Moist Adiabatic Process

The amount of water vapor a sample of air can hold is dependent on the temperature and pressure of the sample. As a parcel of air cools, its relative humidity increases, provided no moisture is either added or removed from the parcel. In general, as a sample cools, its capacity to hold water decreases. When the air reaches a point of saturation, condensation begins. As the water vapor condenses it goes from a higher energy state to a lower one, and as a result, latent heat is released into the air. In an effort to simplify the process, meteorologists assume that an air parcel is bounded by an imaginary balloon-like skin that contains the parcel and does not allow any mixing with its surrounding environment. Therefore, any latent heat released by condensation is contained in the parcel and is utilized to heat only the sample parcel. So, for the case of a rising, expanding, saturated parcel of air, the only heat added to this theoretically closed system is the heat generated within the parcel itself. The energy of the parcel is said to be conserved because the latent heat is a conversion from existing energy and not energy that was added to the system.

If a rising saturated parcel suddenly changes directions and starts to sink (compression), then evaporation of the condensation products will consume the latent heat to re-form water vapor. Since no heat is exchanged between the parcel and the environment, we still refer to this heating and cooling as an adiabatic process. However, during the processes of condensation and evaporation, the cooling and heating of the saturated parcel varies somewhat from the purely dry adiabatic process we discussed above. A rising saturated parcel cools at a slower rate due to the release of latent heat, and a sinking saturated parcel heats more slowly due to the conversion of heat energy during evaporation. This cooling and heating of a rising and sinking saturated parcel is called a moist or saturated adiabatic process.