Stable air masses are exactly what they sound like -- marked by stability or relative calmness within their lower layers. Stable air masses are free from convection and other disturbances typically found in unstable air masses. Due to their static nature, stable air masses are characterized by certain atmospheric conditions.
Because stable air masses are, by nature, calm and free of violent disturbances, they are often marked by the appearance of stratiform clouds or fog. Stratiform clouds can be identified by their smooth, sheetlike nature and do not build vertically like clouds found in unstable air masses with convective activity. Stratiform clouds form in this environment because they are not constantly being disturbed or uprooted and, instead, are free to develop into a smooth sheet. The same goes for fog. Due to the static nature of a stable air mass, there is no activity to uproot or disturb the low clouds, allowing them to settle to the ground as fog.
Stable air masses are also characterized by smooth, undisturbed air. This quality can be seen mainly by pilots or air travelers. People flying through stable air masses will not experience violent turbulence or disturbances as they would in unstable air masses. The undisturbed air is free to flow over the wing optimally and is not interrupted. Polar air masses tend to be more stable, at least until a disturbance pushes them southward.
Rain or other precipitation that accompanies a stable air mass is often more continuous than that coming with unstable air masses. In an unstable air mass, the clouds are constantly being swirled or shifted around by disturbances. Rain in stable air masses, meanwhile, is free of disturbance and has time to settle over a location without being moved or interrupted. The result is more steady precipitation than in an unstable air mass, which often brings showery or sporadic precipitation.
Because stable air masses are free of disturbances, they often allow particles such as dust, smoke or other haze to linger in the air without being shifted or blown around. The result is poor visibility for people on the ground and, especially, people flying in aircraft. Unstable air masses may not have this poor visibility because the air mass is constantly blowing around and disturbing the particles that, if settled, would create haze and reduce visibility.