Elevated portions of weather fronts (which are three-dimensional in nature) force broad areas of upward motion within the Earth's atmosphere which form clouds decks such as altostratus or cirrostratus. Water vapor normally begins to condense on condensation nuclei such as dust, ice, and salt in order to form clouds. The main ways water vapor is added to the air are wind convergence into areas of upward motion, precipitation or virga falling from above, daytime heating evaporating water from the surface of oceans, water bodies or wet land, transpiration from plants, cool or dry air moving over warmer water, and lifting air over mountains. Evaporative cooling occurs when moisture is added to the air through evaporation, which forces the air temperature to cool to its wet-bulb temperature, or until it reaches saturation.
Radiational cooling occurs due to the emission of infrared radiation, either by the air or by the surface underneath. Conductive cooling occurs when the air comes into contact with a colder surface, usually by being blown from one surface to another, for example from a liquid water surface to colder land. The air can rise due to convection, large-scale atmospheric motions, or a physical barrier such as a mountain ( orographic lift). Adiabatic cooling occurs when air rises and expands. There are four main mechanisms for cooling the air to its dew point: adiabatic cooling, conductive cooling, radiational cooling, and evaporative cooling. Streets in Tampere, Finland watered by night rain. 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. Rainfall amounts can be estimated by weather radar. Climate classification systems such as the Köppen classification system use average annual rainfall to help differentiate between differing climate regimes. The globally averaged annual precipitation over land is 715 mm (28.1 in), but over the whole Earth, it is much higher at 990 mm (39 in). Global warming is also causing changes in the precipitation pattern globally, including wetter conditions across eastern North America and drier conditions in the tropics.
The urban heat island effect leads to increased rainfall, both in amounts and intensity, downwind of cities. The movement of the monsoon trough, or intertropical convergence zone, brings rainy seasons to savannah climes. 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. 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. 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. The major cause of rain production is moisture moving along three-dimensional zones of temperature and moisture contrasts known as weather fronts.
It provides water for hydroelectric power plants, crop irrigation, and suitable conditions for many types of ecosystems. Rain is a major component of the water cycle and is responsible for depositing most of the fresh water on the Earth. Rain is water droplets that have condensed from atmospheric water vapor and then fall under gravity.
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