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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 col-umn of air in which the actual lapse rate is less than the saturation adiabatic lapse rate. The ac-tual lapse rate is to the right of the saturation adiabatic lapse rate on the Skew T diagram (fig. 2-4-7). 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 arriv-ing at point B becomes colder than the surround-ing 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 satura-tion adiabatic lapse rate, the column is absolutely



Figure 2-4-5.—Absolute instability (any degree of saturation).



Figure 2-4-6.—Stability (dry air).


Figure 2-4-7.—Absolute stability (any degree of saturation).

 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 satura-tion adiabat. 

INSTABILITY.— Consider now a column of air in which the actual lapse rate is greater than the saturation adiabatic lapse rate (fig. 2-4-8). If a parcel of moist air at point A is displaced up-ward to point B, it cools at the saturation adiabatic lapse rate. Upon arriving at point B the parcel is then warmer than the surrounding air. For this reason, it has a tendency to continue moving farther from its original position. The parcel, therefore, is in a state of INSTABILITY.

The following rule is applicable. If the actual lapse rate for a column of SATURATED (MOIST) AIR is greater than the saturation adiabatic lapse rate, the column is unstable.

NEUTRAL STABILITY.— Consider a col-umn of saturated air in which the actual lapse rate is equal to the saturation adiabatic lapse rate. A parcel of air displaced upward cools at the satura-tion adiabatic lapse rate and is at all times equal in temperature to the surrounding air. On that account, it tends neither to move farther away from nor to return to its original level. Therefore, it is in a state of NEUTRAL STABILITY. The rule for this situation is that if the actual lapse rate for a column of saturated air is equal to the saturation adiabatic lapse rate, the column is neutrally stable.

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