How does the water cycle work?

Solar energy evaporates water from oceans, lakes, and soil, and plants release additional water vapor through transpiration, a combined process called evapotranspiration. Rising air cools, water vapor condenses onto tiny particles to form cloud droplets, and when those droplets grow heavy enough they fall as rain, snow, sleet, or hail. Precipitation either runs off into streams and rivers heading back to the ocean, infiltrates the soil to recharge groundwater, or is intercepted by plants and evaporates again. Ice and snow store water on longer timescales in glaciers and ice sheets.

The residence times differ enormously, which is the part that gives the cycle its character. A water molecule stays in the atmosphere for around nine days on average, in a river for weeks, in a large lake for years, in the deep ocean for centuries to millennia, and locked in an ice sheet or deep aquifer for tens of thousands of years. That last point matters practically: groundwater in some major aquifers is effectively fossil water, recharging far more slowly than it is being extracted, which makes heavy pumping a form of mining rather than a renewable use.

Two consequences worth understanding. The oceans hold about 97 percent of Earth's water, and of the small fresh remainder the majority is locked in ice and groundwater, leaving a very small fraction in the lakes and rivers most people rely on. And a warmer atmosphere holds more water vapor, roughly seven percent more per degree Celsius, which is a large part of why climate change is expected to intensify both heavy rainfall events and drought: more water evaporates from soils, and when precipitation does arrive it arrives in heavier bursts.