Rain
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225809475
2008-07-15T14:39:19Z
ABlake
2388362
clarified and expanded idea
{{otheruses1|precipitation}}
{{Weather nav}}
'''Rain''' is a type of [[precipitation (meteorology)|precipitation]], a product of the condensation of atmospheric water vapor that is released on the Earth's surface. It forms when separate [[droplet|drops]] of [[water]] fall to the [[Earth]] from [[cloud]]s. Not all rain reaches the surface; some of it 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. Rain is the primary source of fresh water for most areas of the world, providing suitable conditions for diverse [[ecosystem|ecosystems]], as well as water for [[hydroelectricity|hydroelectric power plants]] and crop [[irrigation]]. The [[METAR]] code for rain is '''RA'''.
== Formation ==
Rain plays a role in the [[hydrologic cycle]] in which [[moisture]] from the [[ocean]]s evaporates, condenses into drops, precipitates (falls) from the sky, and eventually returns to the ocean via [[river]]s and [[stream]]s to repeat the cycle again. The [[water vapor]] 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.
===Human influence===
[[Image:Rain in Kolkata.jpg|thumb|right|250px|A view of rain falling on a street of [[Kolkata]], [[India]].]]
[[Image:TorrentialRainBallina.jpg|thumb|right|250px|A torrential downpour in [[Ballina]], [[Australia]].]]
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.<ref>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.</ref>
== Classifying the amount of rain ==
When classified according to amount of precipitation, rain can be divided into:<ref>{{cite web |title = Rain Rates |url = http://my.athenet.net/~multiplx/cgi-bin/pics/rain_rate.html |publisher = |accessdate = 2008-04-07
}}</ref>
* Very light rain — when the precipitation rate is < 0.25 mm/hour
* Light rain — when the precipitation rate is between 0.25 mm/hour - 1.0 mm/hour
* Moderate rain — when the precipitation rate is between 1.0 mm/hour - 4.0 mm/hour
* Heavy rain — when the precipitation rate is between 4.0 mm/hour - 16.0 mm/hour
* Very heavy rain — when the precipitation rate is between 16.0 mm/hour - 50 mm/hour
* Extreme rain — when the precipitation rate is > 50.0 mm/hour
== Properties ==
[[Image:GGB reflection in raindrops.jpg|Inverted images of the [[Golden Gate Bridge]] refracted through raindrops|thumb|250px|left]]
Falling raindrops are often depicted in popular culture as "teardrop-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 [[sphere|spherical]]. Larger ones become increasingly flattened on the bottom, like [[hamburger]] buns; very large ones are shaped like [[parachute]]s.<ref>{{cite web |title = Bad Meteorology: Raindrops are shaped like teardrops. |url = http://www.ems.psu.edu/~fraser/Bad/BadRain.html |publisher = Alistair B. Fraser |accessdate = 2008-04-07
}}</ref> The shape of raindrops was studied by [[Philipp Lenard]] in 1898. He found that small raindrops (less than about 2 mm diameter) are approximately spherical. As they get larger (to about 5 mm diameter) they become more doughnut shaped. Beyond about 5 mm they become unstable and fragment. 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. [[Image:Rain ot ocean beach.jpg|thumb|250px|right|Distant rain]]
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.<ref>{{cite web |title = Falling raindrops hit 5 to 20 mph speeds |url = http://www.wonderquest.com/falling-raindrops.htm |publisher = Weather Quest |accessdate = 2008-04-08
}}</ref> The sound of raindrops hitting water is caused by bubbles of air oscillating underwater. See [[Droplet#Sound|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]].
== Effect on agriculture ==
[[Precipitation (meteorology)|Precipitation]], especially rain, has a dramatic effect on [[agriculture]]. All [[plant]]s 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 [[plant]]s, too much or too little rainfall can be harmful, even devastating to [[Crop (agriculture)|crops]]. [[Drought]] can kill crops in massive numbers, while overly wet weather can cause disease and harmful [[fungus]]. [[Plant]]s need varying amounts of rainfall to survive. For example, [[cactus|cacti]] need small amounts of water while tropical plants may need up to hundreds of inches of rain per year to survive.
Agriculture of all nations at least to some extent is dependent on rain. [[India]]n [[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.
== Culture ==
Cultural attitudes towards rain differ across the world. In the largely [[temperate]] [[Europe]], rain metaphorically has a sad and negative connotation — reflected in children's rhymes like [[Rain Rain Go Away]] — in contrast to the bright and happy [[sun]]. Though the traditional notion of rain in the Western World is negative, rain can also bring joy, as some consider it to be soothing or enjoy the aesthetic appeal of it. In dry places, such as parts of [[Africa]], [[Australia]], [[India]], and the [[Middle East]], rain is greeted with [[wiktionary:Euphoria|euphoria]]. (In [[Botswana]], the [[Setswana]] word for rain, "pula," is used as [[Botswana pula|the name of the national currency]], in recognition of the economic importance of rain in this desert country.) [[Image:Umbrella with raindrops.jpg|thumb|right|250px|Rain on an umbrella from passing showers]]
Several cultures have developed means of dealing with rain and have developed numerous protection devices such as [[umbrella]]s and [[raincoat]]s, and diversion devices such as [[rain gutter|gutter]]s and [[storm drain]]s 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 [[thunderstorm]]s or is extremely heavy (as in a [[monsoon]]). Rain may be [[Rainwater harvesting|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 [[flood]]s.
