Rain
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Rain is water droplets that have condensed from atmospheric water vapor and then fall under gravity. Rain is a major component of the water cycle and is responsible for depositing most of the fresh water on the Earth. It provides water for hydroelectric power plants, crop irrigation, and suitable conditions for many types of ecosystems.
The major cause of rain production is moisture moving along three-dimensional zones of temperature and moisture contrasts known as
The
Formation
Water-saturated air
Air contains water vapor, and the amount of water in a given mass of dry air, known as the mixing ratio, is measured in grams of water per kilogram of dry air (g/kg).
There are four main mechanisms for cooling the air to its dew point: adiabatic cooling, conductive cooling, radiational cooling, and evaporative cooling.
The main ways water vapor is added to the air are wind convergence into areas of upward motion,
Coalescence and fragmentation
As these larger water droplets descend, coalescence continues, so that drops become heavy enough to overcome air resistance and fall as rain. Coalescence generally happens most often in clouds above freezing and is also known as the warm rain process.[20] In clouds below freezing, when ice crystals gain enough mass they begin to fall. This generally requires more mass than coalescence when occurring between the crystal and neighboring water droplets. This process is temperature dependent, as supercooled water droplets only exist in a cloud that is below freezing. In addition, because of the great temperature difference between cloud and ground level, these ice crystals may melt as they fall and become rain.[21]
Raindrops have sizes ranging from 0.1 to 9 mm (0.0039 to 0.3543 in) mean diameter but develop a tendency to break up at larger sizes. Smaller drops are called cloud droplets, and their shape is spherical. As a raindrop increases in size, its shape becomes more oblate, with its largest cross-section facing the oncoming airflow. Large rain drops become increasingly flattened on the bottom, like hamburger buns; very large ones are shaped like parachutes.[22][23] Contrary to popular belief, their shape does not resemble a teardrop.[24] The biggest raindrops on Earth were recorded over Brazil and the Marshall Islands in 2004 — some of them were as large as 10 mm (0.39 in). 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.[25]
Raindrops associated with melting hail tend to be larger than other raindrops.[26]
Intensity and duration of rainfall are usually inversely related, i.e., high-intensity storms are likely to be of short duration and low-intensity storms can have a long duration.[27][28]
Droplet size distribution
The final droplet size distribution is an exponential distribution. The number of droplets with diameter between and per unit volume of space is . This is commonly referred to as the Marshall–Palmer law after the researchers who first characterized it.[23][29] The parameters are somewhat temperature-dependent,[30] and the slope also scales with the rate of rainfall (d in centimeters and R in millimeters per hour).[23]
Deviations can occur for small droplets and during different rainfall conditions. The distribution tends to fit averaged rainfall, while instantaneous size spectra often deviate and have been modeled as gamma distributions.[31] The distribution has an upper limit due to droplet fragmentation.[23]
Raindrop impacts
Raindrops impact at their terminal velocity, which is greater for larger drops due to their larger mass-to-drag ratio. At sea level and without wind, 0.5 mm (0.020 in) drizzle impacts at 2 m/s (6.6 ft/s) or 7.2 km/h (4.5 mph), while large 5 mm (0.20 in) drops impact at around 9 m/s (30 ft/s) or 32 km/h (20 mph).[32]
Rain falling on loosely packed material such as newly fallen ash can produce dimples that can be fossilized, called raindrop impressions.[33] The air density dependence of the maximum raindrop diameter together with fossil raindrop imprints has been used to constrain the density of the air 2.7 billion years ago.[34]
The
The METAR code for rain is RA, while the coding for rain showers is SHRA.[37]
Virga
In certain conditions, precipitation may fall from a cloud but then evaporate or sublime before reaching the ground. This is termed virga and is more often seen in hot and dry climates.
