During the day, the sun's heat keeps the air near the surface warm and unsaturated by water vapor. On a cloudless night, the surface radiates heat and cools, eventually reaching a temperature when the air is saturated with water vapor and the water vapor condenses and accumulates in the dew. But if the sky is overcast, the surface cannot be cooled by radiating heat. Because clouds are heat-insulating material, radiation heat cannot be transmitted from the surface. As a result, the surface does not get cold and dew does not freeze.