Warm rises, cool sinks: convection
Air and water are fluids: they can flow. When part of a fluid is warmed, its particles move faster and spread out, so that part becomes less dense and rises. Cooler, denser fluid sinks to take its place. The fluid then goes round in a loop. This movement carries heat, and we call it convection. It happens when the Sun heats the ground and the air above it. The warm air rises, and as it rises it cools. If it contains water vapour, the vapour can condense onto tiny pieces of dust, pollen or smoke to make billions of tiny droplets. These droplets together form a cloud. So convection helps the water cycle by lifting vapour high up.
| Where | What the water does |
|---|---|
| near the heat | warms and rises |
| at the top | cools and spreads |
| at the sides | sinks |
Be the air. With a group of friends, act out air particles. Stand close together and shuffle slowly: this is cool air. Then move faster, spread out and step upward, as warm air does, while another group slides in underneath. Draw arrows to show the loop. Never use a flame or a hot surface.