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This article is about precipitation. For other uses, see Rain (disambiguation).
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Rain is a type of precipitation, a product of the condensation of atmospheric water vapor that is deposited on the Earth\'s surface. It forms when separate drops of water fall to the Earth from clouds. Not all rain reaches the surface; some evaporates while falling through dry air. When none of it reaches the ground, it is called virga, a phenomenon often seen in hot, dry desert regions.
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Rain plays a game in the hydrologic cycle in which moisture from the oceans evaporates, condenses into drops, precipitates (falls) from the sky, and eventually returns to the ocean via rivers and streams to repeat the cycle again. The water vapour from plant respiration also contributes to the moisture in the atmosphere.
A major scientific explanation of how rain forms and falls is called the Bergeron process. More recent research points to the influence of Cloud condensation nuclei released as the result of biological processes.
Distant rain
Based on the reason for precipitation, rain is classified into:
A view of rain falling on a street of Kolkata, India.
The fine particulate matter produced by car exhaust and other human sources of pollution form cloud condensation nuclei, leads to the production of clouds and increases the likelihood of rain. As commuters and commercial traffic cause pollution to build up over the course of the week, the likelihood of rain increases: it peaks by Saturday, after five days of weekday pollution has been built up. In heavily populated areas that are near the coast, such as the United States\' Eastern Seaboard, the effect can be dramatic: there is a 22% higher chance of rain on Saturdays than on Mondays.Cerveny, R. S., and R. C. Balling. Weekly cycles of air pollutants, precipitation and tropical cyclones in the coastal NW Atlantic region. Nature. 394, 561-563.
When classified according to amount of precipitation, rain can be divided into:Rain Rates
Rain falling
Raindrops impact at their terminal velocity, which is greater for larger drops. At sea level and without wind, 0.5 mm drizzle impacts at about 2 m/s, while large 5 mm drops impact at around 9 m/s.Falling raindrops hit 5 to 20 mph speeds The sound of raindrops hitting water is caused by bubbles of air oscillating underwater. See droplet\'s sound.
Generally, rain has a pH slightly under 6. This is because atmospheric carbon dioxide dissolves in the droplet to form minute quantities of carbonic acid, which then partially dissociates, lowering the pH. 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.
Rainfall in Chester County, Pennsylvania
Rainfall is typically measured using a rain gauge. It is expressed as the depth of water that collects on a flat surface, and is routinely measured with an accuracy up to 0.1 mm or 0.01 in. Rain gauges are usually placed at a uniform height above the ground, which may vary depending on the country. There are two types of gauges. Storage rain gauges are used to make daily or monthly measurements. Recording rain gauges measure the intensity of rainfall using a tipping bucket which will only tip when a certain volume of water is in it. An electrical switch can be used to record the tips.
Precipitation, especially rain, has a dramatic effect on agriculture. All plants need at least some water to survive, therefore rain (being the most effective means of watering) is important to agriculture. While a regular rain pattern is usually vital to healthy plants, too much or too little rainfall can be harmful, even devastating to crops. Drought can kill crops in massive numbers, while overly wet weather can cause disease and harmful fungus. Plants need varying amounts of rainfall to survive. For example, cacti need small amounts of water while tropical plants may need up to hundreds of inches of rain to survive.
Agriculture of all nations at least to some extent is dependent on rain. Indian agriculture, for example, (which accounts for 25 percent of the GDP and employs 70 percent of the nation\'s population) is heavily dependent on the rains, especially crops like cotton, rice, oilseeds and coarse grains. A delay of a few days in the arrival of the monsoon can, and does, badly affect the economy, as evidenced in the numerous droughts in India in the 90s.
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 that lead rains to sewers. Many people also prefer to stay inside on rainy days, especially in tropical climates where rain is usually accompanied by thunderstorms or is extremely heavy (as in a monsoon). Rain may be harvested, though rainwater is rarely pure (as acid rain occurs naturally), or used as greywater. Excessive rain, particularly after a dry period that has hardened the soil so that it cannot absorb water, can cause floods.
Many people find the scent during and immediately after rain especially pleasant or distinctive. The source of this scent is petrichor, an oil produced by plants, then absorbed by rocks and soil, and later released into the air during rainfall. Light or heavy rain is sometimes seen as romantic.
A country noted for its raininess is the United Kingdom. The reputation is partly deserved because of the frequency of rain driven into the country by the south-western trade winds following the warm gulf stream currents. Areas along the western coasts can receive between 1016 mm (40 inches, at sea-level) and 2540 mm (100 inches, on the mountains) of rain per year. However, what is less well known is that the eastern and southern half of the country is much drier, with the south east having a lower rainfall average than Jerusalem and Beirut at between 450 and 600 mm per year.
Meanwhile, Bergen in Norway is one of the more famous European rain-cities with its yearly precipitation of 2250 mm (88 inches) on average.
One city that is known for rain is Seattle, Washington. Rain is common in the winter, but mostly the climate is cloudy with little rain. Seattle\'s average rainfall is 942 mm (37.1 inches) per year,Monthly Averages for Seattle, WA. The Weather Channel. Last accessed October 19, 2006. less than New York City\'s 1173 mm (46.2 inches),Monthly Averages for New York, NY. The Weather Channel. Last accessed October 19, 2006. but Seattle has 201 cloudy days per year, compared to 152 in New York. Seattle\'s neighbor to the south, Portland, Oregon, gets more rain with an average of 45 inches (1,100 mm) a year. Cloudiness - Mean Number of Days. National Climatic Data Center. Last accessed October 19, 2006. However, it should be noted that Seattle lies in the rain shadow of the nearby Olympic Mountains, with some locations on the windward sides of the mountains receiving close to 130 inches per year.Average Annual Precipitation in Washington. Oregon Climate Service at Oregon State University. Last accessed October 19, 2006.. The wettest city in the continental United States is Mobile, Alabama, which average 67 inches of rainfall per year
Melbourne has a similar reputation to Vancouver\'s. In the popular imagination it is thought of as being much rainier than Sydney; however, Sydney receives an average of 1094 mm (43.1 inches) of rain per yearAverages for SYDNEY AIRPORT AMO. Australian Government Bureau of Meteorology. Last accessed October 19, 2006. compared to Melbourne\'s 544 mm (21.4 inches).Averages for MELBOURNE AIRPORT. Australian Government Bureau of Meteorology. Last accessed October 19, 2006. Sydney, meanwhile, experiences 53 fewer overcast days per year than Melbourne.
Although Australia is the world\'s driest continent, Mt Bellenden Ker in the north-east of the country records an average of 8000 mm (315 inches) per year, with over 12000 mm (472 inches) of rain recorded in the year 2000.
Cherrapunji, situated on the southern slopes of the Eastern Himalaya in Shillong, India is one of the wettest places on Earth, with an average annual rainfall of 11,430 mm (450 in). The highest recorded rainfall in a single year was 22,987 mm (904.9 inches) in 1861.
A panorama showing relatively clear skies on the left and a rain cloud on the right
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The Ancient Greeks believed that rain was a sign of the gods\' anger towards them. They thought that it symbolised drowning and frustration as it often disturbed what they were doing.
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