Tornadoes

Tornadoes. Written by Keith and Sarah Kordermarten. Focus Question. What are tornadoes and why are they dangerous?

龙卷风。 作者：基思·科德马滕、萨拉·科德马滕。 思考问题。 龙卷风是什么？它为什么很危险？

Introduction. What can lift roofs from buildings, sweep houses into the air, and rip trees out by the roots? Tornadoes can. Tornadoes come in many sizes. Some tornadoes are narrow, only a few feet, one meter across. Others are more than a mile, 1 .6 kilometers wide. The biggest tornado on record was almost 2 .6 miles wide, 4 .2 kilometers. It struck Oklahoma in 2013. Some tornadoes touched down for a short period of time. Others can travel for hundreds of miles, destroying everything in their paths while carrying houses and cars over long distances.

引言。 什么能掀掉屋顶、把房屋卷到空中，还把大树连根拔起？ 龙卷风就能做到。 龙卷风有大有小。 有些龙卷风很窄，直径只有几英尺（1米）。 另一些直径超过1英里（1.6公里）。 有记录以来最大的龙卷风，直径接近2.6英里（4.2公里）。 它在2013年袭击了俄克拉荷马州。 有些龙卷风只在地面停留很短的时间。 另一些能移动数百英里，摧毁沿途一切，还会把房屋和汽车卷到很远的地方。

How tornadoes form. What causes tornadoes? Scientists aren't sure. They know that the strongest and most violent tornadoes come from large thunderstorms called supercells. These thunderstorms form when warm, moist air rises into the atmosphere and mixes with cold, dry air higher in the sky.

龙卷风的形成。 龙卷风是怎么产生的呢？ 科学家们也没有完全弄清楚。 但他们知道，最强最凶猛的龙卷风，来自一种叫做超级单体的大型雷暴。 当温暖湿润的空气上升到大气中，和高空干冷的空气混合，就会形成这种雷暴。

Scientists think that in some supercells, the warm air rises quickly and the cold air rushes in to replace it. This causes the air to rotate. The spinning air begins to form clouds into a funnel shape. The funnel cloud stretches downward with winds up to 300 miles, 483 kilometers per hour. When the funnel touches the ground, it officially becomes a tornado.

科学家认为，在一些超级单体中，暖空气快速上升，冷空气迅速涌入填补位置。 这就会让空气旋转起来。 旋转的空气让云变成了漏斗形状。 漏斗云向下延伸，风速可达每小时300英里（483公里）。 当漏斗接触到地面，它就正式成为龙卷风了。

Some scientists think that tornadoes occur to balance the temperature and moisture levels in the air.

一些科学家认为，龙卷风的出现是为了平衡空气中的温度和湿度。

Measuring tornadoes While scientists are still researching the causes of tornadoes, they have a system for estimating their strength. A scientist named T. Theodore Fujita developed the Enhanced Fujita Scale, known as the EF Scale, as a way to rate a tornado's strength based on wind speed. Because it's difficult to get close to a tornado, scientists estimate its wind speed by examining the damage it causes. The most powerful tornadoes are rated EF5. They have winds greater than 200 miles, 322 kilometers per hour.

测量龙卷风 虽然科学家还在研究龙卷风的成因，但他们已经有了估算龙卷风强度的体系。 科学家T·西奥多·藤田发明了增强藤田分级，简称EF分级，根据风速给龙卷风划分强度等级。 因为很难靠近龙卷风，科学家会通过它造成的破坏来估算风速。 最强的龙卷风被评为EF5级。 它的风速超过每小时200英里（322公里）。