The dew point or dewpoint of a given parcel of air is the temperature to which the parcel must be cooled, at constant barometric pressure, for the water vapor component to condense into water, called dew. When the dew point temperature falls below freezing it is called the frost point, instead creating frost or hoar frost by deposition.

The following graph shows the maximum percentage of water vapor that can exist in air at sea level across a range of temperatures. Note that with higher temperatues the equilibrium partial pressure of water vapor increases thus more water evaporates. The behavior of water vapor does not depend on the presence of air. The formation of dew would occur at the dew point even if the only gas present was water vapor.

The dew point determines relative humidity. When the relative humidity is high, the dew point is closer to the current air temperature. If the relative humidity is 100%, the dew point will be equal to the current temperature. As relative humidity falls, the dew point becomes lower, given the same air temperature.