As you know, water covers nearly three quarters of the Earth's total surface. To be more specific, it is estimated that the Earth contains about 1.46 million cubic km of water. Of this 97.3 per cent is in the oceans and inland seas. The rest 2.7 per cent is found as glaciers and ice caps, fresh water lakes, rivers. Beside this surface water, there is some amount of underground water.
And all the water, from the oceans to the string-like streams and the water that is dug out from beneath, available on the Earth, is called hydrosphere.
Water follows a cycle. Rain falls. Some of it is absorbed by the soil while some evaporates. But most of it moves across the surface of the land, first as brooks, then streams and rivers. The rivers cut and broaden their valleys on the way to the sea. As water follows a downward course due to earth's attraction, it reaches ultimately to the seas.
As water from these bodies, gets evaporated, heated by the sun, it goes up to form clouds. Clouds are also formed when warm air blows over colder land masses or water. Or, when it travels over a colder air mass, or, when it moves up high mountainous slope. However, its only the rain clouds that cause downpour. Rain, snow, fog, mist, and dew, are formed moistures in the air condenses. And it condenses when the air is warm enough to hold the evaporated moistures. Such condensed vapor forms dew, or, sometimes fog, with a sudden drop in temperature. Condensation also occurs when the air rises high enough and the temperature suddenly drops. At that time it becomes too heavy to hold condensed water vapor. This condensed vapor comes down as droplets of rain or crystalises as snow.
Oceans and Seas:
The waters of oceans and seas are always in a great tireless motion and thus causing commotion. Surging, swelling, rolling, breaking, swirling and dancing in rhymes of peaks and troughs. It is the heat from the sun that causes ocean fuss round the clock.
When the sun warms up the water in the equatorial regions water expands and swells up a few inches. But water always finds for a lower level. Thus the extra rise in water level at the Equator cannot hold on. It soon rushes down in two directions - the North Pole and the South Pole.
But this motion is general. Besides this the waves vary in height and length from one region to another. This is mainly because of the variations in the blowing of the wind. The force, the duration, and area across which the wind travels controls the waves.
Again the water in the polar region is heavy because of the cold. So it sinks below the upcoming warm water and slowly crawls along the bottom to the Equatorial region to be warmed. While these two directional flow goes on, the Earth's daily rotation causes more complication.
Oceans merge into one another in such a smooth way that it is hard to demarcate them. However, the oceans come under four names. The Pacific, the largest of all. The Atlantic, Indian, and Arctic - the frozen ocean, come respectively according to their size.
These oceans, by definition, include the seas, bays, gulfs and other ocean inlets attached to them. Some of the major seas include the South China Sea, the world's largest. As also Caribbean Sea, the Mediterranean, to name a few.
Rivers, lakes and ponds:
Among the rivers the Amazon in South America is the world's largest. Others include the Nile of Egypt, Mississippi-Missouri, of United States, and Volga in the CIS.
The lakes may be of sweet water type or of salt water. Sweet water natural lakes re numerous and include the Lake Superior of North America and the Titicaca of South America. The Chilka in India is an example of a salt water lake.
Some of the water that falls on the land as rain evaporates. Some is absorbed by the soil and rocks.Water in the soil and in the pores and cracks of rocks below the surface is called ground water. Most ground water is rain water containing dissolved carbon dioxide and oxygen. There is a little which was trapped in the original sediments and a very small amount comes up from the molten lavas of magmas from deep inside the Earth.