英語聽力：自然百科 Tornadoes 龍卷風的形成

Call them twisters or tornadoes. They are nature's most violent storms with swirling winds that can top 300 miles per hour. About 800 twisters sweep through the US every year, more than anywhere else in the world. The hardest-hit area is a swath of the Great Plains from Texas to South Dakota, known as Tornado Alley.

Here, warm air flows up from the Gulf of Mexico in the spring and summer and crashes into cold air pushing down from Canada. The meeting produces violent thunderstorms called supercells. Scientists don't completely understand how or when tornados form, but they do know a supercell like this one can produce a twister if the conditions are right.

As more moist air flows into a storm, it gets pushed up and twisted by upper level winds. As this rotating column of air gathers force, conditions are right for a collision below. When rain cooled downdrafts hit warm air near the ground, a low hanging revolving cloud forms beneath the cell. A tornado is imminent.

Tornadoes don't last long, anywhere from 20 seconds to an hour. But it can take years to recover from the devastation. These storms kill nearly 90 people each year in the US and cause millions of dollars worth of damage. When a tornado is spotted, experts advise going to a basement staying away from any windows or climbing into a first floor bathtub.

While most people run for safety when a twister appears, some scientists actually race to need it. These storm chasers hunt down tornadoes, trying to get right in the twister's path. They encounter incredible cloud movement, torrential rain, severe winds and hail, lightening, breathtaking storm structures. When they finally locate a twister, they measure it using special tools.

These scientists hope to someday predict exactly when and where tornadoes will strike. Little can prevent the damage caused by tornadoes, but better forecasting could save more lives, givingsurvivors the chance to rebuild after living through one of the most violent storms on earth.