Rain is liquid water in the form of droplets that have condensed from atmospheric water vapor and then precipitated—that is, become heavy enough to fall under gravity. Rain is a major component of the water cycle and is responsible for depositing most of the fresh water on the Earth. It provides suitable conditions for many types of ecosystem, as well as water for hydroelectric power plants and crop irrigation.
The major cause of rain production is moisture moving along three-dimensional zones of temperature and moisture contrasts known as weather fronts. If enough moisture and upward motion is present, precipitation falls from convective clouds (those with strong upward vertical motion) such as cumulonimbus (thunder clouds) which can organize into narrow rainbands. In mountainous areas, heavy precipitation is possible where upslope flow is maximized within windward sides of the terrain at elevation which forces moist air to condense and fall out as rainfall along the sides of mountains. On the leeward side of mountains, desert climates can exist due to the dry air caused by downslope flow which causes heating and drying of the air mass. The movement of the monsoon trough, or intertropical convergence zone, brings rainy seasons to savannah climes.
The urban heat island effect leads to increased rainfall, both in amounts and intensity, downwind of cities. Global warming is also causing changes in the precipitation pattern globally, including wetter conditions across eastern North America and drier conditions in the tropics. Antarctica is the driest continent. The globally averaged annual precipitation over land is 715 millimetres (28.1 in), but over the whole Earth is it much higher at 990 millimetres (39 in). Climate classification systems such as the Köppen climate classification system use average annual rainfall to help differentiate between differing climate regimes. Rainfall is measured using rain gauges. Rainfall amounts can be estimated by weather radar.
Rain is also known or suspected on other planets, where it may be composed of methane, neon, sulfuric acid or even iron rather than water.
Read more about Rain: Forecasting, Global Climatology, Outside of Earth
Famous quotes containing the word rain:
“The mouth of the drowned dog. After long rain the land
Was sodden as the bed of an ancient lake,
Treed with iron and birdless.”
—Ted Hughes (b. 1930)
“The life of a good man will hardly improve us more than the life of a freebooter, for the inevitable laws appear as plainly in the infringement as in the observance, and our lives are sustained by a nearly equal expense of virtue of some kind. The decaying tree, while yet it lives, demands sun, wind, and rain no less than the green one. It secretes sap and performs the functions of health. If we choose, we may study the alburnum only. The gnarled stump has as tender a bud as the sapling.”
—Henry David Thoreau (1817–1862)
“Arise, my love, my fair one, and come away; for now the winter is past, the rain is over and gone. The flowers appear on the earth; the time of singing has come, and the voice of the turtledove is heard in our land. The fig tree puts forth its figs, and the vines are in blossom; they give forth fragrance. Arise, my love, my fair one, and come away.”
—Bible: Hebrew, Song of Solomon 2:10-13.