The hydrologic cycle

Hydrology is the science concerned with the occurrence and distribution of water on and under the earth. Meteorology is the science dealing with the atmosphere and the movement of water, both as vapour and as liquid, in the air. Both sciences are concerned with the hydrologic cycle - the circulation of water from oceans through the atmosphere back to the oceans or to the land and thence to the oceans by overland and subterranean routes. Water is constantly on the move. Evaporation takes place from a multitude of surfaces ranging from a free water surface on the ocean to the moisture on a leaf. Precipitation falls in various forms - snow, rain, hail. Also, moisture is deposited on the surface of the earth by the formation of dew and frost. Water is stored for varying lengths of time in a number of forms including atmospheric moisture, water in swamps and lakes, soil moisture, groundwater, ice in glaciers, and snow on the ground. Water is transferred from one environment to another by surface runoff, infiltration from the surface, groundwater flow, and via atmospheric vapour carried by winds. The most important aspect of the hydrological cycle is not the quantity of water residing in the world's water bodies and atmosphere at any particular instant but rather the rates at which water is transported from one part of the cycle to another. Water is constantly on the move; constantly reacting with its physical, chemical, and biological environment; constantly changing its state, (liquid, vapour, and solid) and constantly reshaping the face of the earth and allowing life as we know it. The plate shows the movement of water from oceans through the atmosphere and returning to the oceans (or to the land) using overland and subterranean routes.