To be ” unstable “, the lowest layers of an air mass must be so warm and/or humid that, if some of the air rises, then that air parcel is warmer than its environment, and so it continues to rise. This is called moist convection.
Weather is strongly affected by how stable or unstable the atmosphere is. Stable air means that the weather is likely to be calm. It may rain or snow slowly and steadily, it may be sunny, but the weather will not change quickly.
Clouds formed by vertical currents in unstable air are cumulus meaning accumulation or heap; they are characterized by their lumpy, billowy appearance. Clouds formed by the cooling of a stable layer are stratus meaning stratified or layered; they are characterized by their uniform, sheet-like appearance.
The temperature of the air mass at different heights is controlled by the environmental lapse rate. The interaction of the surrounding air mass (see left) with the locally heated ground level air (see right) will determine the air’s stability. Temperature difference compared to surroundings. Moisture content.
|Unstable Air||Stable Air|
|Cumuliform clouds||Stratiform clouds and fog|
|Showery precipitation||Continuous precipitation|
|Rough air (turbulence)||Smooth air|
|Good visibility, except in blowing obstructions||Fair to poor visibility in haze and smoke|
Air is considered unstable, in the lowest layers of an air mass when the air is warmer and or more humid than the surrounding air. When this occurs the air will rise, as that air parcel is warmer than the air surrounding it. In an unstable environment, the weather can change suddenly and can be violent.
Indicators of a stable atmosphere are steady winds, clouds in layers, and poor visibility due to haze and smoke hanging near the ground. Unstable conditions are indicated by dust devils, gusty winds, clouds with vertical growth, and good visibility. (
To determine the stability of an air parcel, one compares its temperature to the temperature of the surrounding air mass. If the air parcel’s temperature is greater than the temperature of the surrounding air mass, the air parcel is less dense and tends to rise.
Atmospheric stability is a measure of the atmosphere’s tendency to discourage or deter vertical motion, and vertical motion is directly correlated to different types of weather systems and their severity.
Thus, cumulonimbus are known to be extremely dangerous to air traffic, and it is recommended to avoid them as much as possible. Cumulonimbus can be extremely insidious, and an inattentive pilot can end up in a very dangerous situation while flying in apparently very calm air.
Clouds forming in such unstable environments are of the cumulus variety, which as you may recall, means “heap cloud.” Cumulus clouds are “heap clouds,” and have a bubbly, or billowy appearance.
Because thermal infrared absorption from liquid water is so efficient, many stratus clouds are unstable, because, as the cooling strengthens, condensate builds up, longwave cooling further strengthens, and the layer becomes increasingly susceptible to convective overturning (hence stratocumulus formation).
The warmer, lighter/less dense air will always be forced aloft. With the movement aloft, and the resulting expansion/cooling of the air, the possibility for condensation/precipitation increases. Areas of air mass contact almost always encourage a weather change.
* Which condition is present when a local parcel of air is stable? The parcel of air resists convection.
Air mass having a stable stratification in its lower layer, and consequently free from convection, having a low degree of turbulence, and containing either stratiform clouds, fog, or no clouds at all.