Moisture moving along three-dimensional zones of temperature and moisture contrasts known as weather fronts is the major method of rain production. If enough moisture and upward motion is present, precipitation falls from convective clouds (those with strong upward vertical motion) such as cumulonimbus (thunderstorms) which can organize into narrow rainbands. In mountainous areas, heavy precipitation is possible where upslope flow is maximized within windward sides of the terrain at elevation which forces moist air to condense and fall out as rainfall along the sides of mountains. On the leeward side of mountains, desert climates can exist due to the dry air caused by downslope flow which causes heating and drying of the air mass. The movement of the monsoon trough, or intertropical convergence zone, brings rainy seasons to savannah climes. Rain is the primary source of freshwater for most areas of the world, providing suitable conditions for diverse ecosystems, as well as water for hydroelectric power plants and crop irrigation. Rainfall is measured through the use of rain gauges. Rainfall amounts are estimated actively by weather radar and passively by weather satellites.
The urban heat island effect leads to increased rainfall, both in amounts and intensity, downwind of cities. Global warming is also causing changes in the precipitation pattern globally, including wetter conditions across eastern North America and drier conditions in the tropics. Precipitation is a major component of the water cycle, and is responsible for depositing most of the fresh water on the planet. The globally-averaged annual precipitation is 990 millimetres (39 in). Climate classification systems such as the Köppen climate classification system use average annual rainfall to help differentiate between differing climate regimes. Antarctica is the Earth's driest continent. Rain is also known or suspected on other worlds, composed of methane, iron, neon, and sulfuric acid rather than water.
Contents
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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).[1][2] The amount of moisture in air is also commonly reported as relative humidity; which is the percentage of the total water vapor air can hold at a particular air temperature.[3] How much water vapor a parcel of air can contain before it becomes saturated (100% relative humidity) and forms into a cloud (a group of visible and tiny water and ice particles suspended above the Earth's surface)[4] depends on its temperature. Warmer air can contain more water vapor than cooler air before becoming saturated. Therefore, one way to saturate a parcel of air is to cool it. The dew point is the temperature to which a parcel must be cooled in order to become saturated.[5]There are four main mechanisms for cooling the air to its dew point: adiabatic cooling, conductive cooling, radiational cooling, and evaporative cooling. Adiabatic cooling occurs when air rises and expands.[6] The air can rise due to convection, large-scale atmospheric motions, or a physical barrier such as a mountain (orographic lift). Conductive cooling occurs when the air comes into contact with a colder surface,[7] usually by being blown from one surface to another, for example from a liquid water surface to colder land. Radiational cooling occurs due to the emission of infrared radiation, either by the air or by the surface underneath.[8] Evaporative cooling occurs when moisture is added to the air through evaporation, which forces the air temperature to cool to its wet-bulb temperature, or until it reaches saturation.[9]
The main ways water vapor is added to the air are: wind convergence into areas of upward motion,[10] precipitation or virga falling from above,[11] daytime heating evaporating water from the surface of oceans, water bodies or wet land,[12] transpiration from plants,[13] cool or dry air moving over warmer water,[14] and lifting air over mountains.[15] Water vapor normally begins to condense on condensation nuclei such as dust, ice, and salt in order to form clouds. Elevated portions of weather fronts (which are three-dimensional in nature)[16] force broad areas of upward motion within the Earth's atmosphere which form clouds decks such as altostratus or cirrostratus.[17] Stratus is a stable cloud deck which tends to form when a cool, stable air mass is trapped underneath a warm air mass. It can also form due to the lifting of advection fog during breezy conditions.[18]
Coalescence
Raindrops have sizes ranging from 0.1 millimetres (0.0039 in) to 9 millimetres (0.35 in) mean diameter, above which they tend to break up. 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.[21] Contrary to popular belief, their shape does not resemble a teardrop.[22] The biggest raindrops on Earth were recorded over Brazil and the Marshall Islands in 2004 — some of them were as large as 10 millimetres (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.[23]
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.[24][25] Rain drops associated with melting hail tend to be larger than other rain drops.[26] 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 millimetres (0.020 in) drizzle impacts at 2 metres per second (4.5 mph), while large 5 millimetres (0.20 in) drops impact at around 9 metres per second (20 mph).[27] The sound of raindrops hitting water is caused by bubbles of air oscillating underwater.[28][29] The METAR code for rain is RA, while the coding for rain showers is SHRA.[30]
