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Rain falling
Rain falling
For other uses see Rain (disambiguation).

Rain is a form of precipitation, other forms of which include snow, sleet, hail, and dew. Rain forms when separate drops of water fall to the Earth's surface from clouds. Not all rain reaches the surface, however; some evaporates while falling through dry air. When none of it reaches the ground, it is a precipitation called virga.


Rain in nature

Rain plays a major role in the hydrologic cycle in which moisture from the oceans evaporates, condenses into clouds, precipitates back to earth, and eventually returns to the ocean via streams and rivers to repeat the cycle again. There is also a small amount of water vapor that respires from plants and evaporates to join other water molecules in condensing into clouds.

The amount of rainfall is measured using a rain gauge. It is expressed as the depth of water that collects on a flat surface, and can be measured to the nearest 0.25 mm or 0.01 in. It is sometimes expressed in litres per square metre (1 L/m² = 1 mm).

Rain drops on grass
Rain drops on grass

Falling raindrops are often depicted in cartoons or anime as "tear-shaped", round at the bottom and narrowing towards the top, but this is incorrect (only drops of water dripping from some sources are tear-shaped at the moment of formation). Small raindrops are nearly spherical. Larger ones become increasingly flattened, like hamburger buns; very large ones are shaped like parachutes. [1] On average, raindrops are 1 to 2 mm in diameter. The biggest raindrops on Earth were recorded over Brazil and the Marshall Islands in 2004 - some of them were as large as 10 mm. The large size is explained by condensation on large smoke particles or by collisions between drops in small regions with particularly high content of liquid water.

Generally, rain has a pH slightly under 6, simply from absorption of atmospheric carbon dioxide, which dissociates in the droplet to form minute quantities of carbonic acid. In some desert areas, airborne dust contains enough calcium carbonate to counter the natural acidity of precipitation, and rainfall can be neutral or even alkaline. Rain below pH 5.6 is considered acid rain.

Rain is said to be heavier immediately after a bolt of lightning. The cause of this phenomenon is traceable to the bipolar aspect of the water molecule. The intense electric and magnetic field generated by a lightning bolt forces many of the water molecules in the air surrounding the stroke to line up. These molecules then spontaneously create localized chains of water (similar to nylon or other 'poly' molecules). These chains then form water droplets when the electric/magnetic field is removed. These drops then fall as intensified rain.


Rain on an umbrella
Rain on an umbrella

Cultural attitudes towards rain differ across the world. In the largely temperate Western world, rain traditionally has a sad and negative connotation - reflected in children's rhymes like Rain Rain Go Away - in contrast to the bright and happy sun. In dry places such as India, the rain is greeted with euphoria.

Several cultures have developed means of dealing with rain and have developed numerous protection devices such as umbrellas and raincoats, and diversion devices such as gutters and storm drains. Many people also prefer to stay inside on rainy days, especially in tropical climates where rain is usually accompanied by thunderstorms or rain is extremely heavy (monsoon). Rain may be collected for drinking water, or used as greywater. Excessive rain, particularly after a dry period has hardened the soil so that it cannot absorb water, can cause floods.

Many people find the scent smelt during and immediately after rain especially pleasant or distinctive. The source of this smell is petrichor, an oil produced by plants, then absorbed by rocks and soil, and later released into the air during rainfall.

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