dew point

The temperature to which a volume of air must be cooled, at constant pressure and with no change in its water vapor content, to reach saturation. When a region of the atmosphere reaches the dew point, water droplets spontaneously begin to form, as a consequence of the condensation of the water vapor present in the air. At the level of the Earth's surface, that circumstance manifests itself with the appearance of dew, mist, fog, or - if the temperature is low enough - frost. The dew point can be calculated from air temperature and relative humidity values. In the mathematical equations of atmospheric thermodynamics it is usually expressed as T_d (the subscript refers to dew).