The water cycle — Earth science, 11–13 years
Water keeps moving between the ground, oceans, air and living things through evaporation, condensation, precipitation, collection and infiltration.
Water takes different routes
The same water can be liquid in a river, invisible vapour in the air, or solid ice in a glacier. Sunlight drives evaporation from wet surfaces, cooling makes vapour condense into droplets, and gravity brings water back as rain, snow or hail. Water then flows, freezes, soaks into soil or is used by living things.
Why follow the water
A puddle can vanish without the water being destroyed, and a dry place can receive rain from water that began far away. The water cycle solves this puzzle by tracking matter as it changes place and state. It helps explain clouds, rivers, groundwater, droughts and floods as connected parts of one moving system.
Following one drop
Start with a drop in the Atlantic Ocean. First, sunlight warms the surface and the drop evaporates into water vapour. Next, rising air cools; the vapour condenses on tiny particles and joins a cloud. Winds carry the cloud inland, where the drop falls as rain. It may then enter a stream and eventually return to the ocean.
The cloud-making mistake
It is reasonable to think clouds are made by hot vapour simply rising until it becomes visible. Rising alone is not enough: the air must cool, and water vapour needs tiny surfaces, such as dust or salt, on which droplets can form. A cloud is not smoke or a solid object; it is many small droplets or ice crystals floating in air.
Water planning
Farmers, towns and engineers use the water cycle when planning reservoirs, drainage and irrigation. They ask where rain will fall, how quickly water will run off, and how much can soak into the ground. After heavy rain, paving and compacted soil can send more water into streams quickly, increasing flood risk.
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