Our previous discussion of instability conditions was related to a parcel of air ascending or descending through a layer of air that was at rest along the vertical. But sometimes there are situations when a layer of air, several hundreds of meters thick and thousands of kilometers in area extent, is forced to rise.
Such an extensive and thick layer of air may be moist in its lower part and dry in the upper part. Thus, an anomaly is produced in the vertical distribution of humidity. Such a type of uplift causes the air, which may be initially stable, to become unstable from the adiabatic effect on internal temperature-humidity conditions.
It presents a graphical illustration of the state of convective (or potential) instability. There is a deep layer of air in which the two points A and B represent the temperatures at particular elevations. The lapse rate is indicated by the sloping line AB.
Since air at A is more humid than that at B, after being displaced it reaches the condensation level sooner than that at B. The bottom of the layer cools at the dry-adiabatic lapse rate from 20°C at point A to 16.7°C at 950 mb level where it becomes saturated.
Further cooling carries the bottom of the layer from the condensation level to A along its seudoadiabat. It arrives at 700 mb at a temperature of 3.7°C. On the other hand, the top of the layer cools at the dry- adiabatic lapse rate from 17.5°C at 900 mb to 0"C at its condensation level at about 730 mb.
From this point upward the saturated top layer cools at the retarded adiabatic rate. It arrives at 600 mb level at a temperature of 8.3°C. Thus, after uplift the layer having the initial lapse rate AB has the new lapse rate A'B' which is steeper than before.
Hence, it may be concluded that, in general, saturation lifting decreases the stability even more than does the unsaturated lifting. The mechanism of convective instability is significant in the dynamics of weather conditions.
When convectively or conditionally unstable air masses, in which the surface layers are humid and the moisture content decreases with height, are lifted, there is heavy precipitation from cumulonimbus type of clouds.
Besides, convective instability in the layers of the atmosphere is a contributory factor in the formation of violent storms like tornadoes, thunderstorms and squalls etc.
Ross by made an extensive study of the various aspects of convective instability. According to him, this type of instability is produced in such layers of the atmosphere in which the equivalent potential temperature decreases with increasing altitude.