This site mentions that

...the average cumulus cloud weighs 1.1 million pounds. (Scroll down the page to the cloud picture.)

How, exactly, is the weight of a cloud determined? I thought that clouds are just the visible condensation of water droplets in the atmosphere, but there exists some water in the entire atmosphere, even where there are no clouds.

Is this weight claim based on the additional weight/mass of just the extra water vapor that supersaturates the air and causes the visible manifestation we call a "cloud"?

P.S. I estimate a typical cumulus cloud as having a volume of cir. $1~\mathrm{km^{3}}$, and that volume of Earth's atmosphere centered around altitude $2000 ~\mathrm{m}$ MSL has a mass of $1.00647 \times 10^{9}~\mathrm{kg}$ -- easily three orders of magnitude more massive than 1.1 million $\mathrm{lbs}$ (mass).

  • $\begingroup$ Do you realize how close your PS comes to giving you you own back-of-the-envelope lower bound on the mass of water in the cloud? The next step is to assume that the whole volume has 100% relative humidity and compute the implied partial pressure of water... $\endgroup$ – dmckee --- ex-moderator kitten Aug 11 '16 at 1:43

Weight of a Cloud

This answer is based on the above article, with as much pruning of irrelevant wordage as possible.

The water density of a typical cumulus cloud is 1/2 gram per cubic meter, but the density will vary for different types of clouds. 

By measuring a cloud’s shadow when the sun is directly above it, you can get an idea of its width. A typical cumulus, is about a kilometer across, and generally roughly cubical—so a kilometer long and a kilometer tall. That’s one billion cubic meters in volume. 

enter image description here

Obligatory picture of clouds in lovely blue sky.

Use the density and volume to determine the total water content of the cloud, here it's 500,000,000 grams of water, or 1.1 million pounds.

So how does a several-hundred-ton cloud stay afloat? For one thing, the weight is distributed among trillions of really tiny water droplets spread out over a really big space. Some of these droplets are so small that you would need a million of them to make one raindrop, and gravity’s effect on them is pretty negligible. 

What’s more, the cloud is less dense than dry air, so it's buoyant. It also helps that all those little droplets get some lift from updrafts of warm air. Those droplets don’t float forever, though. When the cloud’s water density increases and the droplets get bigger and heavier, the cloud eventually does fall, bit by little bit, in the form of rain. 

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  • $\begingroup$ Thank you... Should I assume the cloud is less dense than dry air because of what this article mentions? $\endgroup$ – pr1268 Aug 11 '16 at 21:07

Cloud means $liquid$ water in the form of droplets suspended high up in the atmosphere. Water vapor is naturally present everywhere, and not just confined to region of space where clouds are present; so water $vapor$ should not be considered part of the cloud. If you want to calculate weight of cloud then just take liquid water content of cloud into consideration.

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