Clouds are formed from water vapour. The air collects this vapour as it passes over damp places, like the sea. We call the process of a liquid turning to gas ‘evaporation’. Normally, the vapour in the air is invisible, but if the air is cooled then clouds of tiny water droplets are formed. This process we call ‘condensation’.
The amount of water vapour that the air can carry depends on how hot the air is – warmer air can carry more moisture. When warm, moist air rises, either by moving over hills and mountains, or by meeting cooler air, it is cooled. As the air cools, it can carry less vapour. The excess moisture forms clouds of tiny droplets.
Storm clouds gathering
The fluffy, white clouds you see on fine, summer days are called ‘cumulus’.
Your breath
The way clouds form is like the way a misty cloud forms when you breathe out on a cold day. The air that you breathe out contains a lot of moisture. On a cold day, this air is cooled as it meets the cold air outside. As it cools, it can hold less water vapour, and the extra water forms tiny droplets, like a cloud. When a whale breathes out, the water vapour in its warm breath condenses into a misty spray.