国家地理 龙卷风的形成（在线收听）

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.

龙卷风是大自然最为猛烈的风暴，他们自带旋转的狂风，时速最高可达300英里。每年，美国都会遭受近800场龙卷风，没有哪个地方的遭受次数比美国更高。得克萨斯州到南达科塔之间，受创最为惨重的是北美大平原了，这里是龙卷道。这里春天和夏天的时候，暖空气会从墨西哥湾上升，然后与来自加拿大的冷空气相撞。交汇处会产生强烈的雷暴，也称超级单体。

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.

科学家不是十分清楚龙卷风形成的方式和时间点，但他们可以肯定的是：一旦条件合适，类似这样的超级单体就可以产生龙卷风。湿空气进入风暴里之后，就会受到推力作用，被上层的风带动着旋转。这种旋转气柱的力量越来越大，等条件合适的时候，就会发生下面的碰撞。经过雨水冷却的下降气流与地表附近的暖空气碰撞，单体下面的低空就会产生旋转的云。这时候就快产生龙卷风了。

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.

龙卷风持续时间很短，从20秒到1小时不等。但其造成的损害却是数年才能修复的。每年，龙卷风都会导致近90人死亡，其造成的损失达数百万美元。一旦看到龙卷风，专家建议去地下室待着，避开窗户，也不要在一层的浴缸里待着。大多数人对龙卷风避犹不及，一心想着保平安，但有些科学家却对龙卷风趋之若鹜。他们苦苦追寻龙卷风，试图与龙卷风打个照面儿。

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, giving survivors the chance to rebuild after living through one of the most violent storms on earth.

他们会遇到难以想象的云体移动、倾盆大雨、强风、冰雹、闪电以及令人叹为观止的风暴。一旦他们发现了龙卷风，就会用特殊工具来进行测量。这些科学家希望有一天可以准确预测龙卷风发生的时间和地点。很难避免龙卷风造成损失，但预测能力的提高可以挽救更多的生命，让幸存者有机会重建家园，就算他们曾经遭遇了世界上最可怕的风暴之一。