TheInfoList

OR:

Rain is water
droplet A drop or droplet is a small column of liquid, bounded completely or almost completely by free surfaces. A drop may form when liquid accumulates at the lower end of a tube or other surface boundary, producing a hanging drop called a pendant ...
s that have condensed from atmospheric water vapor and then fall under gravity. Rain is a major component of the
water cycle The water cycle, also known as the hydrologic cycle or the hydrological cycle, is a biogeochemical cycle that describes the continuous movement of water on, above and below the surface of the Earth. The mass of water on Earth remains fairly const ...
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 Irrigation (also referred to as watering) is the practice of applying controlled amounts of water to land to help grow crops, landscape plants, and lawns. Irrigation has been a key aspect of agriculture for over 5,000 years and has been devel ...
, and suitable conditions for many types of
ecosystem An ecosystem (or ecological system) consists of all the organisms and the physical environment with which they interact. These biotic and abiotic components are linked together through nutrient cycles and energy flows. Energy enters the syst ...
s. The major cause of rain production is moisture moving along three-dimensional zones of temperature and moisture contrasts known as
weather fronts A weather front is a boundary separating air masses for which several characteristics differ, such as air density, wind, temperature, and humidity. Disturbed and unstable weather due to these differences often arises along the boundary. For ins ...
. If enough moisture and upward motion is present, precipitation falls from
convective Convection is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the conve ...
clouds (those with strong upward vertical motion) such as
cumulonimbus Cumulonimbus (from Latin ''cumulus'', "heaped" and ''nimbus'', "rainstorm") is a dense, towering vertical cloud, typically forming from water vapor condensing in the lower troposphere that builds upward carried by powerful buoyant air currents ...
(thunder clouds) which can organize into narrow
rainbands A rainband is a cloud and precipitation structure associated with an area of rainfall which is significantly elongated. Rainbands can be stratiform or convective, and are generated by differences in temperature. When noted on weather radar ima ...
. In mountainous areas, heavy precipitation is possible where upslope flow is maximized within
windward 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 ...
sides of the
terrain Terrain or relief (also topographical relief) involves the vertical and horizontal dimensions of land surface. The term bathymetry is used to describe underwater relief, while hypsometry studies terrain relative to sea level. The Latin ...
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, brings rainy seasons to
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 does not close. The open canopy allows sufficient light to reach the ground to ...
climes. The
urban heat island An urban heat island (UHI) is an urban or metropolitan area that is significantly warmer than its surrounding rural areas due to human activities. The temperature difference is usually larger at night than during the day, and is most apparen ...
effect leads to increased rainfall, both in amounts and intensity, downwind of cities.
Global warming In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate variability and change, Climate change in a broader sense also includes ...
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 Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on E ...
, neon,
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 form ...
, or even
iron Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in fr ...
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. 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 Convection is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the convec ...
, 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 Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...
, 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 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 Altostratus is a middle-altitude cloud genus made up of water droplets, ice crystals, or a mixture of the two. Altostratus clouds are formed when large masses of warm, moist air rise, causing water vapor to condense. Altostratus clouds are usuall ...
or cirrostratus.
Stratus Stratus may refer to: Weather *Stratus cloud, a cloud type ** Nimbostratus cloud, a cloud type **Stratocumulus cloud, a cloud type ** Altostratus cloud, a cloud type ** Altostratus undulatus cloud, a cloud type ** Cirrostratus cloud, a cloud type ...
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. 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 parachutes. Contrary to popular belief, their shape does not resemble a teardrop. The biggest raindrops on Earth were recorded over Brazil and the
Marshall Islands The Marshall Islands ( mh, Ṃajeḷ), officially the Republic of the Marshall Islands ( mh, Aolepān Aorōkin Ṃajeḷ),'' () is an independent island country and microstate near the Equator in the Pacific Ocean, slightly west of the Internati ...
in 2004 — some of them were as large as . The large size is explained by condensation on large
smoke Smoke is a suspension of airborne particulates and gases emitted when a material undergoes combustion or pyrolysis, together with the quantity of air that is entrained or otherwise mixed into the mass. It is commonly an unwanted by-pr ...
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. 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. The number of droplets with diameter between $d$ and $D+dD$ per unit volume of space is $n\left(d\right) = n_0 e^ dD$. 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 distribution In probability theory and statistics, the gamma distribution is a two- parameter family of continuous probability distributions. The exponential distribution, Erlang distribution, and chi-square distribution are special cases of the gamma di ...
s. The distribution has an upper limit due to droplet fragmentation.

