Unlike the hurricane, the tornado is a very small-scale phenomenon. It is a violent whirlwind, which is typically only a few hundred feet in diameter, moving at between 10 and 20 m.p.h. along a gently curving track some three to six miles long. It is also a short-lived event: an individual hurricane may retain its identity for a week or more, but the average tornado has a life of less than 30 minutes.
Tornadoes commonly form within well-developed thunderclouds. They tend to occur near well-defined cold fronts, where the advancing cold air overruns and displaces a much warmer and more humid air mass underneath, thus causing a quick fall in temperature with height.
They are very common on the Great Plains of the United States, mainly because North America has the right combination of moist air moving up from the Gulf of Mexico and cold air coming down from the northwest.
The breeding ground of the "twisters" lies at the boundary between these air masses, and on some days 20 or more will be reported in Tornado Alley, the flat country of the Midwest stretching through Texas, Kansas and Oklahoma.
Very few people in this world have seen the inside of a tornado and survived. But one such person had that experience 70 years ago in the autumn of 1928. A tornado passed directly over the house of a certain Bill Keller in Wellsford, Kansas, and he lived to give an account of his experience. This is how he graphically described the few seconds during which he found himself at the very centre of the vortex.
"Everything was as still as death, and I could hardly breathe. A screaming hissing sound came directly from the end of the funnel, and when I looked up, I saw to my astonishment right into the very heart of the tornado. There was a circular opening in the centre of the funnel about 50 or 100 feet in diameter and extending straight upwards for a distance of at least half a mile, as best I could judge in the circumstances.
"The walls of this opening were rotating clouds, and the cavity was brilliantly lit with the constant flashes of lightning zigzagging from side to side.
"Around the rim of the great vortex, small tornadoes were constantly forming and breaking away, and writhing their way around the funnel; it was these that made the hissing sound. I noticed that the rotation of the great whirl was anti-clockwise, but that some of the small twisters inside rotated clockwise."