Atmospheric stability is defined as that condition in the atmosphere in which vertical motions are absent or definitely restricted; and, conversely, instability is defined as the state wherein vertical movement is prevalent.”
According to Trewartha, air is said to be stable, and consequently antagonistic to precipitation, if it is non-buoyant and resists vertical displacement.
Voluntary vertical motions are largely absent in stable air. On the other hand, if displacement results in buoyancy and a tendency for further movement away from the original position, the air is unstable”.
The stability of air is determined by the distribution of temperature in the atmosphere at various heights. This measure of the change of temperature is called the lapse rate which is altogether different form the adiabatic lapse rates.
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As we know, the lapse rates always vary with time and place. The dry-adiabatic lapse rate is always the same. By noting at any level the difference in temperature between an air parcel moving upward and the surrounding atmosphere, stability or instability can be ascertained.
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In other words, the environmental lapse rate prevailing in the atmosphere makes it stable or unstable. If the lapse rate exceeds the dry-adiabatic lapse rate, the air is bound to be in the state of unstable equilibrium, and it will tend to rise further.
On the other hand, if the lapse rate is lower than the dry-adiabatic lapse rate, there will be stability in the air. Such an air parcel, even if pushed up strongly, tends to return to its original position. Such a state of equilibrium resists vertical motions in the atmosphere.
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The interrelationship between atmospheric stability and lapse rates has been illustrated ill Figure 32.2. In the left hand side diagram the surface air is at a temperature of 35″C with a lapse rate of 6″C per kilometer.
Imagine that a parcel of air with 35°C temperature at the ground is forced upward as shown in the Figure. After the air has reached a height of 1 kilometer, its temperature has come down to 25°C, while the temperature of the surrounding air is about 29°C.
Obviously the ascending air is colder than the environment at the same level and must sink downward. This parcel of air would tend to come back to its original position unless some outside force is applied to it, because further ascent would cause it to become colder and heavier than the surrounding air.
The relationship between the actual lapse rate and the dry-adiabatic lapse rate is such as to resist vertical movement. Such air is said to be in stable equilibrium. It is to be noted that in this case the existing lapse rate is lower than the dry-adiabatic rate of cooling.
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The right hand side drawing is a diagrammatic representation of the state of unstable equilibrium. In this case the ascending air parcel at the height of 1 kilometer has cooled down to 25°C, while the temperature of the surrounding air at the same level is only about 24° C.
The rising air is warmer and lighter than the surrounding air. In such a situation, the rising air will continue to rise and expand.
Here the rate of cooling of the ascending air is lower than that of the surrounding air, because the lapse rate is higher than the dry-adiabatic lapse rate. Such an air is considered to be unstable. This case illustrates the behaviour of the atmosphere when unstable equilibrium conditions prevail.
Thus, to examine whether an air mass is stable or unstable a comparison should be made between its lapse rate and the dry-adiabatic rate of cooling.
There are occasions when the lapse rate in a certain layer of the atmosphere is found to be about 4.6°C per 1000 meters. Under this situation, when the lapse rate is less than the wet adiabatic rate, even at the point of condensation no vertical motions develop in the atmosphere. In this case the air is dead to be absolutely stable.
Temperature inversion is the typical example of absolute stability. The inversion layer present in the atmosphere acts as a lid to the ascending currents of air. Just beneath the base of the inversion layer the upward rising smoke is forced to spread out in horizontal plane.
In winter, it is a common sight at about sunset near human settlements where rising columns of smoke from domestic chimneys are not allowed to move upward beyond a certain level. This level is provided by the base of the inversion layer.