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Rain

2007 Schools Wikipedia Selection. Related subjects: Climate and the Weather

   Rain falling
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   Rain falling

   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 called virga, a phenomenon often seen in hot,
   dry desert regions. The scientific explanation of how rain forms and
   falls is called the Bergeron process.

Rain in nature

   Rain in the north of Funen, Denmark
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   Rain in the north of Funen, Denmark

   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 0.27 mm or 0.01 in. It is sometimes expressed
   in litres per square metre (1 liter/m² = 1 mm).

   Falling raindrops are often depicted in cartoons or anime 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
   spherical. Larger ones become increasingly flattened on the bottom,
   like hamburger buns; very large ones are shaped like parachutes.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.

   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 .

   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.

   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.

Effect on agriculture

   Precipitation, especially rain, has a dramatic effect on agriculture.
   All but perhaps a couple of 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.

Human influence

   The fine particulate matter produced by car exhaust and other human
   sources of pollution form cloud condensation nuclei, leading to the
   production of clouds and increasing 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.

Culture

   Rain on an umbrella
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   Rain on an umbrella

   Cultural attitudes towards rain differ across the world. In the largely
   temperate Western world, 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, India, and the Middle East, rain
   is greeted with euphoria. (In Botswana, the Setswana word for rain,
   "pula," is used as the name of the national currency, in recognition of
   the economic importance of rain in this desert country.)

   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 rain is extremely heavy ( 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 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 sometimes heavy, rain is
   sometimes also seen as romantic. Rain can also and always will be to
   some people very depressing due to bleak clouds.

   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 (including those in Ireland)
   can receive between 40 inches (sea-level) and 100 inches (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 Beirutat between 450 and
   600mm per year.

   One city that is known for rain is Seattle, Washington, United States.
   Rain is common in the winter, but mostly the climate is cloudy with
   little rain. Seattle's average rainfall is 37.1 inches (942 mm) per
   year, less than New York City with 46.2 inches (1173 mm), but has 201
   cloudy days per year (compared to 152 in New York). 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 200 inches (5080 mm) per year.

   Melbourne, Australia has a similar reputation, in comparison to Sydney,
   Australia; however, Sydney receives an average of 43.1 inches (1094 mm)
   of rain per year compared to Melbourne's 21.4 inches (544 mm). Sydney,
   meanwhile, experiences 53 fewer overcast days per year than Melbourne.
   A panorama showing relatively clear skies on the left and a rain cloud
   on the right
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   A panorama showing relatively clear skies on the left and a rain cloud
   on the right
     * green rain or digital rain

   Retrieved from " http://en.wikipedia.org/wiki/Rain"
   This reference article is mainly selected from the English Wikipedia
   with only minor checks and changes (see www.wikipedia.org for details
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