A cloud can weigh as much as an airplane? Why doesn't it fall?

A cloud can weigh as much as an airplane? Why doesn't it fall?

mass of condensed watery vapor floating in the air at some considerable height above the general surface of the ground." You might ask "If clouds are watery vapor, and vapor has weight, why doesn't it fall on me?" Even though clouds float, the fact is that clouds do have weight - many tons, in fact. And they do "fall" on you, sometimes, when the fog rolls in.

According to columnist Cecil Adams, "a modest-size cloud, one kilometer in diameter and 100 meters thick, has a mass equivalent to one B-747 jumbo jet." ("Can a cloud weigh as much as a 747?", http://www.straightdope.com/classics/a980313a.html, accessed on Sep. 11, 2003). But, with all that mass being spread over such a large volume of space, the density, or weight (mass) for any chosen volume, is very small. If you compressed that cloud into a trash bag, well, in that case, you would not want to be standing below it. Even though a cloud weighs tons, it doesn't fall on you because the rising air responsible for its formation keeps the cloud floating in the air. The air below the cloud is denser than the cloud, thus the cloud floats on top of the denser air nearer the land surface.