英語聽力文摘581 為何夜晚比白天平靜？

Why Are Nights More Calm Compared To The Day?

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 dense, 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 inversion, 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 gust down to the ground again, picking up the most speed on smooth landscapes with little resistance to wind.