The hydrosphere

Approximately 70 percent of the terrestrial surface is covered by saltwater oceans, with a volume of about 1.4 billion cubic kilometres at an average temperature of about 4° C, not far above the freezing point of water. The oceans contain about 97 percent of the planet's water budget. The remaining 3 percent occurs as fresh waters, three-quarters of which are locked up in the form of ice at polar latitudes. Most of the remaining fresh water is groundwater held in soils and rocks; less than 1 percent of it occurs in lakes and rivers. In terms of percentage, atmospheric water vapour is negligible, but the transport of water evaporated from the oceans onto land surfaces is an integral part of the hydrologic cycle that renews and sustains the ecosphere.

The hydrologic cycle involves the transfer of water from the oceans through the atmosphere to the continents and back to the oceans over and beneath the land surface. The cycle involves processes such as precipitation, evaporation, interception, transpiration, infiltration, percolation, and runoff. These processes operate throughout the entire hydrosphere, which extends from about 15 kilometres into the atmosphere to roughly 5 kilometres into the crust.

About one-third of the solar energy that reaches the surface of the Earth is expended on evaporating ocean water. The resulting atmospheric moisture and humidity condense into clouds, rain, snow, and dew. Moisture is a crucial factor in determining weather. It is the driving force behind storms and is responsible for separating electrical charge, which is the cause of lightning. Moisture wets the land, replenishes subterranean aquifers, chemically weathers the rocks, erodes the landscape, nourishes life, and fills the rivers, which carry dissolved chemicals and sediments back into the oceans. For example, water plays a vital role in the carbon dioxide cycle. Calcium is weathered from continental rocks and is then returned to the oceans, where it combines to form calcium carbonates (such minerals constitute the shells of marine life). Eventually the carbonates are deposited on the seafloor and are lithified to form limestones. Some of these carbonate rocks are later dragged deep into the Earth's interior and melted, resulting in a rerelease of carbon dioxide (from volcanoes, for example) into the atmosphere. Cyclical processing of water, carbon dioxide, and oxygen through physical and biological systems on the Earth is fundamental to maintaining balance in the ecosphere.

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<== попередня сторінка	\|	наступна сторінка ==>
The atmosphere	\|	The lithosphere

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