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,