The dew point is the temperature at which air becomes saturated with water vapor and condensation begins. It is a direct measure of atmospheric moisture content.
The dew point temperature is the temperature to which air must be cooled at constant pressure and constant water vapor content for saturation to occur, resulting in the formation of dew, fog, or cloud droplets. Unlike relative humidity, which varies with temperature, the dew point is an absolute measure of the moisture content of the air. When the air temperature equals the dew point, relative humidity is 100 percent and the air is saturated. Dew points above 20 degrees Celsius indicate very moist, tropical air, while dew points below 0 degrees Celsius indicate very dry air (in which case the frost point is used). The dew point is measured using chilled mirror hygrometers or calculated from temperature and relative humidity data. In meteorology, the dew point is critical for predicting precipitation, fog formation, and the height of cloud bases, which can be estimated from the temperature-dew point spread. In hydrology, dew point data are used in evapotranspiration calculations and energy balance models. The dew point depression (difference between air temperature and dew point) is a useful indicator of atmospheric moisture availability for precipitation.