The Water Cycle

Core Idea

The water cycle is nature's way of recycling water over and over. The sun heats water in oceans, lakes, and rivers, causing it to evaporate into the air as water vapor. The vapor rises, cools, and condenses into tiny droplets that form clouds. When the droplets get heavy enough, they fall back to Earth as rain, snow, or hail — that is precipitation. The water collects in rivers and oceans, and the whole cycle starts again. Every phase change you have learned — evaporation, condensation, melting, and freezing — plays a role.

How It's Best Learned

Build a mini water cycle in a sealed clear container: put warm water in the bottom, cover the top with plastic wrap, and place ice cubes on top of the wrap. Students watch evaporation from the warm water, condensation under the cold plastic, and "rain" dripping back down. Label each step with the correct phase change name.

Common Misconceptions

• Thinking clouds are made of water vapor (they are actually tiny liquid droplets or ice crystals — the vapor already condensed).
• Believing the water cycle only happens over oceans.
• Thinking rainwater is "new" water rather than recycled water that has existed for billions of years.
• Assuming the water cycle stops at night or in winter.

Explainer

Everything you have learned about how matter changes state — melting, freezing, evaporation, and condensation — comes together in one of the most important processes on Earth: the water cycle. The water cycle is the continuous journey water takes from the surface of the Earth, up into the sky, and back down again. It has been running for billions of years, and the water you drink today may once have been part of a dinosaur's river.

The cycle starts with evaporation. The sun heats water in oceans, lakes, rivers, and even puddles. Some of that liquid water turns into invisible water vapor and rises into the air. Plants help too — they release water vapor from their leaves in a process called transpiration. All this vapor climbs higher and higher into the atmosphere.

As the vapor rises, the air gets colder. Cold air cannot hold as much water vapor as warm air, so the vapor starts to condense — it turns back into tiny liquid droplets. Billions of these droplets clump together around tiny bits of dust or pollen, and that is what forms a cloud. Clouds may look fluffy and light, but a single cloud can hold millions of gallons of water in the form of those tiny droplets.

When the droplets in a cloud bump into each other and combine, they grow larger and heavier. Eventually, they become too heavy for the air to hold, and they fall to the ground as precipitation — rain, snow, sleet, or hail. Whether it falls as liquid or solid depends on the temperature of the air it passes through. Snow forms when the air is below freezing the whole way down; rain forms when it is warm enough for any ice to melt before hitting the ground.

Once the water hits the ground, it collects in streams, rivers, lakes, and oceans — and the cycle starts all over again. Some water also soaks into the ground and slowly moves underground. No new water is being created, and no water is being destroyed. The same water just keeps cycling through different states and different places, powered by heat from the sun.