

Click here to Order your Radar Equipment Online 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. 247). 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
INSTABILITY.— Consider now a column of air in which the actual lapse rate is greater than the saturation adiabatic lapse rate (fig. 248). If a parcel of moist air at point A is displaced upward 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 column 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 saturation 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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