Conditional Instability

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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,

Figure 2-4-8.—Instability (saturated air).

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.

TYPES OF CONDITIONAL INSTABILITY.— Conditional instability may be one of three types. The REAL LATENT type is a con-dition 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.

NOTE: The computing of positive and nega- tive energy areas and Skew-T analysis is covered in detail in AG2, Vol. 2, Unit 3. Figure 2-4-9

Figure 2-4-9.—Example of positive and negative energy areas (mechanical lifting).

shows an example of analyzed positive and negative energy areas as they would appear on a Skew-T, Log P diagram.

Autoconvection

AUTOCONVECTION is a condition started spontaneously by a layer of air when the lapse rate of temperature is such that density increases with elevation. For density to increase with altitude, the lapse rate must be equal to or exceed 3.42°C per 100 meters. (This is the AUTOCONVECTIVE LAPSE RATE.) An example of this condition is found to exist near the surface of the earth in a road mirage or a dust devil. These conditions occur over surfaces that are easily heated, such as the desert, open fields, etc.; they are usually found during periods of intense sur-face heating.

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