Pseudoadiabatic Process

We have covered the cases of dry air, unsaturated moist air, and saturated air, so what could possibly be left? Well, we all know from personal experience that if condensation continues long enough we get rain. We will see in a later reading that the process of forming a rain drop is more complex than just simple condensation. But for our purposes here, the point is that all that condensing moisture does not remain in the cloud just to be evaporated at some later time. So, we must address the case where the moisture precipitates out of the cloud.

If you remember our earlier discussion of temperature, all substances are composed of molecules in motion which have kinetic energy. As rain, snow, or any other form of precipitation falls out of a cloud, it carries with it that energy it possesses. With this true loss of energy, the process is no longer adiabatic and, therefore, is called pseudoadiabatic. Fortunately, the amount of energy lost through precipitation is very small compared to the energy of the air molecules. The pseudoadiabatic lapse rate is so close to the moist adiabatic lapse rate that meteorologists tend to ignore this difference and often refer to them synonymously. In fact, on adiabatic charts (or pseudoadiabatic charts), the moist adiabatic lapse rate lines are referred to interchangeably as moist, saturated, or pseudoadiabatic.