A measure of absolute humidity. Consider a parcel of gas (typically air) including some water vapor, held under conditions of constant pressure. The dewpoint is the temperature at which the partial pressure of the water vapor is equal to the saturated vapor pressure of water at that temperature. At temperatures above the dewpoint, any dew will tend to evaporate and any clouds will tend to dissipate, whereas below the dewpoint clouds and dew can form, and are likely to form if suitable condensation nuclei are present.
The concept is related to relative humidity but is more useful to pilots because whereas the relative humidity changes radically as the temperature of the air changes, the dewpoint does not.
