Dew appears overnight when cool surfaces pull moisture out of the surrounding air.

The longer version

Overnight, the ground loses heat by radiating it away into a clear sky, and surfaces such as grass and rooftops cool faster than the air around them. When a surface drops to the dew point, the air touching it can no longer hold all its water vapor, and the excess condenses as droplets.

Dew forms most readily on clear, calm nights, since cloud cover sends heat back down and wind keeps mixing warmer air toward the surface. Objects that radiate heat well and face the open sky collect the most, which is why sheltered spots often stay dry.

Where this entry sits

Clouds, wind, rain, and storms shape daily life in ways that most people barely notice. This collection explains snow, lightning, rainbows, the turning seasons, temperature, the instruments forecasters rely on, and scraps of old weather lore. The full list sits on the Weather and Climate page, where every entry is listed in order.