Stability

Ambient and adiabatic lapse rates are a measure of atmospheric stability. Figure 5.6.6 shows these stability conditions. The atmosphere is unstable as long as a parcel of air moving upward cools at a slower rate than the surrounding air and is accelerated upward by buoyancy force. Moving downward, the parcel cools slower and is accelerated downward. Under these conditions, vertical air motions and turbulence are enhanced.

Conversely, when a rising parcel of air is cooler than the surrounding air, the parcel settles back to its original elevation. Downward movement produces a warmer parcel, which rises to its original elevation. Under these conditions, vertical movement is dampened out by adiabatic cooling or warming, and the atmosphere is stable.

Figure 5.6.7 shows that the boundary line between stability and instability is the dry adiabatic lapse line. When the ambient lapse rate exceeds the adiabatic lapse rate, the ambient lapse rate is termed superadiabatic, and the atmosphere is highly unstable. When the two lapse rates are equal, the atmosphere is neutral. When the ambient lapse rate is less than the dry adiabatic lapse rate, the ambient lapse rate is termed subadiabatic, and the atmosphere is stable (Figure 5.6.8). If the air temperature is constant throughout a layer of atmosphere, the ambient lapse rate is zero, the atmosphere is described as isothermal, and the atmosphere is stable (Battan 1979).

When the temperature of ambient air increases (rather than decreases) with altitude, the lapse rate is negative, or inverted, from the normal state. A negative lapse rate occurs under conditions referred to as an inversion, a state in which warmer air blankets colder air. Thermal or temperature inversions represent a high degree of atmospheric

Midnight 6 a.m. Noon

Midnight 6 a.m. Noon

-Temperature -

FIG. 5.6.8 Typical ambient lapse rates during a sunny day and clear night.

-Temperature -

FIG. 5.6.8 Typical ambient lapse rates during a sunny day and clear night.

stability (Battan 1979). An inversion is an extreme suba-diabatic condition, thus almost no vertical air movement occurs.

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