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 Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salts and other total dissolved solids. Although the term specifically excludes seawater and brackish water, it does include ...

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 weather fronts. If enough moisture and upward motion is present, precipitation falls from convective clouds (those with strong upward vertical motion) such as cumulonimbus (thunder clouds) which can organize into narrow rainbands. In mountainous areas, heavy precipitation is possible where

upslope flow An anabatic wind, from the Greek '' anabatos'', verbal of ''anabainein'' meaning moving upward, is a warm wind which blows up a steep slope or mountain side, driven by heating of the slope through insolation.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 Windward () and leeward () are terms used to describe the direction of the wind. Windward is ''upwind'' from the point of reference, i.e. towards the direction from which the wind is coming; leeward is ''downwind'' from the point of reference ...

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 The Intertropical Convergence Zone (ITCZ ), known by sailors as the doldrums or the calms because of its monotonous windless weather, is the area where the northeast and the southeast trade winds converge. It encircles Earth near the thermal e ...

, brings

rainy seasons The wet season (sometimes called the Rainy season) is the time of year when most of a region's average annual rainfall occurs. It is the time of year where the majority of a country's or region's annual precipitation occurs. Generally, the sea ...

savannah A savanna or savannah is a mixed woodland-grassland (i.e. grassy woodland) ecosystem characterised by the trees being sufficiently widely spaced so that the Canopy (forest), canopy does not close. The open canopy allows sufficient light to rea ...

climes. 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. Antarctica is the driest continent. The globally averaged annual precipitation over land is , but over the whole Earth it is much higher at . Climate classification systems such as the Köppen classification system use average annual rainfall to help differentiate between differing climate regimes. Rainfall is measured using rain gauges. Rainfall amounts can be estimated by weather radar. Rain is also known or suspected on other planets, where it may be composed of methane,

neon Neon is a chemical element with the symbol Ne and atomic number 10. It is a noble gas. Neon is a colorless, odorless, inert monatomic gas under standard conditions, with about two-thirds the density of air. It was discovered (along with krypton ...

sulfuric acid Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid ( Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen and hydrogen, with the molecular formu ...

, or even iron rather than water.

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). 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. 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) 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. Raining street in the night - panoramio

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. 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, 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. Evaporative cooling occurs when moisture is added to the air through evaporation, which forces the air temperature to cool to its

wet-bulb temperature The wet-bulb temperature (WBT) is the temperature read by a thermometer covered in water-soaked (water at ambient temperature) cloth (a wet-bulb thermometer) over which air is passed. At 100% relative humidity, the wet-bulb temperature is equal ...

, or until it reaches saturation. The main ways water vapor is added to the air are: wind convergence into areas of upward motion, precipitation or virga falling from above, daytime heating evaporating water from the surface of oceans, water bodies or wet land, transpiration from plants, cool or dry air moving over warmer water, and lifting air over mountains. Water vapor normally begins to condense on

condensation nuclei Cloud condensation nuclei (CCNs), also known as cloud seeds, are small particles typically 0.2 µm, or one hundredth the size of a cloud droplet. CCNs are a unique subset of aerosols in the atmosphere on which water vapour condenses. This ca ...

such as dust, ice, and salt in order to form clouds. Elevated portions of weather fronts (which are three-dimensional in nature) force broad areas of upward motion within the Earth's atmosphere which form clouds decks such as altostratus or cirrostratus. 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.

Coalescence and fragmentation

Coalescence occurs when water droplets fuse to create larger water droplets. Air resistance typically causes the water droplets in a cloud to remain stationary. When air turbulence occurs, water droplets collide, producing larger droplets. Black Rain Clouds

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. 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. Raindrops have sizes ranging from 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

parachute A parachute is a device used to slow the motion of an object through an atmosphere by creating drag or, in a ram-air parachute, aerodynamic lift. A major application is to support people, for recreation or as a safety device for aviators, who ...

s. Contrary to popular belief, their shape does not resemble a teardrop. The biggest raindrops on Earth were recorded over Brazil and the Marshall Islands in 2004 — some of them were as large as . 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. Rain drops associated with melting hail tend to be larger than other rain drops. Water Drop on rose leaf

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.

Droplet size distribution

The final droplet size distribution is an

exponential distribution In probability theory and statistics, the exponential distribution is the probability distribution of the time between events in a Poisson point process, i.e., a process in which events occur continuously and independently at a constant average ...

