Atmospheric Stability

Isothermal process is one that allows heat energy to enter or exit the system. Direct heating or cooling is an isothermal process.
Adiabatic process is one where no heat energy enters or exits the system. Heat is generated by transfer of energy and not by direct heating or cooling.
Warming or cooling of atmospheric air takes place by expansion and contraction of air by lowering and lifting of an air parcel and is hence an adiabatic process.
Air is a bad conductor of heat and hence a certain air parcel can be considered to be insulated from the surrounding air.

Adiabatic cooling refers to the process by which a parcel of air cools down when it is lifted to higher levels.
Since the pressure at higher levels is lower, the volume of the air parcel increases, resulting in adiabatic cooling.
Adiabatic warming refers to the process by which a parcel of air warms up when it is brought down to lower levels.
Since the pressure at lower levels is higher, the volume of the air parcel reduces, resulting in adiabatic warming.

Air is said to be saturated when it holds the maximum amount of water vapour that it can hold at a particular temperature.
Cool air has a lesser capability to hold water vapour compared to warm air.
If a warm, unsaturated parcel of air is forced to climb to higher levels, it cools due to adiabatic cooling and becomes saturated.
Saturation results in condensation of water vapour into water droplets visible as clouds.
Saturation releases latent heat and slows down the temperature lapse rate.

Adiabatic lapse rate refers to the rate of change of temperature with height.
As per ISA conditions, the average lapse rate is taken as 2°C per 1,000 ft or 6.5°C per km.
However, the temperature lapse rate varies with moisture content.
Dry Adiabatic Lapse Rate (DALR) refers to the lapse rate of dry air or air where the relative humidity is less than 100%.
DALR is constant at 3°C per 1,000 ft or 9.8°C per km.

Saturated Adiabatic Lapse Rate (SALR) refers to the lapse rate of saturated air or air where the relative humidity is equal to 100%.
The rate of fall of temperature is lower in saturated air compared to dry air.
The lapse rate of saturated air is not constant and varies with altitude.
SALR varies between 1.2°C per 1,000 ft and 2.2°C per 1,000 ft.
The average value of SALR is taken as 1.8°C per 1,000 ft or 5°C per km.

Environmental Lapse Rate (ELR) refers to the actual or prevailing lapse rate at a particular place and time.
ELR varies depending on atmospheric conditions such as moisture content.
Atmospheric stability is governed by the value of ELR in comparison with DALR and SALR.
A Tephigram is a thermodynamic diagram on which temperatures at various levels are plotted and compared with DALR and SALR.
Tephigram is used to calculate atmospheric stability.

Atmospheric stability is a measure of whether a parcel of air that is pushed upward will continue rising or descend back to its original position.
If the Environmental Lapse Rate (ELR) is very low, or temperature decreases very slowly with height, the atmosphere is said to be stable.
Absolute stability refers to a condition in which the temperature at any height is always higher than that of the air parcel.
The air parcel therefore descends because it is colder and denser than the surrounding environment, irrespective of its moisture content.
Absolute Stability: ELR < SALR < DALR

If the Environmental Lapse Rate (ELR) is very high, or temperature decreases rapidly with height, the atmosphere is said to be unstable.
Absolute instability refers to a condition in which the temperature at any height is always lower than that of the air parcel.
The air parcel therefore continues rising because it is warmer and lighter than the surrounding environment, irrespective of its moisture content.
Absolute Instability: ELR > SALR > DALR

If the Environmental Lapse Rate (ELR) is greater than SALR but less than DALR, the atmosphere is said to be conditionally stable.
A dry air parcel is cooler than the surroundings and therefore descends, while a saturated air parcel is warmer than the surroundings and therefore continues to rise.
Conditional Stability: SALR < ELR < DALR

Neutral Stability refers to a condition when either ELR = DALR or ELR = SALR.
Latent Instability refers to a condition where a stable air parcel becomes unstable due to forced ascent.
Real Latent Instability means that the force required is less than the force released later.
Pseudo Latent Instability means that the force required is greater than the force released later.
Potential Instability means that air is stable at lower levels but unstable at higher levels.

In unstable atmospheric conditions, the air parcel is buoyant and continues to rise.
Rising air expands and cools adiabatically, forming vertically developed clouds such as Cumulus (Cu) and Cumulonimbus (Cb).
Aviation hazards associated with an unstable atmosphere include rain, hail, snow, icing and severe turbulence within clouds.
Visibility outside clouds is generally good because pollutants are carried to higher levels.

In stable atmospheric conditions, the air parcel is not buoyant and tends to descend and settle at lower levels.
Stratiform clouds with large horizontal spread are common in stable atmospheric conditions.
Aviation hazards associated with a stable atmosphere mainly include poor visibility and light rain.