Earth is the only planet in our Solar System on which liquid water is present. More than two-thirds of the Earth's surface is covered by water. In addition to the oceans, the hydrosphere consists of water in lakes and streams, subsurface water, the ice of glaciers and water vapor in the atmosphere.
About 13% of the Earth's sea surface is covered by ice. The amount of ice and its distribution in the polar regions influence how much energy the Earth absorbs from the Sun. This in turn, has an effect on Earth's climate.
Weddell Sea Ice
Radar image of a 45 kilometer x 18 kilometer area of seasonal sea-ice cover in the Weddell Sea, Antarctica. Most of the ice cover consists of smaller rounded ice floes visible as gray-blue circles. These data help scientists estimate the extent and thickness of ice cover.
Satellites have continuously monitored Arctic sea ice since 1979. Over the course of a year, the area covered by the ice changes. The ice melts down to its minimum every summer and builds back up again during the winter. Over the last three decades, scientists have observed a decline per decade of the minimum area and thickness of the sea ice.
In temperate regions, most precipitation in winter is as snow. In higher elevations, some snow may last through the summer months. The melting of this snow may provide water for irrigation and hydroelectric power.
Satellite observations of snow cover aid in predicting how much water will be available during the drier summer months in some areas.
This animation shows the average annual snow cover (1971-1995) and sea ice extent (1978-1995) for the northern hemisphere. Snow is shown in white and sea ice is dark blue. A variety of satellite data were used to create this data set, including data from GOES and AVHRR.
The water of the hydrosphere is in constant motion. Water evaporates from both the land and seas. The water vapor in the atmosphere condenses to form clouds and falls as rain or snow. That which falls on land is carried, eventually, to the sea to begin the cycle anew. This is the water, or hydrologic, cycle.