The stability of the atmosphere depends on its ability to resist vertical motion. A stable atmosphere makes vertical movement difficult, and small vertical disturbances dampen out and disappear. In an unstable atmosphere, small vertical air movements tend to become larger, resulting in turbulent airflow and convective activity. Instability can lead to significant turbulence, extensive vertical clouds, and severe weather.
In meteorology, positive stability refers to an air parcel’s tendency to return to its original height after being disturbed. If air is forced upward, it will expand and cool adiabaticaly. If the parcel of air is colder than the air surrounding it, it will be more dense and sink back to its original position (TParcel < T Air).
An atmosphere that is neutrally stable will consists of air that, if disturbed and forced upward will remain at its new height.
Unstable air is that in which considerable vertical motion occurs. An unstable parcel of air will cool adiabatically as it rises. If however, it remains warmer than the surrounding air, it will subsequently be less dense and continue to rise. (TParcel > T Air)