Water covers about 70% of Earth’s surface, makes up about 70% of your mass,
and is essential for life. This photograph of planet Earth, taken by the Apollo 17 crew as they traveled toward the moon on December 7, 1972, shows an area of the planet from the Mediterranean Sea to Antarctica. Water is visible in this photograph as the Atlantic, Indian and Southern (or Antarctic) Oceans, the south polar icecap, and as heavy cloud cover in the southern hemisphere and scattered along the equator.
Water is the only substance that exists naturally on Earth in all three physical states of matter—gas, liquid, and solid—and
it is always on the move among them. The Earth has oceans of liquid water and polar regions covered by solid water. Energy from the sun is absorbed by liquid water in oceans, lakes, and rivers and gains enough energy for some of it to evaporate and enter the atmosphere as an invisible gas, water vapor. As the water vapor rises in the atmosphere it cools and condenses into tiny liquid droplets that scatter light and become visible as clouds. Under the proper conditions, these droplets further combine and become heavy enough to precipitate (fall out) as drops of liquid or, or if the air is cold enough, flakes of solid, thus returning to the surface of the Earth to continue this cycle of water between its condensed and vapor phases. (Visit http://www.eoearth.org/article/Hydrologic_cycle for more information on the Hydrologic Cycle.)
Water in all three states makes a large contribution to the planet’s climate. Water vapor is a greenhouse gas that traps energy radiated from the surface of the planet and helps to keep the planet warm enough to sustain the complex life that has evolved in this environment. Water vapor is responsible for more than half the Earth’s greenhouse gas warming. On the other hand, clouds and ice fields on the surface reflect a good deal of the radiation from the sun, so this radiation does not reach the surface and warm it. The reflectivity of clouds and ice has a cooling effect on the planet. However, where the earth’s surface has been heated by solar radiation, clouds help trap energy radiated from the heated surface and thus have a warming effect as well. Variations in the amount and form of water in the atmosphere have a complex relationship to our climate that is difficult to model and predict.