The dew point is the temperature to which humid air must cool for water vapor to begin condensing at constant pressure.
At the dew point, air becomes saturated: its relative humidity reaches 100 percent. If cooling continues, excess water vapor can condense into dew on surfaces or form cloud droplets around particles suspended in the air.
Dew point is a measure of the actual amount of moisture in the air, unlike relative humidity, which also depends strongly on temperature. Warm air can have a high moisture content but a moderate relative humidity, while cooler air with the same water vapor can have a much higher relative humidity.
When the dew point is below freezing, meteorologists may refer to the frost point because ice can form directly on surfaces. Dew point is also useful in forecasting fog, estimating atmospheric discomfort, and assessing the likelihood of overnight condensation.