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Hydrosphere refers to that portion of Earth that is composed of water. The hydrosphere represents one component of Earth's system, operating in conjunction with the solid crust (lithosphere) and the air that envelopes the planet (atmosphere). The derivation of the term hydrosphere, from the Greek words for water and ball, is truly descriptive of our world, as it reflects the abundance and importance of water on Earth.

On Earth, water exists in the three primary states of matter; liquid, solid, and gas. The distance of Earth from the Sun, by fortunate coincidence, is such that the amount of energy arriving at the surface of most of the planet is sufficient to elevate the ambient temperature to levels above the freezing point of water, yet insufficient to cause all of the water to evaporate into the gaseous state. The capacity of water to store large quantities of heat energy heavily influences the nature of the global climate. The presence of large bodies of liquid water and the atmosphere restrict the range of temperature fluctuations on Earth. These conditions have allowed the existence of the fourth component of Earth's system, the biosphere.

Water is constantly being cycled through its various manifestations and through the components of Earth's systems by means of the hydrologic cycle. Driven by solar energy, water is evaporated from the ocean surface and distributed over the earth as water vapor. Precipitation returns the water, in liquid and solid forms, to other parts of the globe. Throughout the cycle, water may exist in a number of forms, interact with the atmosphere and lithosphere, or may be utilized by organisms within the biosphere.

One commonly cited statistic asserts that 71% of the surface area of our planet is covered by water, with the largest part covered by oceans. The total volume of sea-water, amounting to 97.2% of all the water on the planet, is 295,000,000 mi3 (1,230,000,000 km3). Usable freshwater constitutes less than 0.5% of all water on Earth. Water in all rivers, lakes and streams totals only 29,800 mi3 (124,200 km3). The amount of groundwater that is within 0.5 mi (0.8 km) of the surface is 960,000 mi3 (4,000,000 km3). Water also exists on Earth in the solid form as icecaps and glaciers, occupying a volume of 6,900,000 mi3 (28,600,000 km3). Straddling the division between hydrosphere and atmosphere is water vapor. A volume of 3,000 mi3 (12,700 km3) of water can be found in the atmosphere.

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Science EncyclopediaScience & Philosophy: Hydrazones to Incompatibility