Clouds and rain

Condensation brings evaporated water back to the Earth. But first: why does water vapour go up at all? Gas balloons filled with lighter gases rise. In the same way, water vapour is lighter than air, so it rises.

The higher air goes above the Earth's surface, the cooler it gets. At certain heights it is so cool that its water vapour turns into tiny droplets. These droplets generally form around dust particles. The small droplets float in the air and form clouds.

Many droplets join together into bigger drops. Some drops become so heavy that they fall: that is rain. Under special conditions the water falls as hail or snow instead.

See the part dust plays:

• Put about a cup of water in a plastic bottle, close it tightly, and squeeze and release it again and again for 2–3 minutes.
• Now drop in a small burnt piece of newspaper, close it and repeat.
• This time a haze, a little cloud, appears above the water.

The burnt paper gives tiny, invisible dust particles, and the water vapour condenses around them to form the cloud.