In the discussion section appended to this column, it is implied that clouds stay put because they are lighter than the underlying air. [q]The reason that a cloud floats is that the water vapor is less dense than the surrounding air, so the surrounding air exerts an upward buoyant force (remember Archimedes?) equal to the weight of the cloud. --Dave Morgan, Ph.D.[/q]
My understanding has always been that a cloud is a two-phase system of gas and liquid, the liquid being microscopic droplets of liquid H2O – these particles are heavier than air, thus they do tend to fall and they will tend to evaporate as they fall. The cloud remains in place however, because H2O vapor rising above the condensation line continues to feed it. A cloud forming under stable conditions doesn’t require the air to be buoyant, as it can be lifted orographically or wedged by a frontal system.
While it is true that unstable air rises buoyantly, you don’t have a cloud until you have condensation, and this produces liquid droplets. If air is unstable, these droplets are kept above the cloud base by strong updrafts.