Rain
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 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. [1] 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 [2].
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.[1]
Artificial rain
There is controversy over whether it is possible to induce artificial rain, a technique known as cloud seeding. According to this theory, when chemicals such as silver iodide or dry ice are shot into clouds they freeze individual droplets of moisture, which fall down to earth, melting as they near and so creating rain.
Culture
Cultural attitudes towards rain differ across the world. In the largely temperate Western world, rain traditionally 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.
The country noted throughout the world for its raininess is Great Britain. 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 sea-board can receive between 40 inches (sea-level) and 200 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 of the country having a lower rain fall average than Jerusalem.
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.
One city that is known for rain is Seattle, Washington, United States. However, the stereotype that it rains all the time is a myth. Contrary to popular belief, rain does not fall perpetually on the city year round. Rain is common in the winter, but mostly the climate is cloudy with little rain. Seattle's average rainfall is 37 inches per year, less than New York City, but 60% of its days are completely overcast (compared to 40% in New York). Melbourne, Australia has a similar reputation, in comparison to Sydney, Australia; but although the weather is generally colder and more overcast, it actually has less rain and fewer rainy days than Sydney.[citation needed]
References
- ^ 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.
See also
- Acid Rain
- Black Rain
- Climate
- Cloud
- Raining animals
- Rain sensor
- Red rain in Kerala
- 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.