A cloud looks soft and weightless, but it isn't — a medium white cloud holds hundreds of tonnes of water. So why doesn't it fall on you? Because the water is split into billions of specks so tiny that air gets in their way and they sink incredibly slowly. And the warm air rising underneath pushes them up faster than they sink, so they just hang there. In the simulator, grow the droplets until they get heavy enough to fall as rain.
Most people think clouds float because they are lighter than air. In fact a white cumulus can hold around 500 tonnes of water; it stays up only because that mass is shattered into billions of droplets so tiny that air is thick and grabby and a gentle updraft cancels their slow fall.
What's actually happening
Ask why a cloud floats and the easy answer is that it must be lighter than air. It isn't. Weigh the water in an ordinary white cumulus cloud and you get something staggering: a cloud a kilometre across can hold around 500 tonnes of water, roughly the weight of a hundred elephants hanging over your garden. The real puzzle isn't that something light is floating. It's that something genuinely heavy refuses to fall.
The trick is how that water is divided. A cloud isn't a pool of water; it's that mass shattered into billions upon billions of microscopic droplets, each only about a hundredth of a millimetre across. At that size, air is no longer a thin nothing to fall through — it's thick and grabby. A droplet that small drifts down at maybe a centimetre per second, so slowly it would take it the better part of a day to fall a single kilometre, and most never get the chance. The same warm air that built the cloud is still gently rising beneath it, and a rising current of just a few centimetres per second lifts the droplets faster than they settle. So the whole heavy mass simply hangs, suspended, like dust caught in a sunbeam that never reaches the floor.
This also quietly explains rain. As long as the droplets stay tiny, the cloud floats indefinitely. But droplets collide and merge, and a merged droplet is bigger, and a bigger droplet falls faster — its fall speed climbs with the square of its size. Once they coalesce past about a tenth of a millimetre, they're falling fast enough to outrun the updraft and drop out of the cloud. The same physics that keeps hundreds of tonnes aloft for hours lets it all come down the moment the droplets get fat enough. Floating and raining are two settings of one machine.
Floating and raining are two settings of one machine: tiny droplets hang for hours, but once they merge and fatten they outrun the updraft and fall.
- 1Drop a small marble and a pinch of flour or talc from the same height at the same moment.
- 2The marble hits the floor almost instantly; the powder drifts down in slow motion, hanging in the air.
- 3Same material falling through the same air — only the size changed. Tiny things fall slowly because, for them, air is thick.
Common questions
A modest white cumulus a kilometre across can hold around 500 tonnes of water, roughly a hundred elephants or a loaded airliner. The puzzle is not that something light floats but that something genuinely heavy refuses to fall.
Droplets collide and merge, and a bigger droplet falls faster, its speed climbing with the square of its size. Once they coalesce past about a tenth of a millimetre they outrun the updraft and drop out of the cloud as rain.
Yes. Fog is simply a cloud touching the ground, made of the same tiny droplets. Walking through fog means walking through the very thing that floats overhead.