How does a stable atmosphere affect visibility near the surface?

Stability describes the atmosphere's resistance to vertical motion, and that resistance is what determines whether the lower air gets mixed and cleaned or left to accumulate. In a stable atmosphere, air displaced upward is cooler than its surroundings and sinks back, so there is little vertical exchange. Haze, smoke, dust and pollutants released near the surface therefore stay near the surface and build up over hours, and moisture concentrates in the same shallow layer, favouring fog and low stratus. The characteristic result is a day that looks calm and benign — smooth air, no convection, no gusts — while surface visibility is poor and slowly worsening. That combination is the hazard for a remote pilot, because the flying conditions are pleasant while the three statute mile visibility requirement is quietly at risk. Instability produces the opposite: vigorous mixing, good visibility, and rough air with convective cloud.

A stable atmosphere resists vertical motion, so haze, smoke and pollutants released near the surface are not mixed upward and accumulate, reducing visibility.

Atmospheric stability describes resistance to vertical motion. Stable air suppresses mixing, trapping haze, smoke and moisture near the surface, producing poor visibility, fog and stratiform cloud with smooth flying conditions.

Smooth Air, Dirty Air. Nothing mixes upward, so everything stays down where you are flying.

Pair stability with the three statute mile visibility requirement. Stable conditions are where a legal flight quietly becomes illegal.

• •Equating smooth air with good visibility
• •Expecting stability to improve surface conditions
• •Checking visibility once rather than watching the trend