Causes
Frontal activity
Stratiform (a broad shield of precipitation with a relatively similar intensity) and dynamic precipitation (convective precipitation which is showery in nature with large changes in intensity over short distances) occur as a consequence of slow ascent of air in
A wide variety of weather can be found along an occluded front, with thunderstorms possible, but usually, their passage is associated with a drying of the air mass. Occluded fronts usually form around mature low-pressure areas.[39] What separates rainfall from other precipitation types, such as ice pellets and snow, is the presence of a thick layer of air aloft which is above the melting point of water, which melts the frozen precipitation well before it reaches the ground. If there is a shallow near-surface layer that is below freezing, freezing rain (rain which freezes on contact with surfaces in subfreezing environments) will result.[40] Hail becomes an increasingly infrequent occurrence when the freezing level within the atmosphere exceeds 3,400 m (11,000 ft) above ground level.[41]
Convection
Orographic effects
Orographic precipitation occurs on the
In
In South America, the
Within the tropics
The wet, or rainy, season is the time of year, covering one or more months, when most of the average annual rainfall in a region falls. and vegetation grows significantly.
Tropical cyclones, a source of very heavy rainfall, consist of large air masses several hundred miles across with low pressure at the centre and with winds blowing inward towards the centre in either a clockwise direction (southern hemisphere) or counterclockwise (northern hemisphere).[58] Although cyclones can take an enormous toll in lives and personal property, they may be important factors in the precipitation regimes of places they impact, as they may bring much-needed precipitation to otherwise dry regions.[59] Areas in their path can receive a year's worth of rainfall from a tropical cyclone passage.[60]
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.[62] The urban heat island effect warms cities 0.6 to 5.6 °C (1.1 to 10.1 °F) above surrounding suburbs and rural areas. This extra heat leads to greater upward motion, which can induce additional shower and thunderstorm activity. Rainfall rates downwind of cities are increased between 48% and 116%. Partly as a result of this warming, monthly rainfall is about 28% greater between 32 and 64 km (20 and 40 mi) downwind of cities, compared with upwind.[63] Some cities induce a total precipitation increase of 51%.[64]
Increasing temperatures tend to increase evaporation which can lead to more precipitation. Precipitation generally increased over land north of 30°N from 1900 through 2005 but has declined over the tropics since the 1970s. Globally there has been no statistically significant overall trend in precipitation over the past century, although trends have varied widely by region and over time. Eastern portions of North and South America, northern Europe, and northern and central Asia have become wetter. The Sahel, the Mediterranean, southern Africa and parts of southern Asia have become drier. There has been an increase in the number of heavy precipitation events over many areas during the past century, as well as an increase since the 1970s in the prevalence of droughts—especially in the tropics and subtropics. Changes in precipitation and evaporation over the oceans are suggested by the decreased salinity of mid- and high-latitude waters (implying more precipitation), along with increased salinity in lower latitudes (implying less precipitation and/or more evaporation). Over the contiguous United States, total annual precipitation increased at an average rate of 6.1 percent since 1900, with the greatest increases within the East North Central climate region (11.6 percent per century) and the South (11.1 percent). Hawaii was the only region to show a decrease (−9.25 percent).[65]
Analysis of 65 years of United States of America rainfall records show the lower 48 states have an increase in heavy downpours since 1950. The largest increases are in the Northeast and Midwest, which in the past decade, have seen 31 and 16 percent more heavy downpours compared to the 1950s. Rhode Island is the state with the largest increase, 104%. McAllen, Texas is the city with the largest increase, 700%. Heavy downpour in the analysis are the days where total precipitation exceeded the top one percent of all rain and snow days during the years 1950–2014.[66][67]
The most successful attempts at influencing weather involve cloud seeding, which include techniques used to increase winter precipitation over mountains and suppress hail.[68]
Characteristics
Patterns
Rainbands are cloud and precipitation areas which are significantly elongated. Rainbands can be stratiform or convective,[69] and are generated by differences in temperature. When noted on weather radar imagery, this precipitation elongation is referred to as banded structure.[70] Rainbands in advance of warm occluded fronts and warm fronts are associated with weak upward motion,[71] and tend to be wide and stratiform in nature.[72]
Rainbands spawned near and ahead of
Once a cyclone occludes an
Rainbands within tropical cyclones are curved in orientation. Tropical cyclone rainbands contain showers and thunderstorms that, together with the eyewall and the eye, constitute a hurricane or tropical storm. The extent of rainbands around a tropical cyclone can help determine the cyclone's intensity.[79]
Acidity
The phrase acid rain was first used by Scottish chemist Robert Augus Smith in 1852.
