Lapse Rate

Stability is determined by comparing the temperature of a rising or sinking air parcel to the environmental air temperature. Imagine the following, at some initial time, an air parcel has the same temperature and pressure as its environment. If you lift the air parcel some distance, its temperature drops due to expansion. If the air parcel is colder than the environment in its new position, it will have a higher density and sink back to its original position. In this case, the air is stable because vertical motion is resisted. If the rising air is warmer and less dense than the surrounding air, it will continue to rise until it reaches some new equilibrium where its temperature matches the environmental temperature. In this case, because an initial change is amplified, the air parcel is unstable. In order to determine if the air parcel is unstable or not the temperature of both the rising air the surrounding environment must be known.

Dry Stability

As long as an air parcel is unsaturated, the rate at which its temperature will change will be constant (3°C / 1,000 feet).

If an air parcel is dry, meaning unsaturated, stability is relatively straightforward. An atmosphere where the environmental lapse rate is the same as the dry adiabatic lapse rate, meaning that the temperature in the environment also drops by 3°C / 1,000 feet, will be considered neutrally stable. After some initial vertical displacement, the temperature of the air parcel will always be the same as the environment so no further change in position is expected.

If the environmental lapse rate is less than the dry adiabatic lapse rate, some initial upward displacement of the air parcel will result in the air parcel being colder and denser than the surrounding environment.

Finally, if the environmental lapse rate is greater than the dry adiabatic lapse rate, an upward displacement of the air parcel will result in the air parcel either being warmer and less dense than the surrounding environment. This is an unstable situation for a dry air parcel.

In general for a dry air parcel, the following is true:

Neutral stability exists when an air parcel’s temperature is equal to that of the surrounding environment (T_Parcel = T_Environment)
Stability exists when an air parcel’s temperature is less than that of the surrounding environment (T_Parcel < T_Environment)
Instability exists when an air parcel’s temperature is greater than that of the surrounding environment (T_Parcel > T_Environment)

Moist Stability

The moist adiabatic lapse rate varies somewhat but an average of 1.5°C/1,000 feet is used as an approximation

As an air parcel is lifted, its temperature initially drops according to the dry adiabatic lapse rate. At some point however, the air may become saturated and water vapour will condense to liquid water and form a cloud. When water vapour condenses, it goes from a higher energy state to a lower energy state and energy is released in the form of latent heat. This has large consequences for the lapse rate of an air parcel and is the reason for the difference between the dry adiabatic lapse rate and the moist adiabatic lapse rate.

As latent heat is added from the process of condensation, it offsets some of the adiabatic cooling from expansion. Therefore, the air parcel will no longer cool at the dry adiabatic lapse rate, but will cool at a slower rate, known as the moist adiabatic lapse rate. To summarize, a parcel will cool at the dry adiabatic rate until it is saturated, after which it won’t cool as quickly due to the release of latent heat during condensation. The moist adiabatic lapse rate varies somewhat but an average of 1.5°C/1,000 feet is used as an approximation.

Determining atmospheric stability is slightly complicated with the introduction of the moist adiabatic lapsre rate, however, the underlying principle is the same – we must compare the lapse rate of the air parcel with the surrounding environmental lapse rate.

The atmosphere is said to be absolutely stable if the environmental lapse rate is less than the moist adiabatic lapse rate. This means that a rising air parcel will always cool at a faster rate than the environment, even after it reaches saturation. If an air parcel is cooler at all levels, then it will not be able to rise, even after it becomes saturated.
The atmosphere is said to be absolutely unstable if the environmental lapse rate is greater than the dry adiabatic lapse rate. This means that a rising air parcel will always cool at a slower rate than the environment, even when it is unsaturated. This means that it will be warmer (and less dense) than the environment, and allowed to rise.
The atmosphere is said to be conditionally unstable if the environmental lapse rate is between the moist and dry adiabatic lapse rates. This means that the buoyancy (the ability of an air parcel to rise) of an air parcel depends on whether or not it is saturated. In a conditionally unstable atmosphere, an air parcel will resist vertical motion when it is unsaturated, because it will cool at the dry adiabatic lapse rate. However, if it is forced to rise and becomes saturated, it will cool at the moist adiabatic lapse rate. In this case, it will cool at a slower rate than the surrounding environment and rise.