The temperature at which air becomes saturated when cooled without addition of moisture or change of pressure. Any further cooling causes condensation; fog and dew are formed in this way.

Frost point is the corresponding temperature of saturation with respect to ice. At temperatures below freezing, both frost point and dew point may be defined because water is often liquid (especially in clouds) at temperatures well below freezing; at freezing (more exactly, at the triple point, +.01°C) they are the same, but below freezing the frost point is higher. For example, if the dew point is −9°C, the frost point is −8°C. Both dew point and frost point are single-valued functions of vapor pressure. Dew Fog Humidity