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Overview |
 As seen from space, Earth has been described as a "blue marble," dominated by sapphire oceans and swirls of white clouds. Continents appear as widely spread outposts, covering less than one-third of the planet's surface. Earth is dominated by water in all its forms: liquid seas, vaporous clouds, and solid ice. The interplay among these forms is depicted as a "water cycle": a water molecule in today's ocean may be found in tomorrow's cloud, then in a glacier during the next Ice Age, and then in the ocean again as climate warms.
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The cycling of water and energy through the atmosphere and oceans is crucial to life on Earth. Yet the ties among the water cycle, Glossary Words |
| atmosphere: Gaseous layer surrounding a planet; the whole mass of air surrounding the earth.
climate: The prevailing or normal pattern of weather at a place, or in a region, averaged over a long period of time; in contrast to weather, which is the state of the atmosphere at a particular time. evaporation: The physical process of converting a liquid to a gas. Commonly considered to occur at a temperature below the boiling point of the liquid. fresh water: Non-saline water. model: System of data, inferences, and relationships, presented as a description of a process or entity. molecule: The simplest structural unit displaying the characteristic physical and chemical properties of a compound. precipitation: Water released from the atmosphere in the form of rain, snow, hail, or sleet from the atmosphere onto Earth's surface. runoff: The downward movement of surface water under gravity in channels ranging from small rills to large rivers. |
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