The total mass of the Earth's hydrosphere is about 1.4 × 1018 tonnes, which is about 0.023% of the Earth's total mass. About 20 × 1012 tonnes of this is in the Earth's atmosphere (the volume of one tonne of water is approximately 1 cubic metre). Approximately 75% of the Earth's surface, an area of some 361 million square kilometers (139.5 million square miles), is covered by ocean. The average salinity of the Earth's oceans is about 35 grams of salt per kilogram of sea water (3.5%) 
Water cycle 
Energy from the sun, in the form of heat and light (insolation), causes evaporation from all wet surfaces including oceans, rivers, lakes, soil and the leaves of plants. Water vapor is further released as transpiration from vegetation and from humans and other animals.
Water is a basic necessity of life. It is inferred that in the solar system water in liquid form exists only on the Earth. Since 2/3 of the Earth is covered by water, the Earth is also called the blue planet and the watery planet. Hydrosphere plays an important role in the existence of the atmosphere in its present form. Oceans are important in this regard. When the Earth was formed it had only a very thin atmosphere rich in hydrogen and helium similar to the present atmosphere of mercury. Later the gases hydrogen and helium were expelled from the atmosphere. The gases and water vapor released at the time of the cooling of the Earth helped in the formation of the atmosphere into its present form. Various types of gases and water vapor released by the volcanoes also reached the atmosphere. When the Earth cooled due to continuous rains the water vapor also condensed and fell as rain. As carbon dioxide in the atmosphere started to dissolve in rain water, atmospheric temperature decreased considerably. This helped the water vapor to condense fast and fall as rain. It is this rain water that filled the depressions on the Earth's surface to result in the formations of the oceans. It is estimated that these oceans formed about 4000 million years ago. It is in the oceans that the first life forms took shape. They did not breath oxygen. Later, when cyanobacteria evolved, the process of conversion of carbon dioxide into food and oxygen began. As a result of the Earth's atmospheric composition became distinctly different from that of the other planets. It is this atmosphere of the Earth that is the basis for the survival of life on the Earth.
See also 
- Earth's atmosphere
- Table of global climate system components
- Water cycle
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