8.18 Atmospheric Stability

8.18.2 Define:

(a) stable air;

A parcel of air that, when disturbed will, attempt to return to its original position (it does not have to succeed).

(b) unstable air;

A parcel of air that, when disturbed, will move further away from its original position.

(c) conditionally unstable air.

A parcel of air that, when disturbed, will remain at its new position.

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.

Stability or instability

There are two main factors that determine whether air will be stable or unstable:

• the temperature lapse rate of the ambient air.
• the adiabatic change in temperature of a vertically moving parcel of air.

Adiabatic process and temperature change

The process whereby air changes temperature by moving vertically up and down is known as the adiabatic process. The air temperature changes adiabatically because of the change in pressure. This resulting expansion process of rising air requires energy, which is taken away from the parcel of air.