Causes
Frontal activity
Main article: Weather fronts
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 synoptic systems (on the order of cm/s), such as in the vicinity of cold fronts and near and poleward of surface warm fronts. Similar ascent is seen around tropical cyclones outside of the eyewall, and in comma-head precipitation patterns around mid-latitude cyclones.[31] 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.[17] 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.[32] Hail becomes an increasingly infrequent occurrence when the freezing level within the atmosphere exceeds 11,000 feet (3,400 m) above ground level.[33]Convection
Orographic effects
Main articles: Orographic lift, Precipitation types (meteorology), and United States rainfall climatology
In Hawaii, Mount Waiʻaleʻale, on the island of Kauai, is notable for its extreme rainfall, as it has the second highest average annual rainfall on Earth, with 460 inches (12,000 mm).[37] Systems known as Kona storms affect the state with heavy rains between October and April.[38] Local climates vary considerably on each island due to their topography, divisible into windward (Koʻolau) and leeward (Kona) regions based upon location relative to the higher mountains. Windward sides face the east to northeast trade winds and receive much more rainfall; leeward sides are drier and sunnier, with less rain and less cloud cover.[39]
In South America, the Andes mountain range blocks Pacific moisture that arrives in that continent, resulting in a desertlike climate just downwind across western Argentina.[40] The Sierra Nevada range creates the same effect in North America forming the Great Basin and Mojave Deserts.[41][42]
Within the tropics
See also: Monsoon and Tropical cyclone
Main article: Wet season
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.[43] The term green season is also sometimes used as a euphemism by tourist authorities.[44] Areas with wet seasons are dispersed across portions of the tropics and subtropics.[45] Savanna climates and areas with monsoon regimes have wet summers and dry winters. Tropical rainforests technically do not have dry or wet seasons, since their rainfall is equally distributed through the year.[46] Some areas with pronounced rainy seasons will see a break in rainfall mid-season when the intertropical convergence zone or monsoon trough move poleward of their location during the middle of the warm season.[24] When the wet season occurs during the warm season, or summer, rain falls mainly during the late afternoon and early evening hours. The wet season is a time when air quality improves,[47] freshwater quality improves,[48][49] 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 counter clockwise (northern hemisphere).[50] 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.[51] Areas in their path can receive a year's worth of rainfall from a tropical cyclone passage.[52]
Human influence
See also: Global warming and Urban heat island
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.[53] The urban heat island effect warms cities 0.6 °C (1.1 °F) to 5.6 °C (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 20 miles (32 km) to 40 miles (64 km) downwind of cities, compared with upwind.[54] Some cities induce a total precipitation increase of 51%.[55]
The most successful attempts at influencing weather involve cloud seeding which include techniques used to increase winter precipitation over mountains and suppress hail.[57]
Characteristics
Patterns
Main article: Rainband
Rainbands are cloud and precipitation areas which are significantly elongated. Rainbands can be stratiform or convective,[58] and are generated by differences in temperature. When noted on weather radar imagery, this precipitation elongation is referred to as banded structure.[59] Rainbands in advance of warm occluded fronts and warm fronts are associated with weak upward motion,[60] and tend to be wide and stratiform in nature.[61]Rainbands spawned near and ahead of cold fronts can be squall lines which are able to produce tornadoes.[62] Rainbands associated with cold fronts can be warped by mountain barriers perpendicular to the front's orientation due to the formation of a low-level barrier jet.[63] Bands of thunderstorms can form with sea breeze and land breeze boundaries, if enough moisture is present. If sea breeze rainbands become active enough just ahead of a cold front, they can mask the location of the cold front itself.[64]
Once a cyclone occludes, a trough of warm air aloft, or "trowal" for short, will be caused by strong southerly winds on its eastern periphery rotating aloft around its northeast, and ultimately northwestern, periphery (also known as the warm conveyor belt), forcing a surface trough to continue into the cold sector on a similar curve to the occluded front. The trowal creates the portion of an occluded cyclone known as its comma head, due to the comma-like shape of the mid-tropospheric cloudiness that accompanies the feature. It can also be the focus of locally heavy precipitation, with thunderstorms possible if the atmosphere along the trowal is unstable enough for convection.[65] Banding within the comma head precipitation pattern of an extratropical cyclone can yield significant amounts of rain.[66] Behind extratropical cyclones during fall and winter, rainbands can form downwind of relative warm bodies of water such as the Great Lakes. Downwind of islands, bands of showers and thunderstorms can develop due to low level wind convergence downwind of the island edges. Offshore California, this has been noted in the wake of cold fronts.[67]
