听力文摘：揭秘车后窗玻璃上的雨滴形状

The next time you're driving during a rain shower, glance back at the rear window. You'll notice that while raindrops batter¹ the front windshield, they seem to avoid the back window as long as the car is moving. How is that possible?

下一次下阵雨而你在开车的时候，看一看后窗玻璃。你会注意到在汽车行驶的过程中，雨滴会连续打击到前面的挡风玻璃上，但是后窗玻璃上则不会出现这样的现象。是什么导致这样的现象发生呢？

When a car is moving at normal highway speed, air flows around it, from front to back. Since air can not go through the glass, when it hits the front window it suddenly changes direction and travels up and over the windshield. A raindrop, however, which is denser² than air, falls from the sky with enough velocity³ so that it cannot be swept up and over the glass by the wind.

当 一辆汽车以正常的速度在高速公路上行驶时，空气漂浮在它周围并且从前往后流动。因为空气并不能穿过玻璃，当它撞击到前方的玻璃上时，它的方向会发生改变， 它会沿挡风玻璃往上运动。然而，由于雨滴的密度比空气大，加之它以足够的速度从天空中滴落下来，所以不会被吹过玻璃的风清除掉。

When raindrops approach the front windshield, they are too heavy and are going too fast to suddenly change direction and be swept up and away from the glass. Instead they burst through the wind stream and hit the windshield.

当雨滴到达前面的挡风玻璃时，它们太重并且速度太快以至于不能够突然改变方向，也就不会被清除掉了。相反，它们冲破流风，打在挡风玻璃上。

Raindrops do tend to miss the back window, however. Wind passing over the rear of the car is accelerated on a downward angle, flowing parallel to and just above the rear window. In this case the velocity of the falling raindrops carries them into the downward flow of the wind, where they skim over the surface of the window without touching⁴ it.

然而，雨滴却不会打到后窗玻璃上。风通过后窗玻璃时会加速往下并在后窗上方平行流动。在这种情况下，落下的雨滴的速度将它们带入向下流动的风中，于是它们就会掠过窗户表面而不会碰到它。

Because the wind is flowing down instead of up, the rain does not have to change direction when it encounters the wind stream. Although the raindrops are still heavier than air, the fact that the wind and rain flow in the same direction enables the wind to sweep the rain away from the window.

因为风是往下而不是往上流动，所以当雨滴碰到流风时，它们不需要改变方向。虽然雨滴还是会比空气重，但是风和雨滴运动方向一致使得风将雨滴吹离玻璃。

1 batter   

v.接连重击；磨损；n.牛奶面糊；击球员

参考例句：

• The batter skied to the center fielder.击球手打出一个高飞球到中外野手。
• Put a small quantity of sugar into the batter.在面糊里放少量的糖。

2 denser   

adj. 不易看透的, 密集的, 浓厚的, 愚钝的

参考例句：

• The denser population necessitates closer consolidation both for internal and external action. 住得日益稠密的居民，对内和对外都不得不更紧密地团结起来。 来自英汉非文学 - 家庭、私有制和国家的起源
• As Tito entered the neighbourhood of San Martino, he found the throng rather denser. 蒂托走近圣马丁教堂附近一带时，发现人群相当密集。

3 velocity   

n.速度，速率

参考例句：

• Einstein's theory links energy with mass and velocity of light.爱因斯坦的理论把能量同质量和光速联系起来。
• The velocity of light is about 300000 kilometres per second.光速约为每秒300000公里。

4 touching   

adj.动人的，使人感伤的

参考例句：

• It was a touching sight.这是一幅动人的景象。
• His letter was touching.他的信很感人。