Stability of Air
The stability of air in the atmosphere depends on the temperature of rising air relative to the temperature of the stationary surrounding air that it passes through, which varies from place to place and with changing atmospheric conditions. Air stability determines whether clouds form when air is uplifted, and the type of cloud.
When a packet of air near the Earth’s surface is heated it rises, being lighter than the surrounding air. Whether or not this air packet continues to rise will depend upon how the temperature in the surrounding air changes with altitude. The rising packet of air will lose heat because it expands as atmospheric pressure falls, and its temperature drops. If the temperature of the surrounding air does not fall as quickly with increasing altitude, the air packet will quickly become colder than the surrounding air, lose its buoyancy, and sink back to its original position. In this case the atmosphere is said to be stable. If the temperature of the surrounding air falls more quickly with increasing altitude, the packet of air will continue to rise. The atmosphere in this circumstance is said to be unstable.
As uplifted air cools, it condenses excess vapour out as cloud. The more unstable the atmosphere the more prolonged the uplift. Small cumulus clouds are evidence of a fairly stable atmosphere. Large cumulonimbus clouds are evidence of a highly unstable atmosphere, conducive to the formation of thunderstorms. Within depressions, atmospheric pressure is low and there is considerable atmospheric uplift and cooling at altitude, increasing atmospheric instability. Low-pressure systems are usually associated with an abundance of cloud and precipitation. In high-pressure systems or anticyclones, air may be descending, compressing and gaining energy, such that temperature at altitude rises, thereby increasing atmospheric stability. Anticyclones are often associated with cloudless skies.