. The number of droplets with diameter between

d

and

D+dD

per unit volume of space is

n(d) = n_0 e^ dD

. This is commonly referred to as the Marshall–Palmer law after the researchers who first characterized it. * The parameters are somewhat temperature-dependent, and the slope also scales with the rate of rainfall

\langle d \rangle^=41 R^

(d in centimeters and R in millimeters per hour). 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. The distribution has an upper limit due to droplet fragmentation.

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, drizzle impacts at or , while large drops impact at around or . Rain falling on loosely packed material such as newly fallen ash can produce dimples that can be fossilized, called

raindrop impressions Raindrop impressions are a geological feature characterized by small crater-like pits with slightly raised edges that are the result of the impact of Rain#Raindrop impacts, raindrop impacts on soft sediment surfaces. Sedimentary structures with si ...

. 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. The sound of raindrops hitting water is caused by bubbles of air oscillating underwater. The

METAR METAR is a format for reporting weather information. A METAR weather report is predominantly used by aircraft pilots, and by meteorologists, who use aggregated METAR information to assist in weather forecasting. Raw METAR is the most common form ...

code for rain is RA, while the coding for rain showers is SHRA.

Virga

In certain conditions precipitation may fall from a cloud but then evaporate or

sublime Sublime may refer to: Entertainment * SuBLime, a comic imprint of Viz Media for BL manga * Sublime (band), an American ska punk band ** ''Sublime'' (album), 1996 * ''Sublime'' (film), a 2007 horror film * SubLime FM, a Dutch radio station dedic ...

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 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 the eyewall, and in comma-head precipitation patterns around

mid-latitude cyclone Extratropical cyclones, sometimes called mid-latitude cyclones or wave cyclones, are low-pressure areas which, along with the anticyclones of high-pressure areas, drive the weather over much of the Earth. Extratropical cyclones are capable o ...

s. 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. What separates rainfall from other precipitation types, such as

ice pellets Ice pellets are a form of precipitation consisting of small, hard, translucent balls of ice. Ice pellets are different from graupel ("soft hail") which is made of frosty white opaque rime, and from a mixture of rain and snow which is a slushy ...

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.

Hail Hail is a form of solid precipitation. It is distinct from ice pellets (American English "sleet"), though the two are often confused. It consists of balls or irregular lumps of ice, each of which is called a hailstone. Ice pellets generally fal ...

becomes an increasingly infrequent occurrence when the freezing level within the atmosphere exceeds above ground level.

Convection

Convective rain, or showery precipitation, occurs from convective clouds (e.g., cumulonimbus or cumulus congestus). It falls as showers with rapidly changing intensity. Convective precipitation falls over a certain area for a relatively short time, as convective clouds have limited horizontal extent. Most precipitation in the tropics appears to be convective; however, it has been suggested that stratiform precipitation also occurs.

Graupel Graupel (; ), also called soft hail, hominy snow, or snow pellets, is precipitation that forms when supercooled water droplets in air are collected and freeze on falling snowflakes, forming balls of crisp, opaque rime. Graupel is distinct from ...

and

hail Hail is a form of solid precipitation. It is distinct from ice pellets (American English "sleet"), though the two are often confused. It consists of balls or irregular lumps of ice, each of which is called a hailstone. Ice pellets generally fal ...

indicate convection. In mid-latitudes, convective precipitation is intermittent and often associated with baroclinic boundaries such as cold fronts, squall lines, and warm fronts.

Orographic effects

Orographic precipitation occurs on the windward side of mountains and is caused by the rising air motion of a large-scale flow of moist air across the mountain ridge, resulting in adiabatic cooling and condensation. In mountainous parts of the world subjected to relatively consistent winds (for example, the trade winds), a more moist climate usually prevails on the windward side of a mountain than on the

or downwind side. Moisture is removed by orographic lift, leaving drier air (see katabatic wind) on the descending and generally warming, leeward side where a

rain shadow A rain shadow is an area of significantly reduced rainfall behind a mountainous region, on the side facing away from prevailing winds, known as its leeward side. Evaporated moisture from water bodies (such as oceans and large lakes) is carrie ...

is observed. In Hawaii,

Mount Waiʻaleʻale Mount Waialeale is a shield volcano and the second highest point on the island of Kauai in the Hawaiian Islands. Its name literally means "rippling water" or "overflowing water" The mountain, at an elevation of , averages more than of rain a ...