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.
== World ==
=== Europe ===
In the [[United Kingdom]] most rain is driven into the country by the south-western trade winds following the warm [[gulf stream]] currents. Areas along the western coasts can receive between 1000 mm (40 inches, at sea-level) and 2500 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.
=== North America ===
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,<ref>{{cite web |title = Monthly Averages for Seattle, WA |url = http://www.weather.com/outlook/recreation/outdoors/wxclimatology/monthly/graph/USWA0395?from=search |publisher = [[The Weather Channel]] |accessdate = 2006-10-19
}}</ref> less than New York City's 1173 mm (46.2 inches),<ref>{{cite web |title = Monthly Averages for New York, NY |url = http://www.weather.com/outlook/recreation/outdoors/wxclimatology/monthly/graph/USNY0996?from=search |publisher = [[The Weather Channel]] |accessdate = 2006-10-19
}}</ref> 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 {{convert|45|in|mm}} a year. <ref>{{cite web |title = Cloudiness - Mean Number of Days |url = http://lwf.ncdc.noaa.gov/oa/climate/online/ccd/cldy.html |publisher = [[National Climatic Data Center]] |accessdate = 2006-10-19
}}</ref> 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.<ref>{{cite web |title = Average Annual Precipitation in Washington |url = http://www.ocs.orst.edu/pub/maps/Precipitation/Total/States/WA/wa.gif |publisher = Oregon Climate Service at [[Oregon State University]] |accessdate = 2006-10-19
}}</ref> The wettest city in the continental United States is [[Mobile, Alabama]], which average 67 inches of rainfall per year.<ref>{{cite web |title = Study Reveals Top 10 Wettest U.S. Cities |url = http://www.livescience.com/environment/070518_rainy_cities.html | Author = Andrea Thompson |publisher = Live Science |accessdate = 2008-04-07
}}</ref> Ketchikan Alaska and other locations in the temperate rainforest of southeast Alaska get an average of 160 inches of rain a year,<ref>{{cite web |title = Ketchikan and Southeast Alaska Weather Information |url = http://www.ketchikanalaska.com/ketchikan_alaska/weather.html |publisher = KetchikanAlaska.Com |accessdate = 2008-04-13}}</ref> sometimes receiving over 200 inches in a year.
=== Australia ===
Although [[Australia]] is the world's driest [[continent]], [[Mount 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.<ref>{{cite web |title = Significant Weather - DECEMBER 2000 (Rainfall) |url = http://www.bom.gov.au/inside/services_policy/public/sigwxsum/sigw1200.shtml#rain |publisher = Australian Government [[Bureau of Meteorology]] |accessdate = 2008-04-08
}}</ref>
[[Melbourne, Australia|Melbourne]] has a similar reputation to Vancouver's. In the popular imagination it is thought of as being much rainier than [[Sydney, Australia|Sydney]]; however, Sydney receives an average of 1094 mm (43.1 inches) of rain per year compared to Melbourne's 544 mm (21.4 inches). Sydney, meanwhile, experiences 53 fewer overcast days per year than Melbourne.<ref name="Sydney Airport">{{cite web |title = Averages for SYDNEY AIRPORT AMO |url = http://www.bom.gov.au/climate/averages/tables/cw_066037.shtml |publisher = Australian Government [[Bureau of Meteorology]] |accessdate = 2006-10-19
}}</ref><ref name="Melbourne Airport">{{cite web |title = Averages for MELBOURNE AIRPORT |url = http://www.bom.gov.au/climate/averages/tables/cw_086282.shtml |publisher = Australian Government [[Bureau of Meteorology]] |accessdate = 2006-10-19
}}</ref>
=== Asia ===
[[Cherrapunji]], situated on the southern slopes of the [[Himalaya|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.
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[[Image:Rain to clear skies panorama.jpg|thumb|700px|center|A panorama showing a rain cloud on the right]]
</div>
==Mythology==
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.
== See also ==
{{commons|Rain}}
*[[Acid rain]]
*[[Basic precipitation]]
*[[Cherrapunji]]
*[[Cloud]]
*[[Johad]]
*[[Mawsynram]]
*[[Meltwater]]
*[[Raining animals]]
*[[Rain dancing]]
*[[Rain sensor]]
*[[Red rain in Kerala]]
*[[Sanitary sewer overflow]]
*[[Water cycle]]
*[[Water resources]]
*[[Weather]]
*[[Yellow rain]]
== References ==
{{reflist}}
== External links ==
*[http://www.geography-site.co.uk/pages/physical/climate/why%20does%20it%20rain.html What are clouds, and why does it rain?]
*[http://news.bbc.co.uk/2/hi/sci/tech/146120.stm BBC article on the weekend rain effect]
*[http://news.bbc.co.uk/2/hi/asia-pacific/3893671.stm BBC article on rain-making]
*[http://www.earthsky.org/humanworld/interviews.php?id=49324 Do we have enough fresh water? Johan Rockstrom says we do, if we use it correctly.] Earth & Sky interview, discusses capturing rainfall and reducing runoff, partly through conservation tillage.
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