Clouds & precipitation

Take a look out of the window: are thick white heaps drifting across the sky, or is a grey blanket covering everything? Clouds look like cotton wool, but they are something quite different. A cloud consists of billions of tiny water droplets or ice crystals so light that the air carries them. They form when moist air rises and cools down on the way: cold air can hold less invisible water vapour than warm air, so the vapour settles on tiny specks of dust as droplets. And if you can read clouds, you often know hours in advance what weather is coming.

Cloud types at different heights

Clouds have names, and each name tells you something about shape and height. Cumulus are the familiar fair weather clouds: white heaps with a flat base floating at about 1 km. Stratus is a grey layer cloud that hangs low and covers the sky like a blanket. Cirrus float right at the top, often at 9 km or higher: they are feathery and thin and made of ice crystals, because it is freezing cold up there. And then there is cumulonimbus, the thunderstorm cloud: it towers from a low hanging base up to over 10 km and brings lightning, thunder and heavy showers.

Rain, snow or hail?

Inside the cloud the tiny droplets bump into each other and merge into bigger drops. Once they get too heavy for the air, they fall: that is precipitation. What arrives at the ground is decided by the temperature on the way down. If it is warmer than 0 °C the whole way, it rains. If it stays colder than 0 °C the whole way, the ice crystals from the cloud arrive as snowflakes. Hail forms in thunderstorm clouds: strong updrafts hurl frozen pellets up again and again, where they collect new ice layer by layer until they fall to the ground as hailstones.

Clouds as a weather forecast

Long before weather apps existed, people read the weather from the sky. Small cumulus in the afternoon usually mean it stays friendly. If high cirrus move in and the sky then turns greyer and greyer, a rain front is often approaching. And when a cumulus towers up into a giant column with an anvil shaped top, it is time to be careful: such a cumulonimbus can turn into a thunderstorm within a short time.