What are clouds made of?

Tiny liquid water droplets, and sometimes ice crystals - not vapour. Water vapour is an invisible gas; a cloud is what happens when air cools enough that the vapour condenses onto microscopic particles of dust, salt or pollen. Without those nuclei to condense on, air can hold far more moisture than it should and simply stays clear.

The droplets are so small - a hundredth of a millimetre - that air resistance almost balances their weight and they fall at a few centimetres a second, which any gentle updraught overcomes. That is why a cloud floats despite weighing a great deal: a modest cumulus holds something like 500 tonnes of water, which is the mass of a hundred elephants, distributed as a fine mist.

Rain requires the droplets to combine into something about a million times larger by volume, which happens either by colliding and merging or, in colder clouds, by ice crystals growing at the expense of surrounding droplets and then melting as they fall. The classification everyone uses - cumulus, stratus, cirrus - was devised by Luke Howard, a London pharmacist, in 1802, and it caught on so completely that Goethe wrote poems in his honour.