The National Weather Service defines a tornado as “ a violently rotating column of air in contact with the ground and pendant from a thunderstorm”. In other words, a thunderstorm is the first step in the creation of a tornado. Then, if other conditions are right, the thunderstorm could spin out one or more tornadoes.

The three key conditions required for thunderstorms to form are:

· Moisture in the lower to mid levels of the atmosphere,

· Unstable air. That is, air that will continue rising once it begins rising from near the ground,

· A lifting force. Something is needed to cause the air to begin rising. The most common lifting force is heating of air near the ground. As the air warms it becomes lighter and begins rising. Advancing masses of cool air, which force warm air upward, also trigger thun

Tornadoes strike in many areas of the world, but nowhere are they as frequent or as fierce as in the United States than in any other country, where an average of 800 per year was reported during the period 2000-2001 (NSSL, 2002). However, tornadic storms are by no means confined to North America. Europe has experienced many and cyclones moving north from the Bay of Bengal often bring them to the eastern part of the Indian subcontinent. Much of Australia is desert, but about 15 tornadoes are reported there each year and the true number may be considerably greater because Australia is so sparsely populated away from the cities (Allaby, 1997). Most tornadoes, however, happen in the American Midwest of America. There is ten states in which tornadoes are fairly common: Alabama, Arkansas, Florida, Iowa, Kansas, Mississippi, Missouri, Nebraska, Oklahoma, and Texas. All of these ten states lie to the east of a line running from Nebraska to Texas. This line also marks the boundary of the Great Plains, with land to the west rising into the Rocky Mountains (Allaby, 1997). The weather systems that generate tornadoes develop over the Great Plains and the area most severely affected by them is often known as ‘Tornado Alley’.

F-5 261-318 Incredible damage

WORLD TORNADOES AND AGRICULTURAL AREAS

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