The heat of the sun vaporizes water from land and sea. Water vapor is given off also in the transpiration of plants and in the breathing of human beings and animals. Under certain conditions the water vapor in the atmosphere condenses and forms clouds. In clouds rain, snow, hail, or sleet sometimes form and, by the force of gravity, fall to earth.

Part of the water that falls on land flows along or near the surface of the ground to rivers, which carry it to the sea. This water, called run-off, is the chief cause of floods and erosion. Another part of the water is absorbed by the soil. Part of this water is evaporated by the sun, part is used by plants, and part seeps down to become groundwater.