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These days, there have been a lot of reports about tornadoes（龙卷风）. A tornado is a very _______ form of weather. In fact, it is a rotating tube（旋转管道）of _______. The tube touches the ground and a cloud at the same time. The reason why tornadoes are so dangerous is that they travel at _______ up to 300 miles per hour! Tornadoes are _______ when warm air and cool air meet in the sky. They can pick up cars, blow _______ trees, and knock over buildings.