17th April 2006
Tornadoes
Source: Excerpt from The Book " Weather "
Distribution: Wherever thunderstorms occur, most
common over
Great Plains of North America.Height:
From base of cumulonimbus cloud to ground level.Cause: Rapid rotation of updrafts within a thunderstorm. Associated Weather: Destructive surface winds. Hazard Warning: Threat to life and property. A tornado is a violently spinning vortex of air that extends from the base of a storm cloud to the ground. It is associated with severe storm activity and is one of nature's most destructive phenomena, capable of generating winds of up to 300 miles per hour (483 kph) in extreme cases.
Exceptional tornadoes may last for hours and travel hundreds of miles. In the United States, the Mattoon-Charleston Tornado of 26 May 1917 covered 293 miles (471 km) in just under seven and a half hours. Most tornadoes, however, are far weaker than this, with some lasting only seconds and generating winds of less than 50 miles per hour (80 kph).
Tornadoes may occur as isolated incidents or in great numbers. In the United States, the so-called "Super Outbreak" of 3-4 April 1974 saw 148 individual tornadoes devastate an area from Alabama to Michigan.
Storms severe enough to generate tornadoes are most likely to occur in the middle latitudes. The United States is by far the most tornado-prone country in the world, enduring around 750 tornadoes annually. Most of these are confined to the Great Plains, with central Oklahoma having the dubious distinction of experiencing more tornadoes per acre than any other location on Earth. Tornadoes also occur regularly in Australia, and occasionally in other countries such as the United Kingdom.
Tornadoes can occur at any time of the year. In the United States, there is an overall peak of activity in May and June on the Great Plains. However, tornadoes occur in different parts of the country at different times of year, with the focus of activity shifting from the Gulf in late winter to the Great Plains in midsummer, and moving southward again in fall.
Tornado Alert
There are two signs to look for when assessing whether a storm is severe enough to generate tornadoes. The first is the "overshoot" phenomenon, where the normally flat top of the storm's anvil displays an ominous bulge. This indicates that the upward rush of air near the center of the storm is so powerful that it has "punched" through the tropopause, bubbling up into the stratosphere. The second feature is an extensive and well-defined Mammatus Formation.
A tornado's movement can be erratic, creating a cycloidal damage path (like the track a spinning top takes on a flat surface). This explains why a tornado can demolish houses either side of one that is left untouched. While this makes it difficult to tell whether you will be caught in an approaching tornado's path, there are some precautions you can take. If possible, shelter indoors, particularly in a bathroom, because this is often the strongest room in the house. If caught outside, try to shelter in a ditch.
Acknowledgement due: John W. Zillman, William J. Burroughs,
Bob Crowder, Ted Robertson, Eleanor Vallier-Talbot and Richard Whitaker.