, on the island of Kauai, is notable for its extreme rainfall, as it is amongst the places in the world with the highest levels of rainfall, with . Systems known as Kona storms affect the state with heavy rains between October and April.Steven Businger and Thomas Birchard Jr
A Bow Echo and Severe Weather Associated with a Kona Low in Hawaii.
Retrieved on 22 May 2007. Local climates vary considerably on each island due to their topography, divisible into windward (''Koolau'') 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. 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. The

Sierra Nevada The Sierra Nevada () is a mountain range in the Western United States, between the Central Valley of California and the Great Basin. The vast majority of the range lies in the state of California, although the Carson Range spur lies primarily ...

range creates the same effect in North America forming the

Great Basin The Great Basin is the largest area of contiguous endorheic basin, endorheic watersheds, those with no outlets, in North America. It spans nearly all of Nevada, much of Utah, and portions of California, Idaho, Oregon, Wyoming, and Baja California ...

and Mojave Deserts.

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. The term ''green season'' is also sometimes used as a

euphemism A euphemism () is an innocuous word or expression used in place of one that is deemed offensive or suggests something unpleasant. Some euphemisms are intended to amuse, while others use bland, inoffensive terms for concepts that the user wishes ...

by tourist authorities. Areas with wet seasons are dispersed across portions of the tropics and

subtropics The subtropical zones or subtropics are geographical and climate zones to the north and south of the tropics. Geographically part of the temperate zones of both hemispheres, they cover the middle latitudes from to approximately 35° north and ...

. 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. Some areas with pronounced rainy seasons will see a break in rainfall mid-season when the

or monsoon trough move poleward of their location during the middle of the warm season. When the wet season occurs during the warm season, or

summer Summer is the hottest of the four temperate seasons, occurring after spring and before autumn. At or centred on the summer solstice, the earliest sunrise and latest sunset occurs, daylight hours are longest and dark hours are shortest, wit ...

, rain falls mainly during the late afternoon and early evening hours. The wet season is a time when air quality improves,

freshwater Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salts and other total dissolved solids. Although the term specifically excludes seawater and brackish water, it does include ...

quality improves, 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). Although

cyclone In meteorology, a cyclone () is a large air mass that rotates around a strong center of low atmospheric pressure, counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere as viewed from above (opposite to an anti ...

s 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. Areas in their path can receive a year's worth of rainfall from a tropical cyclone passage.

Human influence

Change in Average Temperature With Fahrenheit

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

cloud condensation nuclei Cloud condensation nuclei (CCNs), also known as cloud seeds, are small particles typically 0.2 µm, or one hundredth the size of a cloud droplet. CCNs are a unique subset of aerosols in the atmosphere on which water vapour condenses. This c ...

, 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. 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 downwind of cities, compared with upwind. Some cities induce a total precipitation increase of 51%. 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). 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. The most successful attempts at influencing weather involve

cloud seeding Cloud seeding is a type of weather modification that aims to change the amount or type of precipitation that falls from clouds by dispersing substances into the air that serve as cloud condensation or ice nuclei, which alter the microphysical p ...

, which include techniques used to increase winter precipitation over mountains and suppress

Characteristics

Patterns

Rainbands are cloud and precipitation areas which are significantly elongated. Rainbands can be

stratiform Stratiform may refer to: * Any of the stratus family of clouds (fog, stratus clouds, altostratus clouds, cirrostratus clouds, nimbostratus clouds) and the precipitation coming from them. * Any occurrence of layered strata (see stratigraphic unit). ...

or convective, and are generated by differences in temperature. When noted on weather radar imagery, this precipitation elongation is referred to as banded structure. Rainbands in advance of warm occluded fronts and warm fronts are associated with weak upward motion, and tend to be wide and stratiform in nature. Rainbands spawned near and ahead of cold fronts can be squall lines which are able to produce tornadoes. 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. 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. Once a cyclone occludes an occluded front (a trough of warm air aloft) will be caused by strong southerly winds on its eastern periphery rotating aloft around its northeast, and ultimately northwestern, periphery (also termed the warm conveyor belt), forcing a surface trough to continue into the cold sector on a similar curve to the occluded front. The front creates the portion of an occluded cyclone known as its ''comma head'', due to the

comma The comma is a punctuation mark that appears in several variants in different languages. It has the same shape as an apostrophe or single closing quotation mark () in many typefaces, but it differs from them in being placed on the baseline ...

