UniMet

An unstable atmosphere occurs when the environmental lapse rate is greater than the DALR. For example, in the figure below the environmental lapse rate is 11 degrees per 1000m, and the DALR is as normal, 10 degrees per 1000m. A rise parcel of unsaturated air will follow th DALR. As the DALR is less than the environmental lapse rate the parcel will be warmer than he surrounding air and so will continue to rise, moving constantly further away from its original position. This is an unstable atmopshere. It follows then that a parcel of saturated air cooling at the SALR will be even warmer than the air around it - also shown in the figure below.

 

For both cases, unsaturated and saturated air, the parcel continue to rise as they are warmer and less dense than the air around them. The warmer an air parcel is the greater upward force it will have and the more rapidly it will rise. 

 

Absolute instability occurs when the environmental lapse rate is greater than the DALR.