Dew Point

From Pilots Almanac

Dew point is the temperature (in degrees) to which air must be cooled in order to be saturated with water vapor already in the air. When the dew point temperature and air temperature are equal, the air is said to be saturated. Dew point temperature is NEVER GREATER than the air temperature.

Weather reports for pilots usually include the dew point as well as the temperature. When the two are compared, the difference reveals how close the air is to being 100% saturated. This difference is called the temperature-dew point spread. A close spread will indicate poor visibility. Since the temperature and dew point are calculated at the surface, this would suggest the likelihood of fog formation.

As air is cooled toward its dew point, it becomes more saturated even though the gross amount of water vapor is unchanged. Therefore, if the air continues to cool, moisture must be removed from the air and this is accomplished through condensation. This process results in the formation of tiny water droplets that can lead to the development of fog, frost, clouds, or even precipitation.