Dew point temperature indicates the temperature at which water will begin to condense out of moist air. When air is cooled, the relative humidity increases until saturation is reached and condensation occurs. Condensation occurs on surfaces which are at or below the dew point temperature.
Dew point is represented along the 100% relative humidity line on the psychrometric chart. Dew point temperature is determined by moving from a state point horizontally to the left along lines of constant humidity ratio until the upper, curved, saturation temperature boundary is reached. At dew point, dry bulb temperature and wet bulb temperature are exactly the same.
The dew point is closely related to the night time low temperature on still nights. When the air temperature drops to the dew point, energy is added back to the air as frost or dew forms and the temperature stabilizes at the dew point temperature. The dew point temperature is directly related to the actual quantity of moisture in the air and does not change much throughout a day unless a weather front moves through an area and adds or removes large amounts moisture. So the dew point temperature measured during daytime hours can be used as an estimate of the nighttime low temperature.
