Equilibrium of Saturated Air

If the parcel at point A were displaced upward to point B, it would cool at the dry adiabatic lapse rate; and upon arriving at point B, it would be colder than the surrounding air. It would, therefore, have a tendency to return to its original level. Consequently, the column of air becomes stable. From this, the rule is established that if the actual lapse rate of a column of DRY AIR is less than the dry adiabatic lapse rate, the column is stable. NEUTRAL STABILITY.—Consider a column of DRY AIR in which the actual lapse rate is equal to the dry adiabatic lapse rate. The parcel cools at the dry adiabatic lapse rate if displaced upward. It would at all time be at the same temperature and density as the surrounding air. It also has a tendency neither to return to nor to move farther away from its original position. Therefore, the column of dry air is in a state of NEUTRAL STABILITY. Equilibrium of Saturated Air When saturated air is lifted, it cools at a rate different from that of dry air. This is due to release of the latent heat of condensation, which is absorbed by the air. The rate of cooling of moist air is known as the saturation adiabatic lapse rate. This rate is used as a reference for determining the equilibrium of saturated air. ABSOLUTE STABILITY.—Consider a column of air in which the actual lapse rate is less than the saturation adiabatic lapse rate. The actual lapse rate is to the right of the saturation adiabatic lapse rate on the Skew T diagram (fig. 2-12). If the parcel of saturated air at point A is displaced upward to point B, it cools at the saturation adiabatic lapse rate. The air upon arriving at point B becomes colder than the surrounding air. The layer, therefore, would be in a state of ABSOLUTE STABILITY. From this, the following rule is established: If the actual lapse rate for a column of air is less than the saturation adiabatic lapse rate, the column is absolutely stable and the parcel would return to its original position. Dry air cools dry adiabatically and is also colder than the surrounding air. Therefore, this rule applies to all air, as is evidenced when an unsaturated parcel of air is displaced upward dry adiabatically to point B. Here, the parcel is more stable than the parcel displaced along a saturation adiabat. INSTABILITY.—Consider now a column of air in which the actual lapse rate is greater than the saturation adiabatic lapse rate (fig. 2-13). If a parcel of moist air at point A is displaced upward to point B, it cools at the 2-18 AG5f0212 -10 0 10 B B SATURATION ADIABATIC LAPSE RATE DRY ADIABATIC LAPSE RATE ACTUAL LAPSE RATE 1 POINTS B (DRY ADIABATIC) AND B (MOIST ADIABATIC) WARMER THAN SURROUNDING AIR 1 A Figure 2-12.—Absolute stability (any degree of saturation).