8.18 Atmospheric Stability

8.18.2 Define:

(a) stable air;

Stable air is air that resists vertical movement.

If a parcel of air is disturbed upwards or downwards, it will tend to return towards its original level.

The parcel does not have to fully return to its original position, but the important point is that it resists further movement away from that position.

(b) unstable air;

Unstable air is air that encourages vertical movement.

If a parcel of air is disturbed upwards or downwards, it will continue to move further away from its original level.

A rising parcel will continue to rise if it remains warmer and less dense than the surrounding air.

(c) conditionally unstable air.

Conditionally unstable air is air that is stable when unsaturated, but unstable when saturated.

This occurs when the environmental lapse rate lies between the saturated adiabatic lapse rate and the dry adiabatic lapse rate.

In simple form:

SALR < ELR < DALR

This means:

• if the air is dry/unsaturated, it is stable
• if the air becomes saturated, it becomes unstable

8.18.4 Describe how the stability of a rising (or sinking) parcel of air is determined by its temperature compared with the temperature of the surrounding environment.