Water in motion

Water is constantly moving between the Earth and the atmosphere. The sun and wind evaporate water from soil, plant leaves and animals, and from the surfaces of rivers, lakes and oceans.

This process changes water to a gaseous form called water vapour. Water vapour condenses under certain conditions and falls back to earth as rain or snow. Precipitation that ultimately reaches streams and rivers, often transporting other material with it, is known as runoff.

Rain fills up rivers and lakes. Rivers flow into the oceans. Some precipitation penetrates the ground and forms groundwater. In this way water circulates constantly from the earth to the atmosphere and back again, in what is called the hydrological cycle .