It is also known as vapor density of water. It represents the maximum amount of water vapor that air can hold at observed temperature and pressure. For unsaturated air, relative humidity is inversely proportional to the temperature. The mean annual water vapor of the troposphere is about the same over the desert area of the southwestern United States as it is over the Great Lakes, however, the relative humidity is significantly different in the two locations. That is, during its ascent or descent within the atmosphere, an air parcel is neither cooled or heated by radiation, conduction, mixing, or phase changes of water.
P is employed.
If the air parcel is unsaturated, its relative humidity increases during expansional cooling and decreases during compressional warming. The range in moist adiabatic lapse rate depends on the amount of latent heat released during condensation or deposition. The latent heat is the heat released or needed during the phase change of water. It is primarily function of air temperature and also air pressure. It should be noted that the wind speed in heat index as well as humidity in wind chill index, both are not included, do have some effect on the perceived temperature to the human body.