## Raindrop impacts

Raindrops impact at their
terminal velocity Terminal velocity is the maximum velocity (speed) attainable by an object as it falls through a fluid ( air is the most common example). It occurs when the sum of the drag force (''Fd'') and the buoyancy is equal to the downward force of grav ...
, 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. 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 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 before reaching the ground. This is termed
virga In meteorology, a virga, also called a dry storm, is an observable streak or shaft of precipitation falling from a cloud that evaporates or sublimates before reaching the ground. A shaft of precipitation that does not evaporate before re ...
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 front A warm front is a density discontinuity located at the leading edge of a homogeneous warm air mass, and is typically located on the equator-facing edge of an isotherm gradient. Warm fronts lie within broader troughs of low pressure than cold fro ...
s. Similar ascent is seen around tropical cyclones outside the
eyewall The eye is a region of mostly calm weather at the center of tropical cyclones. The eye of a storm is a roughly circular area, typically in diameter. It is surrounded by the ''eyewall'', a ring of towering thunderstorms where the most severe weat ...
, and in comma-head precipitation patterns around mid-latitude cyclones. 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 slush ...
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 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 Cumulonimbus (from Latin ''cumulus'', "heaped" and ''nimbus'', "rainstorm") is a dense, towering vertical cloud, typically forming from water vapor condensing in the lower troposphere that builds upward carried by powerful buoyant air currents ...
or
cumulus congestus Cumulus congestus clouds, also known as towering cumulus, are a form of cumulus that can be based in the low or middle height ranges. They achieve considerable vertical development in areas of deep, moist convection. They are an intermediate stag ...
). 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 The tropics are the regions of Earth surrounding the Equator. They are defined in latitude by the Tropic of Cancer in the Northern Hemisphere at N and the Tropic of Capricorn in the Southern Hemisphere at S. The tropics are also referred ...
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 fr ...
and hail indicate convection. In mid-latitudes, convective precipitation is intermittent and often associated with baroclinic boundaries such as cold fronts,
squall line A squall line, or more accurately a quasi-linear convective system (QLCS), is a line of thunderstorms, often forming along or ahead of a cold front. In the early 20th century, the term was used as a synonym for cold front (which often are accomp ...
s, and warm fronts.

## Orographic effects

Orographic precipitation occurs on the
windward 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 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 wind The trade winds or easterlies are the permanent east-to-west prevailing winds that flow in the Earth's equatorial region. The trade winds blow mainly from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisph ...
s), a more moist
climate Climate is the long-term weather pattern in an area, typically averaged over 30 years. More rigorously, it is the mean and variability of meteorological variables over a time spanning from months to millions of years. Some of the meteorologic ...
usually prevails on the windward side of a mountain than 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 ...
or downwind side. Moisture is removed by orographic lift, leaving drier air (see
katabatic wind A katabatic wind (named from the Greek word κατάβασις '' katabasis'', meaning "descending") is a drainage wind, a wind that carries high-density air from a higher elevation down a slope under the force of gravity. Such winds are someti ...
) on the descending and generally warming, leeward side where a rain shadow is observed. In Hawaii, Mount Waiʻaleʻale, 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 The trade winds or easterlies are the permanent east-to-west prevailing winds that flow in the Earth's equatorial region. The trade winds blow mainly from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisph ...
and receive much more rainfall; leeward sides are drier and sunnier, with less rain and less cloud cover. In South America, the
Andes The Andes, Andes Mountains or Andean Mountains (; ) are the longest continental mountain range in the world, forming a continuous highland along the western edge of South America. The range is long, wide (widest between 18°S – 20°S l ...
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 primar ...
range creates the same effect in North America forming the Great Basin 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 by tourist authorities. Areas with wet seasons are dispersed across portions of the
tropics The tropics are the regions of Earth surrounding the Equator. They are defined in latitude by the Tropic of Cancer in the Northern Hemisphere at N and the Tropic of Capricorn in the Southern Hemisphere at S. The tropics are also referred ...
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 a ...
.
Savanna 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 does not close. The open canopy allows sufficient light to reach the ground t ...
climates and areas with
monsoon A monsoon () is traditionally a seasonal reversing wind accompanied by corresponding changes in precipitation but is now used to describe seasonal changes in atmospheric circulation and precipitation associated with annual latitudinal oscill ...
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 intertropical convergence zone 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, w ...
, 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 includ ...
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 ...
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

