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Tornadoes
龙卷风
A tornado is a violently turning tube of air suspended from a thick cloud.
龙卷风是由雷暴云层伸展至地面的漏斗状云柱的强风涡旋。
It extends from a thunderstorm in the sky down to the ground.
两股空气强烈相对流运动，相互摩擦形成空气漩涡，空气绕龙卷风的轴快速旋转，
The shape is like a funnel:
其形状就像漏斗:
wide at the top, narrower at the bottom.
顶部宽，底部细长，
Tornadoes form when winds blowing in different directions meet in the clouds and begin to turn in circles.
四面八方的气流被吸入漩涡的底部，并随即变为绕轴心向上的涡流。
Warm air rising from below causes the wind tube to reach toward the ground.
来自下面热空气上升使风管朝向地面。
Because of their circular movement,these windstorms are also known as twisters.
由于自身的圆周运动,这些风暴也被称为龙卷风。
The most severe tornadoes can reach wind speeds of 320 kilometers an hour or more.
其最高速度达到每小时320千米。
Weather experts can identify the conditions to form a tornado.
气象专家可以辨别出龙卷风形成的条件具备与否，
And when they observe them,they can advise people that tornadoes might develop.
一旦观测到这些情况，他们会向当地居民发出警报。
But they are not able to tell exactly where or when a tornado will hit.
但是，他们却无法精确地推算出龙卷风会在什么时候袭击哪一个地方。
Tornado warnings still depend in large part on human observations.
龙卷风警告仍然在很大程度上依靠着人类的观察。
