A cloud weighs as much as a hundred elephants

An ordinary fair-weather cloud — a white, fluffy cumulus — measures about one cubic kilometre. Inside, it carries half a gram of water per cubic metre: next to nothing. But multiply that next-to-nothing by a billion cubic metres and you get some 500 tonnes of water floating over your head. The weight of a hundred elephants.

So why doesn't it fall? Because that water isn't a block: it's billions of droplets a hundredth of a millimetre wide, so small they fall at about a centimetre per second, while the warm air rising from the ground pushes them up faster than they sink. A cloud doesn't float: it is falling all the time — and losing the race.

When the droplets merge and fatten, the race changes winners. We call that rain.

And storm clouds play in another league: a cumulonimbus can carry hundreds of thousands of tonnes. An aircraft carrier hovering over your neighbourhood.