When in an ascending air mass the process of consideration starts after the temperature has come down to saturation point, the latent heat of condensation is released leading to warming of the rising air.
Since latent heat is returned to the ascending parcel of air during the process of condensation, the rate of cooling of rising air is reduced. Thus, the reduced rate of temperature change caused by the addition of latent heat of condensation is called the wet/moist adiabatic lapse rate.
The amount of latent head liberated by the process of condensation depends upon the moisture content of an ascending air mass. The rate of cooling in a saturated air mass is, therefore, not always the same.
The wet adiabatic lapse rate varies from 0.5″ to 0.9″ Celsius per 100 meters. Higher moisture content in a rising air lowers down the rate of temperature change, whereas in relatively drier air mass there is higher rate of temperature change.
It is to be noted that any volume of rising air first cools at the dry adiabatic lapse rate from the surface to the condensation level. Beyond this level the rate of the cooling is lowered because of the reasons mentioned above, and the same is known as the wet adiabatic lapse rate.
The figure 32.1 also shows the effect of varying temperatures at the height of condensation level. Further, it is also made clear that depending on the temperatures of the ascending air masses in the initial state, the wet adiabatic lapse rates may be steep, moderate or low.
Thus, higher the temperature of the rising air, slower will be the rate of its cooling from the condensation level upward.
Moreover, the wet adiabatic lapse rate depends on such other factors as the pressure of air, the amount of condensed moisture in the rising air, and its temperature.
However, beyond the condensation level there are two opposing forces that work together: (a) adiabatic cooling by expansion, and (b) warming of the rising air by the addition of latent heat of condensation. The net effect of these processes working simultaneously is to reduce the rate of decrease of temperature.
The wet adiabatic rate of cooling is highly variable and depends on the temperature of the rising air. It is so because the specific humidity of warmer air is far more than that of the colder air. Therefore at higher temperature the latent heat of condensation is greater in quantity.
So the rate of cooling beyond the condensation level is controlled by the latent heat liberated through the process of condensation.
The difference between dry and wet adiabatic lapse rates at higher temperature is much greater. On the contrary, when the temperature of the ascending air is low, the difference between the two adiabatic lapse rates is substantially reduced.
There are occasions when the air parcel ascending through layers of the atmosphere is so cold and dry that both the dry and wet adiabatic lapse rates are almost the same.
In exceptional cases when the rate of cooling in an ascending air mass is more than the dry adiabatic rate, it is called the super-adiabatic lapse rate.