Köppen climate classification
The Köppen classification depends on average monthly values of temperature and precipitation. The most commonly used form of the Köppen classification has five primary types labeled A through E. Specifically, the primary types are A, tropical; B, dry; C, mild mid-latitude; D, cold mid-latitude; and E, polar. The five primary classifications can be further divided into secondary classifications such as
Rain forests are characterized by high rainfall, with definitions setting minimum normal annual rainfall between 1,750 and 2,000 mm (69 and 79 in).
An oceanic (or maritime) climate is typically found along the west coasts at the middle latitudes of all the world's continents, bordering cool oceans, as well as southeastern Australia, and is accompanied by plentiful precipitation year-round.[88] The Mediterranean climate regime resembles the climate of the lands in the Mediterranean Basin, parts of western North America, parts of Western and South Australia, in southwestern South Africa and in parts of central Chile. The climate is characterized by hot, dry summers and cool, wet winters.[89] A steppe is a dry grassland.[90] Subarctic climates are cold with continuous permafrost and little precipitation.[91]
Pollution
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Measurement
Gauges
Rain is measured in units of length per unit time, typically in millimeters per hour,[94] or in countries where imperial units are more common, inches per hour.[95] The "length", or more accurately, "depth" being measured is the depth of rain water that would accumulate on a flat, horizontal and impermeable surface during a given amount of time, typically an hour.[96] One millimeter of rainfall is the equivalent of one liter of water per square meter.[97]
The standard way of measuring rainfall or snowfall is the standard rain gauge, which can be found in 100-mm (4-in) plastic and 200-mm (8-in) metal varieties.[98] The inner cylinder is filled by 25 mm (0.98 in) of rain, with overflow flowing into the outer cylinder. Plastic gauges have markings on the inner cylinder down to 0.25 mm (0.0098 in) resolution, while metal gauges require use of a stick designed with the appropriate 0.25 mm (0.0098 in) markings. After the inner cylinder is filled, the amount inside it is discarded, then filled with the remaining rainfall in the outer cylinder until all the fluid in the outer cylinder is gone, adding to the overall total until the outer cylinder is empty.[99] Other types of gauges include the popular wedge gauge (the cheapest rain gauge and most fragile), the tipping bucket rain gauge, and the weighing rain gauge.[100] For those looking to measure rainfall the most inexpensively, a can that is cylindrical with straight sides will act as a rain gauge if left out in the open, but its accuracy will depend on what ruler is used to measure the rain with. Any of the above rain gauges can be made at home, with enough know-how.[101]
When a precipitation measurement is made, various networks exist across the United States and elsewhere where rainfall measurements can be submitted through the Internet, such as
Remote sensing
One of the main uses of weather radar is to be able to assess the amount of precipitations fallen over large basins for hydrological purposes.[105] For instance, river flood control, sewer management and dam construction are all areas where planners use rainfall accumulation data. Radar-derived rainfall estimates complement surface station data which can be used for calibration. To produce radar accumulations, rain rates over a point are estimated by using the value of reflectivity data at individual grid points. A radar equation is then used, which is
Intensity
Rainfall intensity is classified according to the rate of precipitation, which depends on the considered time.[108] The following categories are used to classify rainfall intensity:
- Light rain — when the precipitation rate is < 2.5 mm (0.098 in) per hour
- Moderate rain — when the precipitation rate is between 2.5 mm (0.098 in) – 7.6 mm (0.30 in) or 10 mm (0.39 in) per hour[109][110]
- Heavy rain — when the precipitation rate is > 7.6 mm (0.30 in) per hour,[109] or between 10 mm (0.39 in) and 50 mm (2.0 in) per hour[110]
- Violent rain — when the precipitation rate is > 50 mm (2.0 in) per hour[110]
Terms used for a heavy or violent rain include gully washer, trash-mover and toad-strangler.[111] The intensity can also be expressed by rainfall erosivity R-factor[112] or in terms of the rainfall time-structure n-index.[108]
Return period
The average time between occurrences of an event with a specified intensity and duration is called the return period.[113] The intensity of a storm can be predicted for any return period and storm duration, from charts based on historic data for the location.[114] The return period is often expressed as an n-year event. For instance, a 10-year storm describes a rare rainfall event occurring on average once every 10 years. The rainfall will be greater and the flooding will be worse than the worst storm expected in any single year. A 100-year storm describes an extremely rare rainfall event occurring on average once in a century. The rainfall will be extreme and flooding worse than a 10-year event. The probability of an event in any year is the inverse of the return period (assuming the probability remains the same for each year).[113] For instance, a 10-year storm has a probability of occurring of 10 percent in any given year, and a 100-year storm occurs with a 1 percent probability in a year. As with all probability events, it is possible, though improbable, to have multiple 100-year storms in a single year.[115]
Forecasting
The Quantitative Precipitation Forecast (abbreviated QPF) is the expected amount of liquid precipitation accumulated over a specified time period over a specified area.
