Dew point
The dew point or dewpoint of a given parcel of air is the temperature to which the parcel must be cooled, keeping barometric pressure constant, for it to become saturated with water vapor. At this temperature condensation will begin if conditions are suitable (presence of a solid surface or other condensation nuclei; otherwise see supersaturation) — water vapor then condenses into liquid 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 dew point determines relative humidity. When the relative humidity is high, the dew point is closer to the current air tempertaure. 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.