Dew point is the temperature at which water vapor in the air begins to condense. The dew point is normally lower than the air temperature. When the air temperature and dew point are equal, the relative humidity is 100 percent. Dew forms when a thin film of air, in contact with the surface, is cooled to below the dew point. This cooling of the air causes dew on the surface or fog in the air, when the dew point is above the freezing temperature. If the air temperature and dew point are below freezing, frost may form on the surface, or ice crystals may form in the air. Fog and clouds occur when large volumes of air are cooled to below the dew point.