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. 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 McAllen is the largest city in Hidalgo County, Texas, United States, and the 22nd-most populous city in Texas. It is located at the southern tip of the state in the Rio Grande Valley, on the Mexico–United States border. The city limits ext ...
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 microphysica ...
, which include techniques used to increase winter precipitation over mountains and suppress hail.

# Characteristics

## Patterns

Rainband A rainband is a cloud and precipitation structure associated with an area of rainfall which is significantly elongated. Rainbands can be stratiform or convective, and are generated by differences in temperature. When noted on weather radar ima ...
s are cloud and precipitation areas which are significantly elongated. Rainbands can be stratiform or
convective Convection is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the conve ...
, 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 front A warm front is a density discontinuity located at the leading edge of a homogeneous warm air mass, and is typically located on the equator-facing edge of an isotherm gradient. Warm fronts lie within broader troughs of low pressure than cold fro ...
s 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 line A squall line, or more accurately a quasi-linear convective system (QLCS), is a line of thunderstorms, often forming along or ahead of a cold front. In the early 20th century, the term was used as a synonym for cold front (which often are accomp ...
s which are able to produce
tornado A tornado is a violently rotating column of air that is in contact with both the surface of the Earth and a cumulonimbus cloud or, in rare cases, the base of a cumulus cloud. It is often referred to as a twister, whirlwind or cyclone, altho ...
es. 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 A sea breeze or onshore breeze is any wind that blows from a large body of water toward or onto a landmass; it develops due to differences in air pressure created by the differing heat capacities of water and dry land. As such, sea breezes a ...
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-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 p ...
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 The Great Lakes, also called the Great Lakes of North America, are a series of large interconnected freshwater lakes in the mid-east region of North America that connect to the Atlantic Ocean via the Saint Lawrence River. There are five lakes ...
. Downwind of islands, bands of showers and thunderstorms can develop due to low level wind convergence downwind of the island edges. Offshore
California California is a U.S. state, state in the Western United States, located along the West Coast of the United States, Pacific Coast. With nearly 39.2million residents across a total area of approximately , it is the List of states and territori ...
, 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 A university () is an institution of higher (or tertiary) education and research which awards academic degrees in several academic disciplines. Universities typically offer both undergraduate and postgraduate programs. In the United States, ...
(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,
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 form ...
(H2SO4) and
nitric acid Nitric acid is the inorganic compound with the formula . It is a highly corrosive mineral acid. The compound is colorless, but older samples tend to be yellow cast due to decomposition into oxides of nitrogen. Most commercially available nitr ...
(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

