plains in the eager hope of sightings, and think nothing of driving 600 or 700 miles to find a promising tornado spawning ground. They are enthusiastic amateur meteorologists, or some would say eccentrics, who have become mesmerised by these violent phenomena, and who relish for its own sake the flow of adrenaline that comes with close contact with a deadly whirlwind.
Storm-chasers often try to cover their expenses by providing video pictures and "on-the-spot" reports to local television channels, so a worthwhile by-product of their peripatetic idiosyncrasy has been some of the best television film of these vicious storms.