Different forms of precipitation (dew, fog, rainfall, frost, snowfall, hailstorm etc.) depend on stability and instability of the atmosphere. The air without vertical movement is called stable air while unstable air undergoes vertical movement (both upward and downward). An airmass ascends and becomes unstable when it becomes warmer than the surrounding airmass while descending airmass becomes stable.
The stability and instability depend on the relationships between ‘normal lapse rate’ and ‘adiabatic change of temperature’. Adiabatic rate is always constant whereas normal lapse rate of air temperature changes. When the normal lapse rate is higher than dry adiabatic rate, the air being warmer rises and becomes unstable. On the other hand, when the normal lapse rate of temperature is lower than dry adiabatic rate, the air being cold descends and becomes stable.
When dry adiabatic lapse rate of an ascending dry air is higher than the normal lapse rate and if it is not saturated and does not attain dew point it becomes colder than surrounding air at certain height with the result it becomes heavier and descends. This process causes stability of atmospheric circulation due to which vertical circulation of air is resisted.
For example, at ground surface if the temperature of a parcel of air is 40°C, the dry adiabatic lapse rate and normal (environmental) lapse rate are 10°C per 1000m and 6.5° C per 1000 m respectively, then at the height of one kilometre (or 1000 m) from the ground surface the temperature of the ascending air would be 30°C (40° -10°= 30°C) while the temperature of surrounding air at that height would be 33.5° C (40°-6.5° = 33.5°C).
Thus, the ascending air being colder than surrounding air would descend and atmospheric stability is caused. Such air (descending) is called to be in stable equilibrium. Sometimes, the normal lapse rate in a certain layer of the atmosphere is about 4.6° C per 1000 metres. In such conditions if the normal lapse rate is less than wet adiabatic lapse rate even at condensation point, further vertical motion of air is stopped and thus such air is said to be absolutely stable and such atmospheric condition is called absolute stability.
When normal lapse rate is greater than dry adiabatic lapse rate of ascending parcel of air the rising air continues to rise upward and expand and thus becomes unstable and is in unstable equilibrium. In other words, atmospheric instability is caused when the rate of cooling of rising air (dry adiabatic lapse rate) is lower than the normal lapse rate.
For example, if the temperature of a certain parcel of air at ground surface is 40°C, the dry adiabatic and normal lapse rates are 10°C and 11°C per 1000m respectively, then the temperature of ascending air at the height of 1000m (one kilometre) would be 30°C (40°-10° = 30°C) while the temperature of the atmosphere at that height would be 29°C (40°-11°C = 29°C).
Thus, the rising air being warmer (30°C) than the surrounding air (29°C) continues to rise and expand to cause atmospheric instability. If the wet adiabatic lapse rate is also less than normal lapse rate, the rising air further continues to rise upward. Such state of continued upward movement of air is called absolute instability.
When the ascending parcel of air reaches such height that its temperature equals the temperature of surrounding air, its further upward movement is stopped. Such air is said to be in the state of neutral equilibrium.
i. Mechanical Instability:
Sometimes, there are abnormal conditions when normal lapse rate is exceptionally very high (15° to 35°C per 1000 metres). In such condition the upper layers of the atmosphere become exceptionally cold and denser than the underlying layers, with the result cold and denser upper layers automatically descend. Such situation is called mechanical instability which helps in the formation of tornadoes.
ii. Conditional Instability:
When a parcel of air is forced to move upward, it cools at dry adiabatic lapse rate (10°C per 1000m or 5.5°F per 1000 feet) whereas normal lapse rate is 6.5°C per 1000m. After rising to certain height the air becomes saturated and latent heat of condensation is added to the rising air so the rising air cools at wet adiabatic lapse rate (5°C per 1000m) whereas the normal lapse rate (6.5°C per 1000m) is greater than it.
Consequently, the air becomes warmer than the surrounding air and hence rises upward automatically. This is called conditional instability because the air is initially forced to move upward but rises automatically due to its own properties after condensation point is reached.
For example, if a parcel of air with 35°C temperature is initially forced to rise up to the height of 1000m, its temperature decreases to 25°C (35°C-10°C, dry adiabatic rate = 25°C) whereas the temperature of surrounding layers of air at the height of 1000m would be 28.5° C (35°C-6.5°C, normal lapse rate) and thus the rising air becomes colder by 3.5°C than the surrounding air.
If the rising air gets saturated at this temperature (25°C), the latent heat of condensation returns back to the rising air and hence it cools at the wet adiabatic lapse rate (5°C per 1000m). Thus, the rising air becomes warmer and unstable, Conditional instability may occur only when the normal lapse rate ranges between dry adiabatic and wet adiabatic lapse rates. In other words, conditional instability occurs when normal lapse rate is greater than dry adiabatic lapse rate but less than wet adiabatic lapse rate.