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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's surface 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. The scientific explanation of how rain forms and falls is called the Bergeron process.

Contents

Rain classification

Orographic rain (relief rain)

Convective rain

Frontal or cyclonic rain

Properties

Measuring rainfall

Effect on agriculture

Human influence

Culture

References

See also

External links

Rain classification

Rain plays a role 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 vapor from plant respiration also contributes to the moisture in the atmosphere.
Rain is classified by ''amount of precipitation'' and ''reason for precipitation''.
When classified according to amount of precipitation, rain can be divided into:^[1]

★ very light rain — when the precipitation rate is < 0.25 mm/hour

★ light rain — when the precipitation rate is between 0.25 mm/hour - 1.0mm/hour

★ moderate rain — when the precipitation rate is between 1.0 mm/hour - 4.0mm/hour

★ heavy rain — when the precipitation rate is between 4.0 mm/hour - 16.0mm/hour

★ very heavy rain — when the precipitation rate is between 16.0 mm/hour - 50mm/hour

★ extreme rain — when the precipitation rate is > 50.0 mm/hour
Based on the reason for precipitation, rain is classified into:

★ Orographic rain

★ Convective rain

★ Frontal or cyclonic rain

Orographic rain (relief rain)

Orographic rain (or relief rain) is caused when the warm moisture-laden wind blowing in to the land from the sea encounters a natural barrier such as mountains. This forces the wind to rise. With gain in altitude, the air expands dynamically due to a decrease in air pressure. Due to this the wind experiences a decrease in temperature (by adiabatic cooling), which results in the increase of the relative humidity. This causes condensation of water vapor into water droplets to form clouds. The relative humidity continues to increase until the dew point reaches the level of condensation, causing air to be saturated. This height where the condensation occurs is called the ''level of condensation''. When the cloud droplets become too heavy to be suspended, rain falls.
As the wind descends on the leeward side of the mountain range, it becomes compressed and warms; which results in the further decrease of the relative humidity of the wind, which is already dry after precipitating its moisture on the windward side of the mountain. Hence the leeward side of the mountains does not receive any rain from these winds and its called the rain shadow region of the mountains.
The Indian Ocean monsoon is a good example of orographic rain. About 80% of the rain that occurs in India is of this category.

Convective rain

Convective rain mainly occurs in the equatorial climatic regions and tropical climatic regions where it is very hot during the day. The rate of evaporation of moisture from the water bodies and respiration from the dense vegetation is very high. The evaporated moisture along with its hot surrounding air begins to ascend. With gain in altitude, the air expands dynamically due to a decrease in air pressure. Due to this the wind experiences a decrease in temperature (per adiabatic cooling), which results in the increase of the relative humidity. This causes condensation of water vapor into water droplets to form unstable towering cumulonimbus clouds. When the cloud droplets become too heavy to be suspended, rain falls.

Frontal or cyclonic rain

Frontal (or cyclonic) rain is caused by cyclonic activity and it occurs along the fronts of the cyclone. It is formed when two masses of air of different temperature, humidity and density meet, e.g., a meeting of moisture laden warm tropical wind with a polar air mass. A layer separating them is called the ''front''. This front has two parts — the ''warm front'' and the ''cold front''. At the warm front, the warm lighter air rises gently over the heavier cold air. As the warm air rises, it cools, and the moisture present in it condenses to form clouds — altostratus clouds. This rain falls steadily for a few hours to a few days.
At the cold front, the cold air forces the warm air to rise rapidly causing its moisture to condense quickly, which results in the formation of cumulonimbus clouds. The rainfall from these clouds is usually heavy and of short duration.

Properties

Falling raindrops are often depicted in cartoons 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.^[2] 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.^[3] 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.

Measuring rainfall

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.

Effect on agriculture

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.

Human influence

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.^[4]

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 . (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 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. Rain can be depressing to some people 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 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.
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,^[5] less than New York City with 1173 mm (46.2 inches),^[6] but has 201 cloudy days per year (compared to 152 in New York).^[7] 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 5080 mm (200 inches) per year.^[8]
Vancouver, British Columbia could be considered the world's capital of rain, despite having some snow during special periods, receiving as much as 40 mm at one time. Almost every day in the winter the Greater Vancouver Area is pummeled by rain.
Melbourne, Australia has a similar reputation, in comparison to Sydney, Australia; however, Sydney receives an average of 1094 mm (43.1 inches) of rain per year^[9] compared to Melbourne's 544 mm (21.4 inches).^[10] 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 8000mm (315 inches) per year, with over 12000mm (472 inches) of rain recorded in the year 2000.
Meanwhile, Bergen in Norway is one of the more famous European rain-cities with its yearly precipitation of 2250 mm (88 inches) on average.
Cherrapunji, situated on the southern slopes of the Eastern Himalaya in Shillong, India is one of the wettest places on Earth.

References

1. http://my.athenet.net/~multiplx/cgi-bin/pics/rain_rate.html
2. http://www.ems.psu.edu/~fraser/Bad/BadRain.html
3. http://www.wonderquest.com/falling-raindrops.htm
4. 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.
5. Monthly Averages for Seattle, WA. ''The Weather Channel''. Last accessed October 19, 2006.
6. Monthly Averages for New York, NY. ''The Weather Channel''. Last accessed October 19, 2006.
7. Cloudiness - Mean Number of Days. National Climatic Data Center. Last accessed October 19, 2006.
8. Average Annual Precipitation in Washington. Oregon Climate Service at Oregon State University. Last accessed October 19, 2006.
9. Averages for SYDNEY AIRPORT AMO. Australian Government Bureau of Meteorology. Last accessed October 19, 2006.
10. Averages for MELBOURNE AIRPORT. Australian Government Bureau of Meteorology. Last accessed October 19, 2006.

See also

★ Acid rain

★ Basic precipitation

★ Cherrapunji

★ Cloud

★ Mawsynram

★ Raining animals

★ Rain dancing

★ Rain sensor

★ Red rain in Kerala

★ Sanitary sewer overflow

★ Water cycle

★ Water resources

★ Weather

External links

★ What are clouds, and why does it rain?

★ BBC article on the weekend rain effect

★ BBC article on rain-making

★ 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.