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.
I wondered a little about the same thing, although I concluded that, since both Cecil and Dave agree that the cloud is less dense than the surrounding air, they must be right.
In any case, it seems to me that, although there are probably pretty strong updrafts in a thunderhead, it doesn’t seem like there are any in a nice fluffy cumulus cloud. Or, at least not any on the scale that would tend to move vapor continuously upward throughout the entire cloud.
Of course, I’m just speculating; I don’t really know.
As a pilot of small planes I can definitely confirm that there are updrafts associated with small cumulus as well as large. Although not nearly as strong as those found in thunderclouds, they can still be strong enough to toss a ton or three of aluminum upward several hundred feet. Holding up individual water droplets should be no problem.