托福閱讀資料之Is Dew In The Morning A Good Way To Predict The Weather?

　　"When the dew is on the grass, rain will never come to pass; When grass is dry at morning light, look for rain before the night.”

　　What’s the science behind this old aphorism? Is the amount of dew in the morning a good way of predicting the weather for the rest of the day?

　　As a matter of fact, this saying is roughly accurate, at least in the spring and fall, but the reason it works is more complicated than we might initially think.

　　We might imagine that the saying works because on a dewy morning, much of the atmosphere’s moisture has already condensed out as dew, so there isn’t a lot left over for a rain storm. This isn’t the case.

　　The amount of water in dew is usually much less than what would fall in a storm, and besides, that dew is likely to evaporate back into the atmosphere as the morning progresses.

　　As it turns out, the formation of dew is linked to the amount of cloudiness in the sky at night.

　　On a clear night the ground cools, radiating its heat away into space. When the ground gets cool enough, dew forms, like beads of condensation on a can of cold soda.

　　If the sky is cloudy at night, however, the Earth’s surface doesn’t cool as much. Some of the heat radiates into space, but much of it bounces off the cloud layer and goes back into the ground. If there are lots of clouds, the ground won’t get cool enough to form dew. The saying works because, chances are, all those nighttime clouds might also cause a rainstorm during the coming day.

　　“如果早晨青草上有露珠，當天就不會下雨;如果青草上沒有露珠，傍晚前就會下雨。”

　　這句古老的格言背后有什么樣的科學依據(jù)呢?朝露的數(shù)量是一種很好的方式來預測當天天氣情況嗎?

　　事實上，這句話大致是準確的。至少在春季和秋季是準確的。但是這句話成立的原因要比我們想象的復雜得多。

　　我們也許會這樣想，早晨的露珠已經(jīng)把空氣中大量的水分凝結成露水，所以沒有足夠的水分產(chǎn)生暴雨。然而情況并非如此。

　　露水中含的水分量遠少于暴雨所降的雨量。另外，在早晨這段時間里，露水可能會蒸發(fā)回到大氣中去。

　　事實證明，露水的形成是與晚上天空中云的數(shù)量有聯(lián)系的。

　　在晴朗的夜晚地面冷卻后，將熱量散發(fā)到空氣中。當?shù)孛胬鋮s充分后露水就形成了，就像裝著冰蘇打水瓶罐表面形成的水珠一樣。

　　如果夜晚天空多云，可是，地球表面冷卻不充分。一些熱量就會散發(fā)到空氣中，但大部分的熱量會彈出云層，回到地面。如果有大量云層，地面冷卻不充分，就不會形成露水。這句格言成立是因為可能所有這些夜間的云也許會在即將到來的一天制造一場暴雨。