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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。