Where Does Dew Come from?
Dew is moisture from the air that collects in drops on blades of grass. During the day, moisture from the earth evaporates into the warm air. But at night, the earth and the air near it cool. So do plants and other outdoor objects.
Cold air cannot hold as much moisture as warm air can. Some of the moisture in the air condenses into drops of water on anything near the ground that is cold. There may be dewdrops on flowers, bushes, and spider webs, too.
Dew does not form when the air doesn’t get cold enough to make the moisture collect in drops. Dew most often forms on evenings or nights when there are few clouds.
So you can see why there is some truth in the old saying: “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.”