Dew Point, the temperature at which water vapor in air will begin to condense—that is, change into a liquid or solid. The condensation occurs because as air is cooled the amount of water vapor it can hold decreases. The dew point varies with the amount of water vapor in the air. In general, the less water vapor there is in the air, the lower is the air's dew point. Air that is cooled to its dew point holds all the water vapor it can and is said to be saturated. Further cooling causes some of the water vapor to condense.

When the condensation occurs in the air itself, the water forms clouds, which may, in turn, produce rain, snow, or other forms of precipitation. If the air is cooled by contact with an object on the earth's surface, the vapor condenses on the object as dew or frost.