Water cycle, illustration

Water cycle, illustration. The natural water cycle (or hydrologic cycle) describes the continuous movement of water on, above and below the surface of the Earth. The cycle begins with the evaporation of water from the surface of the oceans (right). As moist air is lifted, it cools and water vapour condenses to form clouds. Moisture is transported around the globe until it returns to the surface as precipitation (top left). Once the water reaches the ground, some of the water may evaporate back into the atmosphere. Another quantity of water may penetrate the surface and become groundwater (far left). The balance of water that remains on the Earth's surface is runoff, which empties into lakes, rivers and streams and is carried back to the oceans, where the cycle begins again. For a labelled version of this image see F032/9352., Photo by MARK GARLICK/SCIENCE PHOTO LIBRARY