Forecast models show significant sensitivity to humidity levels within the planetary boundary layer, or in the lowest levels of the atmosphere, which decreases with height.[119] QPF can be generated on a quantitative, forecasting amounts, or a qualitative, forecasting the probability of a specific amount, basis.[120] Radar imagery forecasting techniques show higher skill than model forecasts within 6 to 7 hours of the time of the radar image. The forecasts can be verified through use of rain gauge measurements, weather radar estimates, or a combination of both. Various skill scores can be determined to measure the value of the rainfall forecast.[121]
Impact
Agricultural
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 and increase erosion,[122] while overly wet weather can cause harmful fungus growth.[123] Plants need varying amounts of rainfall to survive. For example, certain cacti require small amounts of water,[124] while tropical plants may need up to hundreds of inches of rain per year to survive.
In areas with wet and dry seasons, soil nutrients diminish and erosion increases during the wet season.[27] Animals have adaptation and survival strategies for the wetter regime. The previous dry season leads to food shortages into the wet season, as the crops have yet to mature.[125] Developing countries have noted that their populations show seasonal weight fluctuations due to food shortages seen before the first harvest, which occurs late in the wet season.[126] Rain may be harvested through the use of rainwater tanks; treated to potable use or for non-potable use indoors or for irrigation.[127] Excessive rain during short periods of time can cause flash floods.[128]
Culture and religion
Cultural attitudes towards rain differ across the world. In
Rain holds an important religious significance in many cultures.
Global climatology
Approximately 505,000 km3 (121,000 cu mi) of water falls as precipitation each year across the globe with 398,000 km3 (95,000 cu mi) of it over the oceans.[139] Given the Earth's surface area, that means the globally averaged annual precipitation is 990 mm (39 in). Deserts are defined as areas with an average annual precipitation of less than 250 mm (10 in) per year,[140][141] or as areas where more water is lost by evapotranspiration than falls as precipitation.[142]
Deserts
The northern half of Africa is dominated by the world's most extensive hot, dry region, the
Polar deserts
Since rain only falls as liquid, it rarely falls when surface temperatures are below freezing, unless there is a layer of warm air aloft, in which case it becomes
Rainforests
Rainforests are areas of the world with very high rainfall. Both tropical and temperate rainforests exist. Tropical rainforests occupy a large band of the planet mostly along the equator. Most temperate rainforests are located on mountainous west coasts between 45 and 55 degrees latitude, but they are often found in other areas.
Around 40–75% of all biotic life is found in rainforests. Rainforests are also responsible for 28% of the world's oxygen turnover.
Monsoons
The equatorial region near the Intertropical Convergence Zone (ITCZ), or monsoon trough, is the wettest portion of the world's continents. Annually, the rain belt within the tropics marches northward by August, then moves back southward into the Southern Hemisphere by February and March.[145] Within Asia, rainfall is favored across its southern portion from India east and northeast across the Philippines and southern China into Japan due to the monsoon advecting moisture primarily from the Indian Ocean into the region.[146] The monsoon trough can reach as far north as the 40th parallel in East Asia during August before moving southward thereafter. Its poleward progression is accelerated by the onset of the summer monsoon which is characterized by the development of lower air pressure (a thermal low) over the warmest part of Asia.[147][148] Similar, but weaker, monsoon circulations are present over North America and Australia.[149][150]
During the summer, the Southwest monsoon combined with
Impact of the Westerlies
Westerly flow from the mild north Atlantic leads to wetness across western Europe, in particular Ireland and the United Kingdom, where the western coasts can receive between 1,000 mm (39 in), at sea level and 2,500 mm (98 in), on the mountains of rain per year. Bergen, Norway is one of the more famous European rain-cities with its yearly precipitation of 2,250 mm (89 in) on average. During the fall, winter, and spring, Pacific storm systems bring most of Hawaii and the western United States much of their precipitation.[151] Over the top of the ridge, the jet stream brings a summer precipitation maximum to the Great Lakes. Large thunderstorm areas known as mesoscale convective complexes move through the Plains, Midwest, and Great Lakes during the warm season, contributing up to 10% of the annual precipitation to the region.[156]
The
Wettest known locations
Lloró, a town situated in Chocó, Colombia, is probably the place with the largest rainfall in the world, averaging 13,300 mm (523.6 in) per year.[165] The Department of Chocó is extraordinarily humid. Tutunendaó, a small town situated in the same department, is one of the wettest estimated places on Earth, averaging 11,394 mm (448.6 in) per year; in 1974 the town received 26,303 mm (86 ft 3.6 in), the largest annual rainfall measured in Colombia. Unlike Cherrapunji, which receives most of its rainfall between April and September, Tutunendaó receives rain almost uniformly distributed throughout the year.[166] Quibdó, the capital of Chocó, receives the most rain in the world among cities with over 100,000 inhabitants: 9,000 mm (354 in) per year.[165] Storms in Chocó can drop 500 mm (20 in) of rainfall in a day. This amount is more than what falls in many cities in a year's time.
