Atmospheric Stability

(a) stable air;

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

• Stratiform cloud and smooth flying conditions

(b) unstable air;

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

• Cumuliform cloud and bumpy flying conditions

(c) conditionally unstable air.

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

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.  
  • Environmental Lapse Rate (ELR)
• the adiabatic change in temperature of a vertically moving parcel of air.
  • Adiabatic Lapse Rate (ALR)
    • Dry (DALR)
    • Saturated (SALR)
     
   

Temperature Lapse Rate 

A temperature lapse rate, is the rate of decrease of temperature with altitude.

• if the lapse rate is positive, the temperature decreases with height;
• if negative, the temperature increases with height (inversion).