Look a little closer
A puffy cumulus cloud can contain about 500 metric tonnes of liquid water. That does not mean every cloud weighs exactly 500 tonnes. The familiar estimate described by the U.S. Geological Survey uses an example cloud one cubic kilometre in volume and assumes a particular concentration of water droplets.
A cloud is not a lump of cotton or a visible mass of water vapor. Water vapor is an invisible gas. The white shape we see forms when vapor cools and condenses into countless tiny liquid droplets or ice crystals. Low clouds are mostly made of small water droplets.
The estimate uses simple multiplication. One cubic kilometre contains one billion cubic metres. If each cubic metre holds an average of 0.5 gram of liquid water, multiplying the two gives 500 million grams, or 500,000 kilograms. Each small piece of the cloud contains very little water, but the cloud occupies an enormous volume.
Why does all that water not fall at once? It is not packed into one solid object. The water is spread through a vast volume of air as extremely small droplets, which fall very slowly. Rising and circulating air can also keep those droplets suspended.
Droplets can collide and join together inside the cloud. Once they become large enough that moving air can no longer keep them aloft, gravity pulls them down as rain. In colder conditions, the falling precipitation may instead develop as snow or other forms of ice.
So the phrase ‘a cloud weighs 500 tonnes’ needs a qualifier. The answer changes with the cloud's size and liquid-water content, and it also depends on whether we count only the droplets or include the surrounding air. The 500-tonne figure is a useful estimate for the water in one example cumulus cloud, not a fixed rule for every cloud.