-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 front is unstable enough for convection. Banding within the comma head precipitation pattern of an

extratropical cyclone Extratropical cyclones, sometimes called mid-latitude cyclones or wave cyclones, are low-pressure areas which, along with the anticyclones of high-pressure areas, drive the weather over much of the Earth. Extratropical cyclones are capable of ...

can yield significant amounts of rain. 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. 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. University of Wisconsin–Madison (1998
Objective Dvorak Technique.
Retrieved on 29 May 2006.

Acidity

The phrase ''acid rain'' was first used by Scottish chemist Robert Augus Smith in 1852. The pH of rain varies, especially due to its origin. On America's 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. Rain becomes acidic primarily due to the presence of two strong acids,

(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 raises the pH.

Köppen climate classification

World Köppen Classification (with authors)

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 An oceanic climate, also known as a marine climate, is the humid temperate climate sub-type in Köppen classification ''Cfb'', typical of west coasts in higher middle latitudes of continents, generally featuring cool summers and mild winters ( ...

, Mediterranean climate,

steppe In physical geography, a steppe () is an ecoregion characterized by grassland plains without trees apart from those near rivers and lakes. Steppe biomes may include: * the montane grasslands and shrublands biome * the temperate grasslands, ...

subarctic climate The subarctic climate (also called subpolar climate, or boreal climate) is a climate with long, cold (often very cold) winters, and short, warm to cool summers. It is found on large landmasses, often away from the moderating effects of an ocean, ge ...

, tundra, polar ice cap, and

desert A desert is a barren area of landscape where little precipitation occurs and, consequently, living conditions are hostile for plant and animal life. The lack of vegetation exposes the unprotected surface of the ground to denudation. About on ...

. Rain forests are characterized by high rainfall, with definitions setting minimum normal annual rainfall between . A tropical savanna is a grassland biome located in semi-arid to semi-humid climate regions of

subtropical The subtropical zones or subtropics are geographical zone, geographical and Köppen climate classification, climate zones to the Northern Hemisphere, north and Southern Hemisphere, south of the tropics. Geographically part of the Geographical z ...

and tropical

latitudes In geography, latitude is a coordinate that specifies the north– south position of a point on the surface of the Earth or another celestial body. Latitude is given as an angle that ranges from –90° at the south pole to 90° at the north pol ...

, with rainfall between a year. They are widespread on Africa, and are also found in India, the northern parts of South America, Malaysia, and Australia. The humid subtropical climate zone is 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. Humid subtropical climates lie on the east side continents, roughly between latitudes 20° and 40° degrees away from the equator. 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. The Mediterranean climate regime resembles the climate of the lands in the

Mediterranean Basin In biogeography, the Mediterranean Basin (; also known as the Mediterranean Region or sometimes Mediterranea) is the region of lands around the Mediterranean Sea that have mostly a Mediterranean climate, with mild to cool, rainy winters and w ...

, 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. A steppe is a dry grassland. Subarctic climates are cold with continuous

permafrost Permafrost is ground that continuously remains below 0 °C (32 °F) for two or more years, located on land or under the ocean. Most common in the Northern Hemisphere, around 15% of the Northern Hemisphere or 11% of the global surface ...

and little precipitation.

Pollution

Measurement

Gauges

Rain is measured in units of length per unit time, typically in millimeters per hour, or in countries where imperial units are more common, inches per hour. 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. One millimeter of rainfall is the equivalent of one liter of water per square meter. 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. The inner cylinder is filled by of rain, with overflow flowing into the outer cylinder. Plastic gauges have markings on the inner cylinder down to resolution, while metal gauges require use of a stick designed with the appropriate 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. 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. 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. 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. 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.

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. 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

Z = A R^b ,

where Z represents the radar reflectivity, R represents the rainfall rate, and A and b are constants. Satellite derived rainfall estimates use passive microwave instruments aboard polar orbiting as well as

geostationary A geostationary orbit, also referred to as a geosynchronous equatorial orbit''Geostationary orbit'' and ''Geosynchronous (equatorial) orbit'' are used somewhat interchangeably in sources. (GEO), is a circular geosynchronous orbit in altitude ...

weather satellites to indirectly measure rainfall rates. If one wants an accumulated rainfall over a time period, one has to add up all the accumulations from each grid box within the images during that time.

