Rain is liquid precipitation, as opposed to other kinds of precipitation such as snow, hail and sleet. On Earth, it is the condensation of atmospheric water vapor into drops heavy enough to fall, often making it to the surface. Rain is the primary source of fresh water for most areas of the world, providing suitable conditions for diverse ecosystems, as well as water for hydroelectric power plants and crop irrigation. However, not all rain reaches the surface; some evaporates while falling through dry air. This is called virga, a phenomenon often seen in hot, dry desert regions. The METAR code for rain is RA.

Rain is also known or suspected on other worlds. On Titan, Saturn's largest moon, infrequent methane rain is thought to carve the moon's numerous surface channels. On Venus, sulfuric acid virga evaporates 25 km from the surface. There is likely to be rain of various compositions in the upper atmospheres of the gas giants, as well as precipitation of liquid neon and helium in the deep atmospheres.

Formation

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.

A scientific model known as the Bergeron process explains how rain forms and falls. More recent research points to the influence of Cloud condensation nuclei released as the result of biological processes.

Human influence

The fine particulate matter produced by car exhaust and other human sources of pollution forms 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.

Classifying the amount of rain

When classified according to the rate of precipitation, rain can be divided into:

• 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

Precipitation is measured using a Rain gauge.

Properties

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 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 due to their larger mass. 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. The sound of raindrops hitting water is caused by bubbles of air oscillating underwater. See droplet's sound.

Precipitation with low pH levels, or acid rain, is also a frequent risk produced by lightning. Distilled water, which contains no carbon dioxide, has a neutral pH of 7. Liquids with a pH less than 7 are acidic, and those with a pH greater than 7 are bases. “Clean” or unpolluted rain has a slightly acidic pH of about 5.2, because carbon dioxide and water in the air react together to form carbonic acid, a weak acid (pH 5.6 in distilled water), but unpolluted rain also contains other chemicals. Nitrous oxide present during thunderstorm phenomena, caused by the splitting of nitrogen molecules, can result in the production of acid rain, if nitrous oxide forms compounds with the water molecules in precipitation, thus creating acid rain. Acid rain can damage infrastructures containing calcite, or other solid carbon compounds. In ecosystems, acid rain can dissolve plant tissues of vegetations and increase acidification process in bodies of water, resulting in deaths of marine organisms.

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 per year 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.

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, the Western United States, 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 gutter 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.

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 in, at sea-level) and 2500 mm (100 in, on the mountains) of rain per year.

Meanwhile, Bergen in Norway is one of the more famous European rain-cities with its yearly precipitation of 2250 mm (88 in) 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 in) per year, less than New York City's 1173 mm (46.2 in), 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 1143 mm (45 in) a year. 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 3300 mm (about 130 in) per year. The wettest city in the 48 contiguous United States is Mobile, Alabama, which average 1702 mm (67 in) of rainfall per year. Ketchikan and other locations in the temperate rainforest of southeast Alaska get an average of 4064 mm (160 in) of rain a year, sometimes receiving over 5000 mm (about 200 inches) in a year.

Asia

[[File:Raindrops sizes.svg|thumb|A. Raindrops are not tear-shaped, as most people think.

B. Very small raindrops are almost spherical in shape.

C. Larger raindrops become flattened at the bottom, like that of a hamburger bun, due to air resistance.

D. Large raindrops have a large amount of air resistance, which makes them begin to become unstable.

E. Very large raindrops split into smaller raindrops due to air resistance.]]

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 in) in 1861.

The 38-year average at Mawsynram, Meghalaya, India is 11,873 mm (467.4 in). Like in Cherrapunji, Mawsynram's rainfall is concentrated in the monsoon season, while the rain at other famously humid places, like Tutunendo and Waialeale is more evenly distributed through the year. However, as there has been no meteorological department office, but a peon of the Meghalaya Public Works Department posted there in the area who takes the figure without supervision, Cherrapunji has been presented frequently as the wettest place in the world for statistical purposes. Doubts have been expressed by the knowledgeable about the rainfall data. In 1850, Joseph Dalton Hooker, a Royal Navy doctor turned naturalist, who spent the monsoon months at Cherrapunji, recorded such variations. "He was puzzled by the curiously localised patterns of rain; move your gauge a few hundred yards and it registered only half as much as before", writes Alexander Frater in his book, Chasing the Monsoon. This shows that recording the rainfall at the most humid places on earth has always been a difficult task for weathermen.

Year	Cherrapunji Rainfall (mm)	Mawsynram Rainfall (mm)
2002	12,262	11,300
2001	9,071	10,765
2000	11,221	13,561
1999	12,503	13,444
1998	14,536	16,090

South America

Lloró, a town situated in Chocó, Colombia, is probably the place with the largest measured rainfall in the world, averaging 13,300 mm per year (523.6 in). In fact, the whole Department of Chocó is extraordinarily humid. Tutunendo, a small town situated in the same department, is one of the wettest places on earth, averaging 11,394 mm per year (448 in); in 1974 the town received 26,303 mm (86 ft 3½ in), the largest annual rainfall measured in Colombia. Unlike Cherrapunji, which receives most of its rainfall between April and September, Tutunendo receives rain almost uniformly distributed throughout the year. The months of January and February have somewhat less frequent storms. On average, Tutunendo has 280 days with rainfall per year. Over ⅔ of the rain (68%) falls during the night. The average relative humidity is 90% and the average temperature is 26.4 °C. Quibdó, the capital of Chocó, receives the most rain in the world among cities with over 100,000 inhabitants: 9000 mm (29 ft 6 in) per year. Storms in Chocó can drop 500 mm (19.7 in) of rainfall in a day. This amount is more than falls in many cities in a year's time.

Oceania

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 in) per year, with over 12,000 mm (472 in) of rain recorded in the year 2000.

Melbourne has a similar reputation to Seattle's. In the popular imagination it is thought of as being much rainier than Sydney; however, Sydney receives an average of 1213 mm (47.8 in) of rain per year compared to Melbourne's 648 mm (25.5 in). Sydney, meanwhile, experiences 46 fewer overcast days per year than Melbourne, with 133 cloudy days, compared to Melbourne's 179.

Mount Waialeale on the island of Kauaʻi in the Hawaiian Islands averages more than 11,680 mm (38 ft 4 in or 460 in) of rain over the last 32 years, with a record 17,340 mm (683 in) in 1982. Its summit is considered one of the rainiest spots on earth. It has been promoted in tourist literature for many years as the wettest spot in the world.

See also

• Acid rain
• Cloud
• Johad
• Meltwater
• Rainbow
• Raining animals
• Rain dancing
• Rain sensor
• Red rain in Kerala
• Sanitary sewer overflow
• Sediment precipitation
• Water cycle
• Water resources
• Weather

References

1. 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.

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.