How much does a cloud weigh?

The calculation is simpler than it sounds. A typical cumulus is about a kilometre across and roughly as tall as it is wide, giving it a volume near one cubic kilometre — a billion cubic metres. Cloud water density runs around half a gram per cubic metre. Multiply, and you get 500 million grams of water droplets.

That is 500 tons of water, suspended over your head, arranged into something that reads to the eye as weightless. Peggy LeMone at the US National Center for Atmospheric Research did much of the work of getting people to take the question seriously.

So why doesn't it fall? Because weight is the wrong frame. The cloud is less dense than the drier air beneath it, and the droplets are tiny enough that air resistance and updrafts hold them. It is not defying gravity. It is floating, the same way a boat does.