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"When the dew is on the grass, rain will never come to pass;When grass is dry at morning light, look for rain before thenight.”
“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。”
What's the science behind this old aphorism?
這句古老的格言背后有什么樣的科學依據呢？
Is the amount ofdew in the morning a good way of predicting the weather forthe rest of the day?
朝露的數量是一種很好的方式來預測當天天氣情況嗎？
As a matter of fact, this saying is roughly accurate, at least in the spring and fall, but the reason itworks is more complicated than we might initially think.
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。
We might imagine that the saying works because on a dewy morning, much of the atmosphere'smoisture has already condensed out as dew, so there isn't a lot left over for a rain storm.
我們也許會這樣想，早晨的露珠已經把空氣中大量的水分凝結成露水，所以沒有足夠的水分產生暴雨。
This isn't the case.
然而情況并非如此。
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.
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，露水可能會蒸發回到大氣中去。
As it turns out, the formation of dew is linked to the amount of cloudiness in the sky at night.
事實證明，露水的形成是與晚上天空中云的數量有聯系的。
On a clear night the ground cools, radiating its heat away into space.
在晴朗的夜晚地面冷卻后，將熱量散發到空氣中。
When the ground gets cool enough, dew forms, like beads of condensation on a can of cold soda.
當地面冷卻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。
If the sky is cloudy at night, however, the Earth's surface doesn't cool as much.
如果夜晚天空多云，可是，地球表面冷卻不充分。
Some of the heatradiates into space, but much of it bounces off the cloud layer and goes back into the ground.
一些熱量就會散發到空氣中，但大部分的熱量會彈出云層，回到地面。
If there are lots of clouds, the ground won't get cool enough to form dew.
如果有大量云層，地面冷卻不充分，就不會形成露水。
The saying worksbecause, chances are, all those nighttime clouds might also cause a rainstorm during the comingday.
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。
