Water cycle, computer illustration. The natural water cycle (or hydrologic cycle) describes the continuous movement (arrows) of water on the Earth. The cycle begins (from left to right) with the evaporation of water from the surface of the oceans (lower left). The moist air cools as it rises and water vapour condenses to form clouds. Moisture is transported around the globe until it returns to the surface as precipitation. Once the water reaches the ground, some may evaporate back into the atmosphere, whilst more may penetrate the surface and become groundwater. 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.