The dew point is the temperature at which air becomes saturated and dew can form.
Air contains water vapour, and warmer air can generally hold more vapour than colder air. If moist air cools without losing water vapour, its relative humidity rises. When the air reaches its dew point, relative humidity is 100 percent, and additional cooling can cause water vapour to condense into liquid droplets on surfaces or into clouds in the atmosphere.
A high dew point indicates that the air contains substantial moisture and often feels humid. A low dew point indicates drier air, even if the air temperature itself is high. Dew point is not the same as relative humidity: relative humidity changes when temperature changes, while dew point more directly reflects the amount of moisture present. Fog, dew, frost, and cloud formation all depend on cooling toward saturation, although frost forms when surfaces are at or below freezing.