full text of the classic FAA guide
WHAT DOES IT ALL MEAN?
Can we fly in unstable air? Stable air? Certainly we can and ordinarily do since air is seldom neutrally stable. The usual convection in unstable air gives a “bumpy” ride; only at times is it violent enough to be hazardous. In stable air, flying is usually smooth but sometimes can be plagued by low ceiling and visibility. It behooves us in preflight planning to take into account stability or instability and any associated hazards. Certain observations you can make on your own:
Thunderstorms are sure signs of violently unstable air. Give these storms a wide berth.
Showers and clouds towering upward with great ambition indicate strong updrafts and rough (turbulent) air. Stay clear of these clouds.
Fair weather cumulus clouds often indicate bumpy turbulence beneath and in the clouds. The cloud tops indicate the approximate upper limit of convection; flight above is usually smooth.
Dust devils are a sign of dry, unstable air, usually to considerable height. Your ride may be fairly rough unless you can get above the instability.
Stratiform clouds indicate stable air. Flight generally will be smooth, but low ceiling and visibility might require IFR.
Restricted visibility at or near the surface over large areas usually indicates stable air. Expect a smooth ride, but poor visibility may require IFR.
Thunderstorms may be embedded in stratiform clouds posing an unseen threat to instrument flight.
Even in clear weather, you have some clues to stability, viz.:
When temperature decreases uniformly and rapidly as you climb (approaching 3° C per 1,000 feet), you have an indication of unstable air.
If temperature remains unchanged or decreases only slightly with altitude, the air tends to be stable.
If the temperature increases with altitude through a layer—an inversion—the layer is stable and convection is suppressed. Air may be unstable beneath the inversion.
When air near the surface is warm and moist, suspect instability. Surface heating, cooling aloft, converging or upslope winds, or an invading mass of colder air may lead to instability and cumuliform clouds.