Intensity

Rainfall intensity is classified according to the rate of precipitation, which depends on the considered time.(pdf)
The following categories are used to classify rainfall intensity: * Light rain — when the precipitation rate is < per hour * Moderate rain — when the precipitation rate is between – or per hour * Heavy rain — when the precipitation rate is > per hour, or between and per hour * Violent rain — when the precipitation rate is > per hour Euphemisms for a heavy or violent rain include gully washer, trash-mover and toad-strangler. The intensity can also be expressed by rainfall erosivity ''R-factor'' or in terms of the rainfall time-structure ''n-index''.

Return period

The average time between occurrences of an event with a specified intensity and duration is called the return period. The intensity of a storm can be predicted for any return period and storm duration, from charts based on historic data for the location. 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). 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.

Forecasting

The Quantitative Precipitation Forecast (abbreviated QPF) is the expected amount of liquid precipitation accumulated over a specified time period over a specified area. A QPF will be specified when a measurable precipitation type reaching a minimum threshold is forecast for any hour during a QPF valid period. Precipitation forecasts tend to be bound by synoptic hours such as 0000, 0600, 1200 and 1800 GMT. Terrain is considered in QPFs by use of topography or based upon climatological precipitation patterns from observations with fine detail. Starting in the mid to late 1990s, QPFs were used within hydrologic forecast models to simulate impact to rivers throughout the United States. 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. QPF can be generated on a quantitative, forecasting amounts, or a qualitative, forecasting the probability of a specific amount, basis. 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.

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, while overly wet weather can cause harmful fungus growth. Plants need varying amounts of rainfall to survive. For example, certain

cacti A cactus (, or less commonly, cactus) is a member of the plant family Cactaceae, a family comprising about 127 genera with some 1750 known species of the order Caryophyllales. The word ''cactus'' derives, through Latin, from the Ancient Greek ...

require small amounts of water, 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. 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. 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. Rain may be harvested through the use of rainwater tanks; treated to potable use or for non-potable use indoors or for irrigation. Excessive rain during short periods of time can cause

flash flood A flash flood is a rapid flooding of low-lying areas: washes, rivers, dry lakes and depressions. It may be caused by heavy rain associated with a severe thunderstorm, hurricane, or tropical storm, or by meltwater from ice or snow flowing o ...

Culture and religion

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. 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, or during periods of drought, 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 its country, since it has a desert climate. 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 drain A storm drain, storm sewer (United Kingdom, United States, U.S. and Canada), surface water drain/sewer (United Kingdom), or stormwater drain (Australia and New Zealand) is infrastructure designed to Drainage, drain excess rain and ground water ...

s that lead rains to sewers. 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. Nuremberg Chronicle f 029r 2

Rain holds an important religious significance in many cultures. The ancient

Sumer Sumer () is the earliest known civilization in the historical region of southern Mesopotamia (south-central Iraq), emerging during the Chalcolithic and early Bronze Ages between the sixth and fifth millennium BC. It is one of the cradles of c ...

ians believed that rain was the semen of the sky-god An, which fell from the heavens to inseminate his consort, the earth-goddess Ki, causing her to give birth to all the plants of the earth. The Akkadians believed that the clouds were the breasts of Anu's consort Antu and that rain was milk from her breasts. According to Jewish tradition, in the first century BC, the Jewish miracle-worker Honi ha-M'agel ended a three-year drought in Judaea by drawing a circle in the sand and praying for rain, refusing to leave the circle until his prayer was granted. In his '' Meditations'', the Roman emperor Marcus Aurelius preserves a prayer for rain made by the

Athenians Athens ( ; el, Αθήνα, Athína ; grc, Ἀθῆναι, Athênai (pl.) ) is both the capital and largest city of Greece. With a population close to four million, it is also the seventh largest city in the European Union. Athens dominates a ...

to the Greek sky-god Zeus. Various Native American tribes are known to have historically conducted rain dances in effort to encourage rainfall. Rainmaking rituals are also important in many African cultures. In the present-day United States, various state governors have held Days of Prayer for rain, including the Days of Prayer for Rain in the State of Texas in 2011.

Global climatology

Approximately of water falls as precipitation each year across the globe with of it over the oceans. Given the Earth's surface area, that means the globally averaged annual precipitation is . Deserts are defined as areas with an average annual precipitation of less than per year, or as areas where more water is lost by evapotranspiration than falls as precipitation.

