How the Water Cycle Works: Evaporation, Condensation and Precipitation

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Earth has a fixed supply of water that has been recycled for billions of years. The water cycle is how solar energy moves that water between oceans, air and land — and why the glass you drink from has probably passed through a cloud, a river and an ocean more times than you can count.

A Closed Loop Powered by the Sun

There is roughly the same amount of water on Earth today as there was when the dinosaurs walked — about 1.4 billion cubic kilometres of it. What changes is where that water sits and what form it takes. The water cycle (hydrological cycle) is the endless rearrangement of that fixed stock between three reservoirs: the oceans, the atmosphere and freshwater on land (rivers, lakes, ice and groundwater). The engine driving the whole system is solar energy, with gravity pulling water back down.

Evaporation and Transpiration: Water Goes Up

The sun warms the surface of oceans, lakes and rivers, and a small fraction of that liquid water gains enough energy to escape as invisible water vapour. Over land, plants do the same thing on purpose: they draw water up through their roots and release it from their leaves, a process called transpiration. Together, evaporation and transpiration are the main route by which water enters the air. Warm air can hold far more vapour than cold air, which is why humid tropical air is so water-rich.

Condensation: Vapour Becomes Cloud

As moist air rises it expands and cools, and once it drops to its dew point the vapour can no longer stay a gas. It condenses onto microscopic dust and salt particles (cloud condensation nuclei), forming tiny droplets or ice crystals — a cloud. The same physics is why a cold drink sweats, why dew forms on grass at dawn, and why your bathroom mirror fogs after a shower. Clouds are not floating water in the sci-fi sense; they are droplets so small they stay suspended in rising air.

Precipitation: Water Comes Down

When droplets inside a cloud collide and merge until they grow too heavy for the updraft to support, they fall as precipitation — rain, snow, sleet or hail, depending on the temperature profile below the cloud. Most precipitation that falls over the ocean returns there directly; what falls on land is where the interesting journeys begin. Understanding which clouds make rain is covered in our guide to cloud types and what they mean for weather.

On Land: Runoff, Infiltration and Storage

Falling water does several things at once. Some is caught by vegetation and evaporates on the spot. Some infiltrates the soil and percolates down to become groundwater, which can sit for years or millennia in aquifers before emerging in a spring or well. The rest flows over the surface as runoff, gathering into streams and rivers that carry it toward the sea — the slow, gravity-driven journey described in how a river forms. Ice sheets and glaciers are the cycle's pause button, locking freshwater away as solid ice for thousands of years.

Why It Never Really Ends

There is no start or finish to the water cycle — only continuous movement between states. Water molecules spend very different amounts of time in each reservoir: days in the atmosphere, weeks in a river, centuries in the deep ocean, and up to hundreds of thousands of years trapped in ice or groundwater. That is why the cycle is also a giant storage and sorting system, moving salt out of freshwater (evaporation leaves the salt behind) and spreading heat around the planet as it goes.

See also

• Why You Get Jet Lag: The Body Clock Explained
• Why We Have Seasons — It Is Not Because Earth Gets Closer to the Sun
• Why the Sky Is Blue and Sunsets Are Red: One Effect Explains Both
• Why the Sea Rises and Falls: How Tides Work
• Why Is the Sea Salty? (And Is It Getting Saltier?)
• Why Drizzle Is Not Just Light Rain: The Physics of Raindrops