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-马丁和莎拉·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 measure only a few feet, one meter across. Others are more than a mile, 1 .6 kilometers wide. Some tornadoes touch down for a short period of 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？ 答案就是龙卷风。 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后只停留很短的时间。 还有些龙卷风会一路席卷数百英里远。

How Tornadoes Form Why do tornadoes happen? Scientists aren't sure. They know that strong tornadoes come from huge thunderstorms called supercells. These storms form when warm, moist air rises in the atmosphere and mixes with cold, dry air. Scientists think that in some supercells, the mixture of air at different temperatures causes the air to spin. The spinning air forms a cloud in a funnel shape. When the funnel touches the ground, it becomes a tornado.

龙卷风是怎么形成的？为什么会出现龙卷风呢？ 科学家们还没有确定的答案。 他们知道强烈的龙卷风来自名为“超级单体”的巨型雷暴。 当温暖潮湿的空气上升到大气层，和寒冷干燥的空气混合时，就会形成这类雷暴。 科学家认为，在部分超级单体中，不同温度的空气混合会让空气开始旋转。 旋转的空气会形成漏斗形状的云。 当这个漏斗云碰到地面的时候，就变成了龙卷风。

Some scientists think that tornadoes happen in order to balance the temperature and moisture in the air.

有些科学家认为，龙卷风的出现是为了平衡空气中的温度和湿度。

Measuring Tornadoes Scientists are still studying the causes of tornadoes, but they have a system to estimate their strength. They use a special scale called the Enhanced Fujita EF Scale. It uses wind speed to rate a tornado's strength. Scientists look at the damage caused by a tornado to estimate wind speed. EF5 tornadoes, the most powerful tornadoes, have winds greater than 200 miles per hour, 322 kilometers per hour.

龙卷风的测量 科学家们还在研究龙卷风的成因，不过他们已经有了一套估算龙卷风强度的体系。 他们使用一种叫做“增强藤田级数（EF级）”的特殊量表。 它通过风速来评定龙卷风的强度等级。 科学家会通过龙卷风造成的破坏来估算它的风速。 EF5级龙卷风是威力最强的龙卷风，风速超过每小时200英里，也就是每小时322公里。

where tornadoes form. Tornadoes may be hard to measure, but scientists have a good idea where they will strike. It's possible for a tornado to happen anywhere in the world. However, most tornadoes occur in the central part of the United States, also known as Tornado Alley. More than 1 ,000 tornadoes develop there each year.

龙卷风在哪里形成 龙卷风或许很难测量，但科学家很清楚它们可能会在哪里出现。 世界上任何地方都有可能出现龙卷风。 不过绝大多数龙卷风都出现在美国中部地区，这片区域也叫龙卷风 走廊。 这里每年会生成超过1000场龙卷风。