Science Puzzle
Why Don’t Clouds Fall Out of the Sky?
A fluffy white cumulus cloud on a summer's day can hold around 500 tonnes of water, about as much as 80 elephants weigh. Yet it drifts across the sky for hours without coming down.
What stops all that water falling out of the sky?
The Answer
Clouds are falling, just extremely slowly. Their droplets are so small that the air barely lets them drop, and gentle rising air is enough to keep them up.
A cloud isn't water vapour: vapour is invisible. It's made of liquid droplets, or tiny ice crystals, each around 0.02 millimetres across, far thinner than a hair. Anything falling through air is slowed by air resistance; the smaller it is, the more surface it has for its weight. A typical cloud droplet falls at only about a centimetre per second; it would take most of a day to drop one kilometre in perfectly still air.
The air is rarely still. Cumulus clouds form on top of rising currents of warm air, which easily lift droplets faster than they fall. And droplets that do sink out of the bottom of a cloud meet drier air there and evaporate.
Rain begins when droplets bump into each other and merge. A typical raindrop holds as much water as about a million cloud droplets. It's heavy enough to fall at several metres per second, too fast for the rising air to hold it, and down it comes.
The principle: Air resistance and terminal velocity. Tiny particles have a large surface for their weight, so they fall very slowly through air and can be held up by even gentle upward currents.