英语听力文摘 English Digest 581、为何夜晚比白天平静？

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　　More often than not, the wind dies down at night after a blustery day. Where does the wind go? Why are nights generally more calm than days?

 

　　The difference between calm nights and windy days is the sun. During the day, the sun warms the surface of the Earth, which in turn warms the atmosphere above it. But at sunset, the earth’s surface and the layers of the atmosphere directly above it cool faster than higher layers of the atmosphere. That creates a stable temperature pattern, with dense¹, cool air on the ground and less dense, warmer air above.

 

　　And that’s a little crazy, because normally the atmosphere gets cooler as you go up! But with cooler air sitting tight at ground level, the usual temperature pattern turns upside down for a few hundred feet above the ground. Known as a temperature inversion², this upside-down pattern acts as a barrier, dividing the cool, calm air on the ground from whatever’s happening above.

 

　　On many calm nights, the wind still blows far overhead, but a temperature inversion prevents it from coming down to earth.

 

　　At sunrise, everything changes.The sun warms the earth and the lower layers of the atmosphere, mixing everything up. Without the temperature inversion as a boundary, the winds gust³ down to the ground again, picking up the most speed on smooth landscapes with little resistance to wind.

 

 

 

　　为什么夜晚要比白天平静？

 

　　大多数情况下，大风刮了一天以后会在夜晚时平息下来。风去了哪里呢？为什么夜晚通常会比白天更平静呢？

 

　　平静的夜晚与多风的白天之间的差异就在于太阳的影响。白天的时候，太阳照射会使地球表面温度升高，进而会使大气温度也随之上升。但日落时，地球表面和表面 上方的大气温度比高层大气下降得更快。由此会产生一种稳定的气温状态，地面空气的密度大，温度较低，而地面上方的空气较为稀薄，温度较高。

 

　　这听起来有点不可思议，因为通常情况下，气温是随着高度的增加而降低的。但是随着冷空气停留在地表，地表上方几百英尺处的气温模式就会发生颠倒。这就是逆温现象，这种倒置的气温模式就像是屏障一样，使地表较冷而平静的空气从上层大气中分离出来。

 

　　在许多个平静的夜晚中，风依旧在我们头顶上方远远吹过，但逆温现象会阻止风吹向地面。

 

　　日出的时候，一切都变了。太阳的照射会使地球和低层大气温度升高，所有气流都混合在一起。没有了逆温层作为阻隔物，风又会吹向地面，在几乎不受阻力的情况下加速吹过平坦的地面。

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