Rain water is liquid water in the form of droplets that have condensed from the water vapor in the atmosphere and then become heavy enough to fall under the action of gravity. Rainwater is the main component of water cycle and the cause of most of the fresh water deposition on the earth. It provides suitable conditions for various types of ecosystems, as well as for water for hydroelectric power plants and crop irrigation.

The main reason for rain is the movement of water along the three-dimensional region of temperature and water contrast, which is called the weather front. If there is enough water and upward movement, precipitation will form from convective clouds (precipitation with strong vertical movement), such as cumulonimbus clouds (thunderstorms), which will be organized into narrow rain belts. In mountainous areas, there may be a large amount of precipitation. In windward areas of high altitude areas, the flow on the uphill is the largest, which forces humid air to condense and rain along the hillside. On the leeward side of the mountain, the downhill flow leads to dry and dry air, which leads to desert climate and the heating and drying of air masses. The movement of monsoon trough or tropical convergence zone brings rainy season to savanna climate.

Urban heat island effect results in downwind increased rainfall, both in quantity and intensity. Global warming also causes changes in the pattern of global precipitation, including wetter conditions in eastern North America and drier conditions in the tropics. Antarctica is the driest continent. The global average annual precipitation on land is 715 mm (28.1 in.), but higher on the whole planet, 990 mm (39 in.). Climate classification system (e.g. K? Ppen classification system) uses annual average rainfall to help distinguish different climate systems. Rainfall is measured with a rain gauge. Rainfall can be estimated by weather radar.

Rain is known or suspected on other planets, where it may consist of methane, neon, sulfuric acid or even iron rather than water.

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