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.[68]
Acidity
See also: Acid rain
The pH of rain varies, especially due to its origin. On Americas East Coast, rain that is derived from the Atlantic Ocean typically has a pH of 5.0-5.6; rain that comes across the continental from the west has a pH of 3.8-4.8; and local thunderstorms can have a pH as low as 2.0.[69] Rain becomes acidic primarily due to the presence of two strong acids, sulfuric acid (H2SO4) and nitric acid (HNO3). Sulfuric acid is derived from natural sources such as volcanoes, and wetlands (sulfate reducing bacteria); and anthropogenic sources such as the combustion of fossil fuels, and mining where H2S is present. Nitric acid is produced by natural sources such as lightning, soil bacteria, and natural fires; while also produced anthropogenically by the combustion of fossil fuels and from power plants. In the past 20 years the concentrations of nitric and sulfuric acid has decreased in presence of rainwater, which may be due to the significant increase in ammonium (most likely as ammonia from livestock production), which acts as a buffer in acid rain and raising the pH.[70]Köppen climate classification
Main article: 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 forest, monsoon, tropical savanna, humid subtropical, humid continental, oceanic climate, Mediterranean climate, steppe, subarctic climate, tundra, polar ice cap, and desert.Rain forests are characterized by high rainfall, with definitions setting minimum normal annual rainfall between 1,750 millimetres (69 in) and 2,000 millimetres (79 in).[72] A tropical savanna is a grasslandbiome located in semi-arid to semi-humid climate regions of subtropical and tropical latitudes, with rainfall between 750 millimetres (30 in) and 1,270 millimetres (50 in) a year. They are widespread on Africa, and are also found in India, the northern parts of South America, Malaysia, and Australia.[73] The humid subtropical climate zone where winter rainfall is associated with large storms that the westerlies steer from west to east. Most summer rainfall occurs during thunderstorms and from occasional tropical cyclones.[74] Humid subtropical climates lie on the east side continents, roughly between latitudes 20° and 40° degrees away from the equator.[75]
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.[76] 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.[77] A steppe is a dry grassland.[78] Subarctic climates are cold with continuous permafrost and little precipitation.[79]
Measurement
Gauges
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 CoCoRAHS or GLOBE.[84][85] If a network is not available in the area where one lives, the nearest local weather or met office will likely be interested in the measurement.[86]
One millimeter of rainfall is the equivalent of one liter of water per square meter. This makes computing the water requirements of crops simple.[87]
Remote sensing
See also: Weather radar
- Z = ARb,
Intensity
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- Light rain — when the precipitation rate is < 2.5 millimetres (0.098 in) per hour
- Moderate rain — when the precipitation rate is between 2.5 millimetres (0.098 in) - 7.6 millimetres (0.30 in) or 10 millimetres (0.39 in) per hour[91][92]
- Heavy rain — when the precipitation rate is > 7.6 millimetres (0.30 in) per hour,[91] or between 10 millimetres (0.39 in) and 50 millimetres (2.0 in) per hour[92]
- Violent rain — when the precipitation rate is > 50 millimetres (2.0 in) per hour[92]
Return period
See also: 100-year flood
The likelihood or probability of an event with a specified intensity and duration, is called the return period or frequency.[93] The intensity of a storm can be predicted for any return period and storm duration, from charts based on historic data for the location.[94] The term 1 in 10 year storm describes a rainfall event which is rare and is only likely to occur once every 10 years, so it has a 10 percent likelihood any given year. The rainfall will be greater and the flooding will be worse than the worst storm expected in any single year. The term 1 in 100 year storm describes a rainfall event which is extremely rare and which will occur with a likelihood of only once in a century, so has a 1 percent likelihood in any given year. The rainfall will be extreme and flooding to be worse than a 1 in 10 year event. As with all probability events, it is possible, though improbable, to have multiple "1 in 100 Year Storms" in a single year.[95]Forecasting
Main article: Quantitative precipitation forecast
Impact
Effect on agriculture
In areas with wet and dry seasons, soil nutrients diminish and erosion increases during the wet season.[24] 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.[105] 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.[106] Rain may be harvested through the use of rainwater tanks; treated to potable use or for non-potable use indoors or for irrigation,.[107] Excessive rain during short periods of time can cause flash floods.[108]
In culture
Cultural attitudes towards rain differ across the world. In temperate climates, people tend to be more stressed when the weather is unstable or cloudy, with its impact greater on men than women.[109] Rain can also bring joy, as some consider it to be soothing or enjoy the aesthetic appeal of it. In dry places, such as India,[110] or during periods of drought,[111] rain lifts people's moods. 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.[112] 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.[113] Many people find the scent during and immediately after rain 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.[114]Global climatology
