Because an unstable atmosphere can lead to violent storms and turbulence, flight crew members must be aware of the nature of the atmosphere in which they are flying. What are the characteristics of a stable versus an unstable atmosphere? How can atmospheric stability be predicted?
Properties of the atmosphere are partially determined by its stability or resistance to change. Change in air temperature relative to altitude is a major factor in predicting atmospheric stability.
Smooth air, stratus clouds, and gentle continuous precipitation are characteristic of a stable atmosphere. The unstable atmosphere tends to be more turbulent and filled with rain showers and thunderstorms. Turbulence dissipates fog, smoke, and haze, however, leaving the air clearer immediately after it occurs. This increased visibility is good for flying. Since severe thunderstorms are associated with unstable air, weather forecasters are always on the lookout for warm, moist air at the surface with colder, dry air aloft that will create instability.
Atmospheric Stability: Pilots can identify or expect stable atmospheric conditions in the presence of stratus cloud, inversions and behind warm fronts. Stable conditions will last longer in the morning when overcast conditions persist through the day by blocking the sun from heating the ground. During flight conditions are often calm with poor visibility due to haze, fog, smoke, dust and pollution trapped in inversions layers.
Atmospheric Instability: Pilots can identify or expect unstable atmospheric conditions in the presence of cumulus cloud, thermals, wind shear and cold fronts. During flight, conditions are often bumpy with good visibility due to rising hot air currents dissipate fog, smoke, haze, dust and pollution.