The dew point is the temperature at which dew (condensation) forms and is a measure of atmospheric moisture. It is the temperature to which air must be cooled at constant pressure and water content to reach saturation. Dew points are expressed as a temperature. Higher dew points correlate to higher moisture content, also known as absolute humidity.

The dew point represents the lowest temperature to which air having specific values of temperature and relative humidity (RH) can be cooled. At the dew point, air has a relative humidity of 100%, and additional cooling produces condensation rather than lowering the air temperature.

The figure below shows a constant dewpoint line of 12 °C, illustrating that a dew point can be represented by many combinations of RH and temperature. This dew point corresponds to a well-controlled lab of 23 °C and 50% RH (yellow star), an environment of 12 °C and 100% RH (dew point equals temperature by definition when RH is 100%), and all other points on the line. A tester can only meet conditions with higher dew point (hotter, more humid) than ambient lab conditions. Lower dew point conditions (cooler, drier) can only be met with conditioned air.