Water Cycle

How to use it

Press Watch the cycle and the landscape comes alive. Water vapour rises from the ocean and trees, gathers into clouds and falls as rain or snow, then runs off into the river or soaks into the ground. Turn up the sun for faster evaporation. Lower the air temperature to make snow pile up on the mountain, then raise it and watch the snowmelt fill the river. Turn up the wind to push clouds over the land, where the mountains force them up and they drop their water.

Tap any label, or any part of the picture, for a short explanation. Follow a drop tells the story of one drop step by step, with random turns: it might be back in the ocean in a week, or stuck in a glacier or deep groundwater for thousands of years. The Quiz has eight questions; some ask you to click the right part of the picture. With reduced motion turned on, the picture starts paused and moves on with Next step.

The water cycle explained

The water cycle is the endless journey of water between the oceans, the air and the land. The sun drives it: its energy makes water evaporate from oceans, lakes and soil, and plants give off vapour through their leaves in transpiration. As the vapour rises, it cools and condenses into the tiny droplets and ice crystals that make clouds. When they grow heavy, they fall as precipitation: snow if the air is below freezing all the way down, rain if the snow melts on the way. On land, water runs off into rivers, soaks into the soil (infiltration) or is stored as snow, ice and groundwater, and gravity slowly brings it back to the sea, where it collects.

Water is never used up. It only changes place, and changes between solid, liquid and gas. How long it stays depends on the place: about 9 days in the air, weeks to months in a river, a season in a snowpack, about 3,000 years in the ocean, and from months to more than 10,000 years underground or in ice sheets. The picture is a simple model: real clouds hold far more water, and real groundwater moves much more slowly.

The water cycle is also the engine of our weather: clouds, rain, snow, fog and dew are all part of it. The mountains show why the side facing the wind is often the wettest.

• Why is there more rain on the mountain than over the low land?
• Put a bowl of water in a sunny window and one in the shade. Which empties first, and where did the water go?
• Follow three drops each. Who had the longest trip? Who got stuck?

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