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Tornado

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A tornado is a violent windstorm characterized by a twisting, funnel-shaped cloud.

File:Tornado thumbnail.jpg
From the NOAA Photo Library

It is spawned by a thunderstorm (or sometimes as a result of a hurricane) and produced when cool air overrides a layer of warm air, forcing the warm air to rise rapidly. The damage from a tornado is a result of the high wind velocity and wind-blown debris. Tornado season in North America is generally March through August, although tornadoes can occur at any time of year. They tend to occur in the afternoons and evenings: over 80 percent of all tornadoes strike between noon and midnight.

Tornadoes can be nearly invisible, marked only by swirling debris at the base of the funnel. Some are composed almost entirely of windblown dust and still others are composed of several mini-funnels. Tornadoes are most destructive when they touch ground. Normally a tornado will stay on the ground for no more than 20 minutes; however, one tornado can touch ground several times in different areas.

Tornadoes do occur throughout the world. However, the United States experiences the most intense and devastating tornadoes. On average, the United States experiences 100,000 thunderstorms each year. Approximately 1,000 tornadoes develop from these storms.

The intensity of tornadoes is measured by the Fujita - Pearson Tornado Scale (also known simply as Fujita scale).

See also

Tropical cyclone, curl