Dew, a deposit of moisture that forms on the surface of a cool object. Dew is most commonly seen on leaves and grass in the early morning. The foggy beads of mist appearing on the outer surface of a glass of ice water are also dew.
Dew appears when an object is cooled to a temperature below the dew point of the air surrounding it. When the temperature falls below the freezing point of water, frost forms instead of dew. Dew usually forms in the evening, at night, or in the early morning because at these times objects on the earth are cooler than the air around them. During the day, the heat of the sun causes dew to evaporate.