"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.

Why It's Slightly True

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.

Dew And Cloudiness

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.