The hydrosphere is the water layer on the surface of the earth e.g. oceans, lakes, rivers and other water bodies. Water covers 71 % of the total surface area of the earth. Of the total water, 97 % is in the oceans, 2 % is stored as ice-sheets and less than 1% is available as fresh water.
Differential heating by the sun is responsible for water circulation in the hydrosphere, like the circulation of air in the atmosphere. When the surface water in the oceans, lakes etc., is heated by the sunrays, evaporation happens and water vapour is added to the lower atmosphere. The water vapours in the atmosphere may get cooled, leading to their condensation into tiny droplets, which form clouds. Such clouds may cause precipitation as rainfall or snowfall. Rainfall on the land leads to surface run-off in the form of rivers, which reach the ocean finally. In this process, water is consumed by plants and animals in the biosphere. Water may be temporarily stored on the land or as underground water. This circulation of water among hydrosphere, atmosphere and lithosphere is called the hydrological cycle.
Water circulates both horizontally and vertically in the oceans. When the wind blows on the ocean surface, it drags the water in its direction resulting in the formation of currents and waves. The movement of ocean water also happens due to gravitational force of the moon and the sun, called the tides. Normally, tides occur twice a day at regular intervals. Oceans are teeming with plant and animal life. Oceans also have a moderating influence on the climate of coastal areas. Valuable reserves of oil have been tapped in off-shore areas e.g. The Bombay High, Gulf of Mexico.