This is the VOA Special English Environment Report.
Every year in the United States people watch for dangerous windstorms called tornadoes. A tornado is a violently turning pipe of air suspended from a dense cloud. It forms when winds blowing in separate directions meet in the clouds and begin to turn in circles. Warm air rising from below causes the wind pipe to reach toward the ground. It is not officially a tornado unless it has touched the ground. A tornado can destroy anything in its path.
Tornadoes come in many sizes. They can be thin pipes with openings on the ground just a few meters across. Or they can be huge pipes that stretch as far as one-and-a-half kilometers. A tornado's size is not linked to its strength. Large tornadoes can be very weak, and some of the smallest can be the most damaging. No matter how big or small, however, the strongest winds on Earth are in tornadoes.
Tornadoes are most common in the central part of the United States called "Tornado Alley." This area stretches south from western Iowa down to Texas.
Weather experts have done a lot of research in Tornado Alley. They have discovered that unlike severe ocean storms, tornadoes can strike without warning. Usually weather experts can report days before a severe ocean storm hits. However, tornadoes can form within minutes. There is almost no time for public warnings before they strike.
The force of a tornado is judged not by its size, but by the total damage caused to human-made structures. The Fujita Scale is the device used to measure tornadoes. It is named after Ted Fujita. He was a University of Chicago weather expert who developed the measure in the nineteen-seventies. There are six levels on the measure. Tornadoes that cause only light damage are an F-zero. The ones with the highest winds that destroy well-built homes and throw vehicles more than one-hundred meters are an F-five.
In the nineteen-sixties, about six-hundred-fifty tornadoes were reported each year in the United States. Now, more than one-thousand tornadoes are seen yearly. Weather experts do not think the increase is caused by climate changes. Instead, they say Americans are moving away from cities into more open farming areas. This means that they see and report tornadoes more often.
This VOA Special English Environment Report was written by Jill Moss.