Water Cycle
The water cycle is the continuous movement of water between the oceans, atmosphere, and land through evaporation, condensation, precipitation, and runoff.
Definition
The water cycle is the continuous movement of water among the oceans, atmosphere, and land: the Sun evaporates water into the air, the vapor cools into clouds, the clouds release rain or snow, and the water flows back to rivers and oceans to start again. Water evaporates from oceans, lakes, and soil and transpires from plants, then returns as precipitation that becomes runoff, infiltrates as groundwater, is stored as ice, or evaporates once more, with oceans holding about $97\%$ of Earth's water and dominating global evaporation and precipitation. The global cycle moves roughly $500{,}000\ \text{km}^3$ of water through the atmosphere each year, with an average atmospheric residence time of only about $9$ days compared to thousands of years in the ocean and beyond $10{,}000$ years in deep groundwater, and warming intensifies the whole cycle because saturation vapor pressure rises about $7\%$ per degree Celsius.
Example
A puddle that disappears on a sunny day has not vanished, it evaporated and may return as rain somewhere else the next week, and rain falling on the Piedmont either runs off into the Catawba River system, soaks in to recharge wells and springs, or evaporates and transpires back into the air from forests and lawns. Stable isotope ratios in precipitation record its temperature and transport history, which is how ice cores reveal past climate, and atmospheric rivers can carry as much water as the Mississippi across the Pacific, delivering much of California's yearly rainfall in just a few events.
Key Insight
Earth has recycled the same water for billions of years, so the water in a glass today may once have been drunk by a dinosaur, and the cycle also acts like a giant heat pump, since evaporation absorbs energy at the ocean surface and condensation releases it high in the atmosphere, often thousands of kilometers away. Of all that water, only about $2.5\%$ is fresh, and most of that is locked in ice or deep aquifers, leaving rivers, lakes, and shallow groundwater as the small, fast-moving slice that is actually accessible to people.