The temperature at which air becomes saturated with water vapor is called the dew point.
If air cools to its dew point, its relative humidity reaches 100 percent. Further cooling can cause water vapor to condense into dew, fog, cloud droplets, or frost, depending on the surface temperature and the presence of ice-forming conditions.
Dew point is a useful measure of atmospheric moisture because it does not change merely because air temperature changes. A hot afternoon can have the same dew point as a cooler morning, even though its relative humidity is different. Higher dew points generally mean more moisture is available for clouds and precipitation.
Dew point is often confused with relative humidity. Relative humidity is a percentage that depends on both moisture and temperature; dew point is a temperature measurement that directly indicates the amount of water vapor in the air.