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    This is the VOA SpecialEnglish Environment Report.

    Every year in the United States people watch for dangerouswindstorms called tornadoes. A tornado is a violently turning pipeof air suspended from a dense cloud. It forms when winds blowing inseparate directions meet in the clouds and begin to turn in circles.Warm air rising from below causes the wind pipe to reach toward theground. It is not officially a tornado unless it has touched theground. A tornado can destroy anything in its path.

    Tornadoes come in many sizes. Theycan be thin pipes with openings on the ground just a few metersacross. Or they can be huge pipes that stretch as far asone-and-a-half kilometers. A tornado's size is not linked to itsstrength. Large tornadoes can be very weak, and some of the smallestcan be the most damaging. No matter how big or small, however, thestrongest winds on Earth are in tornadoes.

    Tornadoes are most common in the central part of the UnitedStates called "Tornado Alley." This area stretches south fromwestern Iowa down to Texas.

    Weather experts have done a lot of research in Tornado Alley.They have discovered that unlike severe ocean storms, tornadoes canstrike without warning. Usually weather experts can report daysbefore a severe ocean storm hits. However, tornadoes can form withinminutes. There is almost no time for public warnings before theystrike.

    The force of a tornado is judged not by its size, but by thetotal damage caused to human-made structures. The Fujita Scale isthe device used to measure tornadoes. It is named after Ted Fujita.He was a University of Chicago weather expert who developed themeasure in the nineteen-seventies. There are six levels on themeasure. Tornadoes that cause only light damage are an F-zero. Theones with the highest winds that destroy well-built homes and throwvehicles more than one-hundred meters are an F-five.

    In the nineteen-sixties, aboutsix-hundred-fifty tornadoes were reported each year in the UnitedStates. Now, more than one-thousand tornadoes are seen yearly.Weather experts do not think the increase is caused by climatechanges. Instead, they say Americans are moving away from citiesinto more open farming areas. This means that they see and reporttornadoes more often.

    This VOA Special English Environment Report was written by JillMoss.