Why is the grass wet in the morning?
On clear, calm nights the ground and grass cool faster than the air. When they drop to the air's dew point, water condenses on them as dew. If that happens below freezing, it forms frost instead, sometimes even when the thermometer says 36°F.
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Where the water comes from
It doesn't rain down. The water was in the air as vapor all along. Grass blades radiate heat to the clear sky and get colder than the air around them. Once they reach the dew point, the air touching them can't hold all its water, and droplets form on the surface, the same way a cold drink sweats on a summer day.
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Frost on a 36°F morning
Official temperatures are measured about five to six feet above the ground. On a still, clear night the ground and car roofs can be several degrees colder than that, so they drop below freezing while the thermometer still reads in the mid-30s. Water vapor then turns straight to ice crystals on them: frost.
Why some nights stay dry
Clouds act like a blanket and keep the ground from cooling enough. Wind keeps mixing slightly warmer air down to the ground. And very dry air has a dew point so low the grass never reaches it. Dew needs clear, calm and fairly moist all at once.
Questions people ask
- Why does frost form on car roofs but not under trees?
- Open surfaces see the cold, clear sky and lose heat fast. Trees, carports and clouds block that heat loss.
- Is dew a sign of rain?
- No, the opposite: heavy dew usually means a clear, calm night, often under high pressure.