Tornadoes

Tornadoes. Written by Keith and Sarah Corte -Martin. Focus question. What are tornadoes? And why are they dangerous?

《龙卷风》。作者：基思·科特-马丁与萨拉·科特-马丁。 核心问题：什么是龙卷风？它们为什么很危险？

Introduction. What can lift roofs from buildings and sweep houses into the air? Tornadoes can. Tornadoes come in many sizes. Some tornadoes are only a few feet, one meter, across. Others are more than a mile, 1 .6 kilometers wide. Some tornadoes touch down for a short time. Others travel for hundreds of miles.

引言： 什么东西能掀掉建筑的屋顶，还能把房子卷到空中？ 龙卷风就可以。 龙卷风有大有小， 有些龙卷风的直径只有几英尺（也就是1米），宽。 还有些龙卷风的宽度超过1英里（也就是1.6公里）。 有些龙卷风接触地面的时间很短， 还有些能绵延数百英里。

How Tornadoes Form Why do tornadoes happen? Scientists are not sure. Tornadoes come from giant thunderstorms called supercells. A supercell happens when warm, moist air rises to mix with cold, dry air.

龙卷风是怎么形成的 龙卷风为什么会出现呢？ 科学家们目前还没有确定的答案。 龙卷风诞生自名为超级单体的巨型雷暴云。 当暖湿气流上升，和干冷气流混合时，就会形成超级单体。

The mixing of cold and warm air causes the air to spin. The spinning wind turns into a cloud in a funnel shape. As the cloud turns, the wind becomes stronger. When the funnel cloud touches the ground, it is a tornado.

冷暖气流的混合会让空气开始旋转。 旋转的风会变成漏斗形状的云。 随着云不断旋转，风力会变得越来越强。 当漏斗云接触到地面的时候，就变成了龙卷风。

Measuring Tornadoes Scientists have a way to measure the strength of tornadoes. They look at the harm caused by a tornado. They use the amount of harm to estimate the wind speed. They use a special scale called the EF scale. The EF scale measures the strength of the tornado.

龙卷风的测量 科学家们有办法测量龙卷风的强度 他们会观察龙卷风造成的破坏程度。 再根据破坏的严重程度估算当时的风速。 他们用的是一种叫做EF级的特殊分级标准。 EF级是用来衡量龙卷风强度的。

Where Tornadoes Form Tornadoes may be hard to measure, but scientists have a good idea where they'll strike. It's true that a tornado can hit anywhere in the world at any time. Most tornadoes happen in the central part of the United States. This area is called Tornado Alley.

龙卷风的形成地点 龙卷风也许很难测量， 但科学家们很清楚它们会在哪里出现。 确实，龙卷风随时可能在世界上任何地方出现。 不过大部分龙卷风都出现在美国的中部地区。 这片区域叫做龙卷风走廊。