Continent | Highest average | Place | Elevation | Years of record | ||
---|---|---|---|---|---|---|
in | mm | ft | m | |||
South America | 523.6 | 13,299 | Lloró, Colombia (estimated)[a][b] | 520 | 158[c] | 29 |
Asia | 467.4 | 11,872 | Mawsynram, India[a][d] | 4,597 | 1,401 | 39 |
Africa | 405.0 | 10,287 | Debundscha, Cameroon | 30 | 9.1 | 32 |
Oceania | 404.3 | 10,269 | Hawaii (US)[a]
|
5,148 | 1,569 | 30 |
South America | 354.0 | 8,992 | Quibdo , Colombia
|
120 | 36.6 | 16 |
Australia | 340.0 | 8,636 | Mount Bellenden Ker, Queensland | 5,102 | 1,555 | 9 |
North America | 256.0 | 6,502 | Hucuktlis Lake, British Columbia | 12 | 3.66 | 14 |
Europe | 183.0 | 4,648 | Crkvice, Montenegro | 3,337 | 1,017 | 22 |
Source (without conversions): Global Measured Extremes of Temperature and Precipitation, National Climatic Data Center. 9 August 2004.[167] |
Continent | Place | Highest rainfall | ||
---|---|---|---|---|
in | mm | |||
Highest average annual rainfall[168] | Asia | Mawsynram, India
|
467.4 | 11,870 |
Highest in one year[168] | Asia | Cherrapunji, India
|
1,042 | 26,470 |
Highest in one calendar month[169] | Asia | Cherrapunji, India | 366 | 9,296 |
Highest in 24 hours[168] | Indian Ocean | Foc Foc, La Réunion
|
71.8 | 1,820 |
Highest in 12 hours[168] | Indian Ocean | Foc Foc, La Réunion | 45.0 | 1,140 |
Highest in one minute[168] | North America | Unionville, Maryland, US | 1.23 | 31.2 |
See also
- Atmospheric river
- Blue roof
- Cistern
- Hydropower
- Intensity-duration-frequency curve
- Johad
- Petrichor – the cause of the scent during and after rain
- Precipitation types
- Rain dust
- Rain sensor
- Rain water harvesting
- Rainbow
- Raining animals
- Rainmaking
- Rainwater management
- Red rain in Kerala
- Sanitary sewer overflow
- Sediment precipitation
- Water resources
- Weather
Notes
- a b c The value given is the continent's highest, and possibly the world's, depending on measurement practices, procedures and period of record variations.
- ^ The official greatest average annual precipitation for South America is 900 cm (354 in) at Quibdó, Colombia. The 1,330 cm (523.6 in) average at Lloró [23 km (14 mi) SE and at a higher elevation than Quibdó] is an estimated amount.
- ^ Approximate elevation.
- Guinness Book of World Records.[170]
- Mount Snowdon, about 500 yards (460 m) from Glaslyn, is estimated to have at least 200.0 inches (5,080 mm) per year.
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External links
- BBC article on the weekend rain effect
- BBC article on rain-making
- BBC article on the mathematics of running in the rain
- What are clouds, and why does it rain?