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About Tornadoes

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Although tornadoes are occasionally reported in other parts of the world, most occur in the United States east of the Rocky Mountains during the spring and summer. However, tornadoes can occur in any state at any time of the year. Nationally, an average of 800 tornadoes are sighted each year, causing about 80 deaths and more than 1,500 injuries.

Tornado Basics

A tornado is a violent whirlwind--a rotating funnel of air that extends from a cloud to the ground. Tornadoes can travel for many miles at speeds of 250 miles per hour or more. These storms change direction without warning, randomly destroying homes and power lines, uprooting trees, and even hurling large objects--such as automobiles--over long distances.

Tornadoes usually accompany severe thunderstorms. Occasionally, tornadoes occur during tropical storms or hurricanes. The path of damage left behind by a tornado averages 9 miles long by 200 yards wide, but a severe tornado can damage an area up to 50 miles long and a mile wide.

Tornadoes that occur over oceans and lakes are called waterspouts. Because they rotate less vigorously and affect less-populated areas, waterspouts are usually not as destructive as tornadoes; however, waterspouts can move inland and become tornadoes. Waterspouts are more common in the Southeast, particularly along the Gulf Coast, but can form over any body of warm water.

The most destructive force in a tornado is the updraft in the funnel. As this unstable air moves upward at high speed, it can suction up houses and trees and move them hundreds of feet.

How Tornadoes are Formed

When unseasonably warm humid air collides with a cold front, intense thunderstorm clouds form and tornadoes may develop.

As warm air rises within the storm clouds, cooler air rushes in from the sides, creating a whirling wind that draws surrounding air toward its center.

An area of strong rotation develops, 2 to 6 miles wide. Next to appear is a dark, low cloud base called a rotating wall cloud.

Moments later, as rotation becomes even stronger, a funnel develops.