An average of 800 tornados occur every year in the USA.
美国每年平均有八百个龙卷风。

They can generate wind speeds faster than 250 mph.
龙卷风的风速可以超过时速二百五十英里。
At times, they devastate whole communities.
有时候，龙卷风会毁灭整个社区。
Some are over a mile in diameter and can travel distances of 100 miles or more.
有些龙卷风的直径超过一英里，而且吹袭的距离可以达到一百英里甚至更远。