Deserts

The northern half of Africa is dominated by the world's most extensive hot, dry region, the Sahara Desert. Some deserts also occupy much of southern Africa: the Namib and the Kalahari. Across Asia, a large annual rainfall minimum, composed primarily of deserts, stretches from the

Gobi Desert The Gobi Desert (Chinese: 戈壁 (沙漠), Mongolian: Говь (ᠭᠣᠪᠢ)) () is a large desert or brushland region in East Asia, and is the sixth largest desert in the world. Geography The Gobi measures from southwest to northeast an ...

in Mongolia west-southwest through western Pakistan ( Balochistan) and Iran into the Arabian Desert in Saudi Arabia. Most of Australia is semi-arid or desert, making it the world's driest inhabited continent. 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. The drier areas of the United States are regions where the Sonoran Desert overspreads the Desert Southwest, the Great Basin and central Wyoming.

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 freezing rain. Due to the entire atmosphere being below freezing most of the time, very cold climates see very little rainfall and are often known as polar deserts. A common biome in this area is the tundra which has a short summer thaw and a long frozen winter. Ice caps see no rain at all, making Antarctica the world's driest continent.

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 The equator is a circle of latitude, about in circumference, that divides Earth into the Northern and Southern hemispheres. It is an imaginary line located at 0 degrees latitude, halfway between the North and South poles. The term can als ...

. 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 The Intertropical Convergence Zone (ITCZ ), known by sailors as the doldrums or the calms because of its monotonous windless weather, is the area where the northeast and the southeast trade winds converge. It encircles Earth near the thermal e ...

(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. 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. 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. Similar, but weaker, monsoon circulations are present over North America and Australia. During the summer, the Southwest monsoon combined with Gulf of California and Gulf of Mexico moisture moving around the

subtropical ridge The horse latitudes are the latitudes about 30 degrees north and south of the Equator. They are characterized by sunny skies, calm winds, and very little precipitation. They are also known as Subtropics, subtropical ridges, or highs. It is a h ...

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 The Great Plains (french: Grandes Plaines), sometimes simply "the Plains", is a broad expanse of flatland in North America. It is located west of the Mississippi River and east of the Rocky Mountains, much of it covered in prairie, steppe, an ...

. The eastern half of the contiguous United States east of the 98th meridian, the mountains of the Pacific Northwest, and the

range are the wetter portions of the nation, with average rainfall exceeding per year.NationalAtlas.go
Precipitation of the Individual States and of the Conterminous States.
Retrieved on 9 March 2008. Tropical cyclones enhance precipitation across southern sections of the United States, as well as Puerto Rico, the United States Virgin Islands, the Northern Mariana Islands, Guam, and American Samoa.

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 , at sea level and , on the mountains of rain per year. Bergen, Norway is one of the more famous European rain-cities with its yearly precipitation of on average. During the fall, winter, and spring, Pacific storm systems bring most of Hawaii and the western United States much of their precipitation.J. Horel
Normal Monthly Precipitation, Inches.
Retrieved on 19 March 2008. 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.Walker S. Ashley, Thomas L. Mote, P. Grady Dixon, Sharon L. Trotter, Emily J. Powell, Joshua D. Durkee, and Andrew J. Grundstein
Distribution of Mesoscale Convective Complex Rainfall in the United States.
Retrieved on 2 March 2008. The El Niño-Southern Oscillation affects the precipitation distribution, by altering rainfall patterns across the western United States, Midwest, the Southeast, 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 Cherrapunji () or Sohra is a subdivisional town (Proposed District) East Khasi Hills district in the Indian state of Meghalaya. It is the traditional capital of ka ''hima'' Sohra (Khasi tribal kingdom). Sohra has often been credited as being t ...

, situated on the southern slopes of the Eastern Himalaya in Shillong, India is the confirmed wettest place on Earth, with an average annual rainfall of . The highest recorded rainfall in a single year was in 1861. The 38-year average at nearby Mawsynram, Meghalaya, India is . The wettest spot in Australia is

Mount Bellenden Ker Mount Bellenden Ker is the second-highest mountain in Queensland, Australia, with a height of . It is named after the botanist John Bellenden Ker Gawler. Located south of Cairns near Babinda, it is adjacent to Mount Bartle Frere, the state's ...

in the north-east of the country which records an average of per year, with over of rain recorded during 2000. The Big Bog on the island of

Maui The island of Maui (; Hawaiian: ) is the second-largest of the islands of the state of Hawaii at 727.2 square miles (1,883 km2) and is the 17th largest island in the United States. Maui is the largest of Maui County's four islands, which ...

