Conditional Instability

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. Conditional Instability In the treatment of stability and instability so far, only air that was either dry or saturated was considered. Under normal atmospheric conditions natural air is unsaturated to begin with, but becomes saturated if lifted high enough. This presents no problem if the actual lapse rate for the column of air is greater than the dry adiabatic lapse rate (absolutely unstable) or if the actual lapse rate is less than the saturation adiabatic lapse rate (absolutely stable). However, if the lapse rate for a column of natural air lies between the dry adiabatic lapse rate and the saturation adiabatic lapse rate, the air may be stable or unstable, depending upon the distribution of moisture. When the actual lapse rate of a column of air lies between the saturation adiabatic lapse rate and the dry adiabatic lapse rate, the equilibrium is termed CONDITIONAL INSTABILITY, because the stability is conditioned by the moisture distribution. The equilibrium of this column of air is determined by the use of positive and negative energy areas as analyzed on a Skew-T, Log P diagram. The determination of an area as positive or negative depends upon whether the parcel is being lifted mechanically (by a front or orographic barriers) or by convective means and whether the environment is colder or warmer than the ascending parcel. Positive areas are conducive to instability. Negative areas are conducive to stability. Conditional instability may be one of three types. The REAL LATENT type is a condition in which the positive area is larger than the negative area (potentially unstable). The PSEUDOLATENT type is a condition in which the positive area is smaller than the negative area (potentially STABLE). The STABLE type is a condition in which there is no positive area. Figure 2-19 AG5f0213 -10 0 10 B SATURATION ADIABATIC LAPSE RATE ACTUAL LAPSE RATE POINT B WARMER THAN THE SURROUNDING AIR A Figure 2-13.—Instability (saturated air).