Moist Adiabatic Lapse Rate

MOIST ADIABATIC LAPSE RATE

Of course, the dry adiabatic lapse rate does not work if the air is moist. Remember the saturation level of any air mass depends on its temperature and its moisture content. Lower temperature air masses have less moisture saturation. What happens then is that an air parcel laden with moisture that rises and cools will not be able to hold that moisture. Under the right conditions, a cloud will form.

Remember too that condensation draws energy from the area because it takes energy to condense something. The latent heat of condensation is approximately 2260 kilojoules per kilogram or with respect to water, about 40.8 kilojoules per mole. This means that some of the adiabatic cooling you get when its air mass expands is offset and doesn't cool as much as you would expect. This slower rate of temperature loss is called the moist or wet adiabatic lapse rate. It is only 4.5 degrees per kilometer because condensation adds heat to the air mass. Figure 16 shows you what this looks like:

Figure 16.