5.10 and 5.11 in Ahrens. The partial pressure of water vapor is called the actual vapor pressure, which is a fairly good measure of the total amount of water vapor in the air. High actual vapor pressure indicates large numbers of water vapor molecules, whereas low actual vapor pressure indicates comparatively small numbers of vapor molecules. Saturation vapor pressure describes how much water vapor is necessary to make the air saturated at any given temperature. Recall, saturation occurs when the total number of evaporating water molecules balances the total number of condensing molecules.
Energy is necessary to drive the evaporation process.
Note also that the saturation vapor pressure over water is greater than the saturation vapor pressure over ice . RH is the ratio of the air s water vapor content to its capacity. RH can change by adding moisture to a parcel or by altering its temperature. The dew point temperature is related to fog and cloud formation.