See also: Earth rainfall climatology
Approximately 505,000 cubic kilometres (121,000 cu mi) of water falls as precipitation each year across the globe with 398,000 cubic kilometres (95,000 cu mi) of it over the oceans.[115] Given the Earth's surface area, that means the globally-averaged annual precipitation is 990 millimetres (39 in). Deserts are defined as areas with an average annual precipitation of less than 250 millimetres (10 in) per year,[116][117] or as areas where more water is lost by evapotranspiration than falls as precipitation.[118]Deserts
Main article: Desert
Wetlands
See also: Monsoon and Monsoon trough
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.[121] 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.[122] 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.[123][124] Similar, but weaker, monsoon circulations are present over North America and Australia.[125][126] During the summer, the Southwest monsoon combined with Gulf of California and Gulf of Mexico moisture moving around the subtropical ridge in the Atlantic ocean bring the promise of afternoon and evening thunderstorms to the southern tier of the United States as well as the Great Plains.[127] The eastern half of the contiguous United States east of the 98th meridian, the mountains of the Pacific Northwest, and the Sierra Nevada range are the wetter portions of the nation, with average rainfall exceeding 30 inches (760 mm) per year.[120] Tropical cyclones enhance precipitation across southern sections of the United States,[128] as well as Puerto Rico, the United States Virgin Islands,[129] the Northern Mariana Islands,[130] Guam, and American Samoa.Impact of the Westerlies
See also: Westerlies
Westerly flow from the mild north Atlantic leads to wetness across western Europe, in particular 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. In 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.[127] 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.[131]The El Niño-Southern Oscillation affects the precipitation distribution, by altering rainfall patterns across the western United States,[132] Midwest,[133][134] the Southeast,[135] and throughout the tropics. There is also evidence that global warming is leading to increased precipitation to the eastern portions of North America, while droughts are becoming more frequent in the tropics and subtropics.
Wettest known locations
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 (905.0 in) in 1861. The 38-year average at nearby Mawsynram, Meghalaya, India is 11,873 mm (467.4 in).[136] The wettest spot in Australia is Mount Bellenden Ker in the north-east of the country records an average of 8,000 millimetres (310 in) per year, with over 1,200 millimetres (47 in) of rain recorded during 2000.[137] Mount Waialeale on the island of Kauaʻi in the Hawaiian Islands averages more than 11,680 millimetres (460 in) of rain over the last 32 years, with a record 17,340 millimetres (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.[138] Lloró, a town situated in Chocó, Colombia, is probably the place with the largest measured rainfall in the world, averaging 13,300 mm (520 in) per year.[139] The Department of Chocó is extraordinarily humid. Tutunendo, 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, Tutunendo receives rain almost uniformly distributed throughout the year.[140] Quibdó, the capital of Chocó, receives the most rain in the world among cities with over 100,000 inhabitants: 9,000 millimetres (350 in) per year.[139] Storms in Chocó can drop 500 mm (20 in) of rainfall in a day. This amount is more than falls in many cities in a year's time.| Continent | Highest average (inches/mm) | Place | Elevation (feet/m) | Years of Record |
|---|---|---|---|---|
| South America | 523.6 in/13,299 mm | Lloró, Colombia[a][b] | 520 ft/158 m[c] | 29 |
| Asia | 467.4 in/11,872 mm | Mawsynram, India[a][d] | 4,597 ft/1,401 m | 39 |
| Oceania | 460.0 in/11,684 mm | Mount Waiʻaleʻale, Kauai, Hawaii (USA)[a] | 5,148 ft/1,569 m | 30 |
| Africa | 405.0 in/10,287 mm | Debundscha, Cameroon | 30 ft/9.1 m | 32 |
| South America | 354.0 in/8,992 mm | Quibdo, Colombia | 120 ft/36.6 m | 16 |
| Australia | 340.0 in/8,636 mm | Mount Bellenden Ker, Queensland | 5,102 ft/1,555 m | 9 |
| North America | 256.0 in/6,502 mm | Henderson Lake, British Columbia | 12 ft/3.66 m | 14 |
| Europe | 183.0 in/4,648 mm | Crkvice, Montenegro | 3,337 ft/1,017 m | 22 |
| Source (without conversions): Global Measured Extremes of Temperature and Precipitation, National Climatic Data Center. August 9, 2004.[141] | ||||
| Continent | Place | Highest rainfall | References | |
|---|---|---|---|---|
| Highest average annual rainfall | Asia | Mawsynram, India | 467.4 in/11,872 mm | [142] |
| Highest in one year | Asia | Cherrapunji, India | 1,042 in/26,467 mm | [143] |
| Highest in one Calendar month | Asia | Cherrapunji, India | 366 in/9,296 mm | [143] |
| Highest in 24 hours | Indian Ocean | Fac Fac, La Reunion Island | 73 in/1,854 mm | [144] |
| Highest in 12 hours | Indian Ocean | Belouve, La Reunion Island | 53 in/1,346 mm | [143] |
| Highest in one minute | North America | Guadeloupe, Caribbean Islands | 1.5 in/38 mm | [144] |
