Air can hold only a limited amount of water vapor at any temperature. The dew point is the temperature to which that air must be cooled, at constant pressure, for saturation to occur. Once the air temperature reaches the dew point, condensation begins.

A high dew point means the air is already moist; a low dew point means it is dry. Dew points above 20 °C feel muggy to most people. Values near or below freezing indicate dry air that can produce frost when temperatures drop further.

Forecasters use dew point rather than relative humidity when describing moisture because it does not change with temperature alone. Rising dew points often signal increasing rain potential. Aircraft icing forecasts and fog predictions also rely on dew-point information.

• Temperature of saturation for given moisture
• Direct measure of atmospheric water vapor
• High values feel humid
• Key for fog, dew, and precipitation forecasts