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 A monsoon () is traditionally a seasonal reversing wind accompanied by corresponding changes in precipitation but is now used to describe seasonal changes in atmospheric circulation and precipitation associated with annual latitudinal oscill ...
, tropical savanna, humid subtropical, humid continental, oceanic climate, 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 In physical geography, tundra () is a type of biome where tree growth is hindered by frigid temperatures and short growing seasons. The term ''tundra'' comes through Russian (') from the Kildin Sámi word (') meaning "uplands", "treeless m ...
,
polar ice cap A polar ice cap or polar cap is a high-latitude region of a planet, dwarf planet, or natural satellite that is covered in ice. There are no requirements with respect to size or composition for a body of ice to be termed a polar ice cap, nor ...
, 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 one ...
. Rain forests are characterized by high rainfall, with definitions setting minimum normal annual rainfall between . A tropical savanna is a grassland
biome A biome () is a biogeographical unit consisting of a biological community that has formed in response to the physical environment in which they are found and a shared regional climate. Biomes may span more than one continent. Biome is a broader ...
located in
semi-arid A semi-arid climate, semi-desert climate, or steppe climate is a dry climate sub-type. It is located on regions that receive precipitation below potential evapotranspiration, but not as low as a desert climate. There are different kinds of semi-a ...
to semi-humid climate regions of
subtropical 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 a ...
and
tropical The tropics are the regions of Earth surrounding the Equator. They are defined in latitude by the Tropic of Cancer in the Northern Hemisphere at N and the Tropic of Capricorn in the Southern Hemisphere at S. The tropics are also referred ...
latitudes, with rainfall between a year. They are widespread on Africa, and are also found in India, the northern parts of South America,
Malaysia Malaysia ( ; ) is a country in Southeast Asia. The federal constitutional monarchy consists of thirteen states and three federal territories, separated by the South China Sea into two regions: Peninsular Malaysia and Borneo's East Malaysi ...
, and Australia. The humid subtropical climate zone is where winter rainfall is associated with large
storm A storm is any disturbed state of the natural environment or the atmosphere of an astronomical body. It may be marked by significant disruptions to normal conditions such as strong wind, tornadoes, hail, thunder and lightning (a thunderstorm ...
s that the
westerlies The westerlies, anti-trades, or prevailing westerlies, are prevailing winds from the west toward the east in the middle latitudes between 30 and 60 degrees latitude. They originate from the high-pressure areas in the horse latitudes and tren ...
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
latitude 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 pole ...
s 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 wa ...
, parts of western North America, parts of
Western Western may refer to: Places *Western, Nebraska, a village in the US * Western, New York, a town in the US *Western Creek, Tasmania, a locality in Australia * Western Junction, Tasmania, a locality in Australia *Western world, countries that ...
and
South Australia South Australia (commonly abbreviated as SA) is a state in the southern central part of Australia. It covers some of the most arid parts of the country. With a total land area of , it is the fourth-largest of Australia's states and territories ...
, in southwestern
South Africa South Africa, officially the Republic of South Africa (RSA), is the southernmost country in Africa. It is bounded to the south by of coastline that stretch along the South Atlantic and Indian Oceans; to the north by the neighbouring countr ...
and in parts of central
Chile Chile, officially the Republic of Chile, is a country in the western part of South America. It is the southernmost country in the world, and the closest to Antarctica, occupying a long and narrow strip of land between the Andes to the east ...
. 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.

# Measurement

## Gauges

Rain is measured in units of length per unit time, typically in millimeters per hour, or in countries where
imperial units The imperial system of units, imperial system or imperial units (also known as British Imperial or Exchequer Standards of 1826) is the system of units first defined in the British Weights and Measures Act 1824 and continued to be developed thro ...
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 Hydrology () is the scientific study of the movement, distribution, and management of water on Earth and other planets, including the water cycle, water resources, and environmental watershed sustainability. A practitioner of hydrology is call ...
purposes. For instance, river
flood control Flood control methods are used to reduce or prevent the detrimental effects of flood waters."Flood Control", MSN Encarta, 2008 (see below: Further reading). Flood relief methods are used to reduce the effects of flood waters or high water leve ...
, 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 orbit A polar orbit is one in which a satellite passes above or nearly above both poles of the body being orbited (usually a planet such as the Earth, but possibly another body such as the Moon or Sun) on each revolution. It has an inclination of abo ...
ing as well as geostationary
weather satellite A weather satellite or meteorological satellite is a type of Earth observation satellite that is primarily used to monitor the weather and climate of the Earth. Satellites can be polar orbiting (covering the entire Earth asynchronously), or geo ...
s 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 A return period, also known as a recurrence interval or repeat interval, is an average time or an estimated average time between events such as earthquakes, floods, landslides, or river discharge flows to occur. It is a statistical measurement typ ...
. 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 Greenwich Mean Time (GMT) is the mean solar time at the Royal Observatory in Greenwich, London, counted from midnight. At different times in the past, it has been calculated in different ways, including being calculated from noon; as a cons ...
. 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 A skill is the learned ability to act with determined results with good execution often within a given amount of time, energy, or both. Skills can often be divided into domain-general and domain-specific skills. For example, in the domain of ...
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 A drought is defined as drier than normal conditions.Douville, H., K. Raghavan, J. Renwick, R.P. Allan, P.A. Arias, M. Barlow, R. Cerezo-Mota, A. Cherchi, T.Y. Gan, J. Gergis, D.  Jiang, A.  Khan, W.  Pokam Mba, D.  Rosenfeld, J. Tierney, an ...
can kill crops and increase erosion, while overly wet weather can cause harmful
fungus A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from ...
growth. Plants need varying amounts of rainfall to survive. For example, certain cacti 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 Soil, also commonly referred to as earth or dirt, is a mixture of organic matter, minerals, gases, liquids, and organisms that together support life. Some scientific definitions distinguish ''dirt'' from ''soil'' by restricting the former te ...
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 tank A rainwater tank (sometimes called a rain barrel in North America in reference to smaller tanks, or a water butt in the UK) is a water tank used to collect and store rain water runoff, typically from rooftops via pipes. Rainwater tanks are device ...
s; 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 ov ...
s.

