"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，当天就不会下雨;如果青草上没有露珠，傍晚前就会下雨。”

　　这句古老的格言背后有什么样的科学依据呢?朝露的数量是一种很好的方式来预测当天天气情况吗?

　　事实上，这句话大致是准确的。至少在春季和秋季是准确的。但是这句话成立的原因要比我们想象的复杂得多。

　　我们也许会这样想，早晨的露珠已经把空气中大量的水分凝结成露水，所以没有足够的水分产生暴雨。然而情况并非如此。

　　露水中含的水分量远少于暴雨所降的雨量。另外，在早晨这段时间里，露水可能会蒸发回到大气中去。

　　事实证明，露水的形成是与晚上天空中云的数量有联系的。

　　在晴朗的夜晚地面冷却后，将热量散发到空气中。当地面冷却充分后露水就形成了，就像装着冰苏打水瓶罐表面形成的水珠一样。

　　如果夜晚天空多云，可是，地球表面冷却不充分。一些热量就会散发到空气中，但大部分的热量会弹出云层，回到地面。如果有大量云层，地面冷却不充分，就不会形成露水。这句格言成立是因为可能所有这些夜间的云也许会在即将到来的一天制造一场暴雨。