What are clouds made of?

Clouds are billions of water droplets, each too small to see on its own, made when rising air cools. And on the greyest day, the sun is still shining above them.

The little aeroplane climbs among towering white clouds lit from above, the patchwork land and a winding river now very small far below. — Chapter four of the storybook: climbing among clouds as tall as mountains, with the land growing small below. Tap the picture to open the chapter.

The short version

The air always carries some water you cannot see, mixed into it as a gas called water vapour. Warm air can hold a lot of it; cold air can hold less. Air cools as it rises, so when it goes up far enough, some of that invisible water turns back into liquid: tiny droplets, each formed around a speck of dust or a grain of salt from the sea, so small that about a hundred of them side by side would stretch a millimetre.

A cloud is billions of those droplets together. They scatter sunlight in every direction, which is why a cloud looks white, and a thick one lets so little light through that it looks grey underneath.

Why some clouds look like mountains

On a sunny day the ground warms up, and bubbles of warm air rise from it like bubbles in a pan of water. Where a bubble has cooled enough, a cloud forms on top of it. These are cumulus clouds, the puffy heaps (cumulus is Latin for heap), and on a warm afternoon they can keep building until they tower higher than any mountain on Earth.

A big heap cloud holds hundreds of tonnes of water. It floats because the water is spread through an enormous space, in drops so small that the rising air keeps them up.

What clouds mean to a pilot

Inside a cloud you cannot see anything at all. It is like flying through thick fog: the ground, the horizon and every other aeroplane disappear. So pilots who fly by looking out of the window have to keep clear of clouds, by distances written into the rules. Pilots trained to fly by their instruments can go through them, steering by the dials alone.

The towering clouds are bumpy inside, because of the rising air that builds them, and the tallest of all become thunderstorms, which every pilot keeps well away from (here is what they do).

Above the grey

Clouds have tops. Climb up through a grey layer and you come out into sunshine that was there the whole time: on the dullest day, the sun is shining above the cloud. And from up there the fields below look like patches and the rivers like threads, because everything looks smaller the further away it is.

In the story

In chapter four Little Blue climbs until the fields are patches below him and the clouds “rise around him like white mountains.” It is a beautiful sight, and for the first time the sky feels a little too big for a very small pilot. He breathes, and goes on up.

Higher Than the Clouds sings the other side of that climb, right through the grey into the blue, and its chorus looks down on the same small world Little Blue sees: Rivers turned to ribbons, mountains small below.

Three words to keep

Water vapour: water as an invisible gas, mixed into the air.
Condense: to turn from a gas into tiny drops, the way your breath mists a cold window.
Cumulus: the heaped, puffy clouds of a sunny day.