## Culture and religion

Cultural attitudes towards rain differ across the world. In
temperate climate In geography, the temperate climates of Earth occur in the middle latitudes (23.5° to 66.5° N/S of Equator), which span between the tropics and the polar regions of Earth. These zones generally have wider temperature ranges througho ...
s, 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 A drought is defined as drier than normal conditions.Douville, H., K. Raghavan, J. Renwick, R.P. Allan, P.A. Arias, M. Barlow, R. Cerezo-Mota, A. Cherchi, T.Y. Gan, J. Gergis, D.  Jiang, A.  Khan, W.  Pokam Mba, D.  Rosenfeld, J. Tierney, an ...
, rain lifts people's moods. In Botswana, the
Setswana Tswana, also known by its native name , and previously spelled Sechuana in English, is a Bantu language spoken in Southern Africa by about 8.2 million people. It belongs to the Bantu language family within the Sotho-Tswana branch of Zone ...
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, U.S. and Canada), surface water drain/sewer (United Kingdom), or stormwater drain (Australia and New Zealand) is infrastructure designed to drain excess rain and ground water from impervious surfac ...
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 Petrichor () is the earthy scent produced when rain falls on dry soil. The word is constructed , the ethereal fluid that is the blood of the gods in Greek mythology. Origins Long before this phenomenon received its name in 1964, it had been ...
, an oil produced by plants, then absorbed by rocks and soil, and later released into the air during rainfall. 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 ...
ians believed that rain was the
semen Semen, also known as seminal fluid, is an organic bodily fluid created to contain spermatozoa. It is secreted by the gonads (sexual glands) and other sexual organs of male or hermaphroditic animals and can fertilize the female ovum. Semen i ...
of the
sky-god The sky often has important religious significance. Many religions, both polytheistic and monotheistic, have deities associated with the sky. The daytime sky deities are typically distinct from the nighttime ones. Stith Thompson's ''Motif-I ...
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 The Akkadian Empire () was the first ancient empire of Mesopotamia after the long-lived civilization of Sumer. It was centered in the city of Akkad () and its surrounding region. The empire united Akkadian and Sumerian speakers under one ru ...
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 Judea or Judaea ( or ; from he, יהודה, Standard ''Yəhūda'', Tiberian ''Yehūḏā''; el, Ἰουδαία, ; la, Iūdaea) is an ancient, historic, Biblical Hebrew, contemporaneous Latin, and the modern-day name of the mountainous sout ...
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 Marcus Aurelius Antoninus (Latin: áːɾkus̠ auɾέːli.us̠ antɔ́ːni.us̠ English: ; 26 April 121 – 17 March 180) was Roman emperor from 161 to 180 AD and a Stoic philosopher. He was the last of the rulers known as the Five Good E ...
preserves a prayer for rain made by the Athenians to the Greek sky-god
Zeus Zeus or , , ; grc, Δῐός, ''Diós'', label= genitive Boeotian Aeolic and Laconian grc-dor, Δεύς, Deús ; grc, Δέος, ''Déos'', label= genitive el, Δίας, ''Días'' () is the sky and thunder god in ancient Greek religi ...
. 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 , photo = Sahara real color.jpg , photo_caption = The Sahara taken by Apollo 17 astronauts, 1972 , map = , map_image = , location = , country = , country1 = , ...
. Some deserts also occupy much of southern Africa: the
Namib The Namib ( ; pt, Namibe) is a coastal desert in Southern Africa. The name is of Khoekhoegowab origin and means "vast place". According to the broadest definition, the Namib stretches for more than along the Atlantic coasts of Angola, Nami ...
and the
Kalahari The Kalahari Desert is a large semi-arid sandy savanna in Southern Africa extending for , covering much of Botswana, and parts of Namibia and South Africa. It is not to be confused with the Angolan, Namibian, and South African Namib coastal de ...
. 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 The Andes, Andes Mountains or Andean Mountains (; ) are the longest continental mountain range in the world, forming a continuous highland along the western edge of South America. The range is long, wide (widest between 18°S – 20°S l ...
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 The Sonoran Desert ( es, Desierto de Sonora) is a desert in North America and ecoregion that covers the northwestern Mexican states of Sonora, Baja California, and Baja California Sur, as well as part of the southwestern United States (in Arizona ...
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 Freezing rain is rain maintained at temperatures below freezing by the ambient air mass that causes freezing on contact with surfaces. Unlike a mixture of rain and snow or ice pellets, freezing rain is made entirely of liquid droplets. The rai ...
. 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 In physical geography, tundra () is a type of biome where tree growth is hindered by frigid temperatures and short growing seasons. The term ''tundra'' comes through Russian (') from the Kildin Sámi word (') meaning "uplands", "treeless m ...
which has a short summer thaw and a long frozen winter. Ice caps see no rain at all, making
Antarctica Antarctica () is Earth's southernmost and least-populated continent A continent is any of several large landmasses. Generally identified by convention rather than any strict criteria, up to seven geographical regions are commonly ...
the world's driest continent.