has the highest average annual rainfall in the Hawaiian Islands, at .

on the island of

Kauaʻi Kauai, () anglicized as Kauai ( ), is geologically the second-oldest of the main Hawaiian Islands (after Niʻihau). With an area of 562.3 square miles (1,456.4 km2), it is the fourth-largest of these islands and the List of islands of th ...

achieves similar torrential rains, while slightly lower than that of the Big Bog, at of rain per year over the last 32 years, with a record in 1982. Its summit is considered one of the rainiest spots on earth, with a reported 350 days of rain per year.
Lloró, a town situated in Chocó,

Colombia Colombia (, ; ), officially the Republic of Colombia, is a country in South America with insular regions in North America—near Nicaragua's Caribbean coast—as well as in the Pacific Ocean. The Colombian mainland is bordered by the Car ...

, is probably the place with the largest rainfall in the world, averaging per year. 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 per year; in 1974 the town received , 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. Quibdó, the capital of Chocó, receives the most rain in the world among cities with over 100,000 inhabitants: per year. Storms in Chocó can drop of rainfall in a day. This amount is more than what falls in many cities in a year's time.

Outside Earth

Rainfalls of diamonds have been suggested to occur on the gas giant planets, Jupiter and

Saturn Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It is a gas giant with an average radius of about nine and a half times that of Earth. It has only one-eighth the average density of Earth; h ...

, as well as on the ice giant planets, Uranus and

Neptune Neptune is the eighth planet from the Sun and the farthest known planet in the Solar System. It is the fourth-largest planet in the Solar System by diameter, the third-most-massive planet, and the densest giant planet. It is 17 times ...

. There is likely to be rain of various compositions in the upper atmospheres of the gas giants, as well as precipitation of liquid

in the deep atmospheres. On

Titan Titan most often refers to: * Titan (moon), the largest moon of Saturn * Titans, a race of deities in Greek mythology Titan or Titans may also refer to: Arts and entertainment Fictional entities Fictional locations * Titan in fiction, fictiona ...

's largest natural satellite, infrequent methane rain is thought to carve the moon's numerous surface channels. On Venus,

virga evaporates from the surface. Extrasolar planet OGLE-TR-56b in the constellation

Sagittarius Sagittarius ( ) may refer to: *Sagittarius (constellation) *Sagittarius (astrology), a sign of the Zodiac Ships *''SuperStar Sagittarius'', a cruise ship * USS ''Sagittarius'' (AKN-2), a World War II US Navy cargo ship Music *Sagittarius (ban ...

is hypothesized to have iron rain. Accordingly, research carried out by the

European Southern Observatory The European Organisation for Astronomical Research in the Southern Hemisphere, commonly referred to as the European Southern Observatory (ESO), is an intergovernmental organization, intergovernmental research organisation made up of 16 mem ...

shows that

WASP-76b WASP-76b is a Hot Jupiter exoplanet orbiting the star WASP-76 in the constellation Pisces. Wasp-76b orbits its parent star at a distance of 0.033 AU with a period of 1.8 days. Its mass is 0.92 times that of Jupiter. WASP-76b was discovered on Oc ...

can produce showers of burning liquid iron droplets once temperature decreases during the planet's night hours. There is evidence from samples of basalt brought back by the Apollo missions that the Moon has been subject to lava rain.Taylor, G. Jeffrey
"Finding distant chips from distant maria"
pp. 8–9, ''Planetary Science Research Discoveries'', 30 April 2006.

Notes

* 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 at Quibdó, Colombia. The average at Lloró

SE and at a higher elevation than Quibdó SE, Se, or Sé may refer to: Arts and entertainment * ''Sé'' (album), by Lúnasa, 2006 * Se (instrument), a traditional Chinese musical instrument Businesses and organizations * Sea Ltd (NYSE: SE), tech conglomerate headquartered in Singapore ...

is an estimated amount. * Approximate elevation. * Recognized as "The Wettest place on Earth" by the ''

Guinness Book of World Records ''Guinness World Records'', known from its inception in 1955 until 1999 as ''The Guinness Book of Records'' and in previous United States editions as ''The Guinness Book of World Records'', is a reference book published annually, listing world ...

''. * This is the highest figure for which records are available. The summit of Mount Snowdon, about from Glaslyn, is estimated to have at least per year.

References

External links

BBC article on the weekend rain effectBBC article on rain-makingBBC article on the mathematics of running in the rain
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