Tornadoes are the whirling dervishes of meteorology small in scale and very localised, but the most violent of all windstorms.
Each individual tornado is a fierce maelstrom of swirling air, anything from a few yards to several hundred in diameter, it forms and dissipates quickly, having a life span of anything from a few minutes to several hours, and it moves along a relatively short and narrow path with winds around it at speeds up to 300 m.p.h.
The essential requirement for the formation of a tornado Is the presence of strong persistent updraughts In the atmosphere, and the availability of plenty of moisture near the pound. It so happens that these same conditions facilitate the development of thunderstorms, hail and heavy showers, so a tornado often coincides with these phenomena.
In such conditions, the anomalous buoyancy of the air at lower levels causes it to rise and if, as it ascends, it is forced to "turn" because of variations in the strength and direction of the wind with height, sometimes this turning motion rapidly accelerates, and the result is a tornado.
They are common near well-defined cold fronts, where advancing cold air displaces a much warmer and more humid air-mass, and there occurs a very rapid decrease in temperature with height. The Great Plains of the US provide a favourite breeding pound, 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. One of the most devastating outbreaks of tornadoes ever recorded occurred in that region almost exactly 25 years ago. During a 12-hour period on April 3rd and 4th, 1974, 148 of them were counted in a wide corridor stretching from the Great Lakes down to Alabama and Georgia and caused great loss of life and devastation: some 2,500 buildings were laid low, 315 people were killed, and 5,000 injured.