Definition

The temperature to which a given parcel of humid air must be cooled, at constant atmospheric pressure, for water vapor to be deposited on a surface as ice crystals without undergoing the liquid phase. (The gas to solid process is called deposition as opposed to the gas to liquid process know as condensation.) The temperature must be below freezing and the air must have cooled enough to become saturated, given that the relative humidity of the air will increase as the temperature falls until it reaches 100%.