What type of clouds is associated with stable air?

Cloud shape is a direct read-out of atmospheric stability, because a cloud's form records the motion of the air that made it. Stable air resists vertical displacement, so moisture that condenses spreads horizontally along the level where it reached saturation, producing flat layered stratiform cloud. Precipitation from such cloud is steady and light — drizzle or continuous light rain — because there is no vigorous updraft to build large droplets. The accompanying conditions follow the same logic: smooth air, poor surface visibility from trapped haze, and a tendency toward fog and low stratus. Unstable air produces the opposite family: cumuliform cloud building vertically, showery precipitation, turbulence, and good visibility from vigorous mixing. Lenticular cloud is the special case worth knowing separately, since it forms in stable air flowing over terrain but signals severe mountain wave turbulence rather than the benign conditions stratiform cloud usually implies.

Stable air resists vertical motion, so condensation spreads horizontally into flat stratiform layers producing steady light precipitation.

Stable air produces stratiform cloud, steady precipitation, smooth conditions and reduced surface visibility. Unstable air produces cumuliform cloud, showers, turbulence and good visibility. Lenticular cloud indicates mountain wave activity in stable flow over terrain.

Layers Mean Stable, Lumps Mean Unstable. The shape of the cloud is the shape of the air's motion.

Lenticular cloud is the exception to the calm-stable association. Stable air over mountains produces severe wave turbulence.

• •Matching stratiform cloud to unstable air
• •Assuming all stable conditions are benign, including lenticular cases
• •Reading light steady rain as a sign of convective activity