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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.

Fujita - Pearson Tornado Scale

The strength of tornadoes is measured by the Fujita - Pearson Tornado Scale. Tornadoes are classified into the following categories (mph stands for miles per hour):

F-0: 40-72 mph (65-115 km/h) chimney damage, tree branches broken
F-1: 73-112 mph (116-180 km/h), mobile homes pushed off foundation or overturned
F-2: 113-157 mph (181-250 km/h), considerable damage, mobile homes demolished, trees uprooted
F-3: 158-205 mph (251-330 km/h), roofs and walls torn down, trains overturned, cars thrown
F-4: 207-260 mph (331-415 km/h), well-constructed walls leveled
F-5: 261-318 mph (416-510 km/h), homes lifted off foundation and carried considerable distances, automobiles thrown as far as 100 meters

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See also: Tropical cyclone, curl