## Rainforests

Rainforests are areas of the world with very high rainfall. Both
tropical The tropics are the regions of Earth surrounding the Equator. They are defined in latitude by the Tropic of Cancer in the Northern Hemisphere at N and the Tropic of Capricorn in the Southern Hemisphere at S. The tropics are also referred ...
and
temperate In geography, the temperate climates of Earth occur in the middle latitudes (23.5° to 66.5° N/S of Equator), which span between the tropics and the polar regions of Earth. These zones generally have wider temperature ranges throughout t ...
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 a ...
. 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. 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 The Indian Ocean is the third-largest of the world's five oceanic divisions, covering or ~19.8% of the water on Earth's surface. It is bounded by Asia to the north, Africa to the west and Australia to the east. To the south it is bounded by th ...
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 Thermal lows, or heat lows, are non- frontal low-pressure areas that occur over the continents in the subtropics during the warm season, as the result of intense heating when compared to their surrounding environments.Glossary of Meteorology (20 ...
) 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 The Gulf of Mexico ( es, Golfo de México) is an ocean basin and a marginal sea of the Atlantic Ocean, largely surrounded by the North American continent. It is bounded on the northeast, north and northwest by the Gulf Coast of the United Sta ...
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 subtropical ridges, or highs. It is a high-press ...
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, ...
. The eastern half of the contiguous United States east of the 98th meridian, the mountains of the Pacific Northwest, and 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 primar ...
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 Puerto Rico (; abbreviated PR; tnq, Boriken, ''Borinquen''), officially the Commonwealth of Puerto Rico ( es, link=yes, Estado Libre Asociado de Puerto Rico, lit=Free Associated State of Puerto Rico), is a Caribbean island and unincorporated ...
, the
United States Virgin Islands The United States Virgin Islands,. Also called the ''American Virgin Islands'' and the ''U.S. Virgin Islands''. officially the Virgin Islands of the United States, are a group of Caribbean islands and an unincorporated and organized territory ...
, the
Northern Mariana Islands The Northern Mariana Islands, officially the Commonwealth of the Northern Mariana Islands (CNMI; ch, Sankattan Siha Na Islas Mariånas; cal, Commonwealth Téél Falúw kka Efáng llól Marianas), is an unincorporated territory and commonwea ...
, 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 Bergen (), historically Bjørgvin, is a city and municipality in Vestland county on the west coast of Norway. , its population is roughly 285,900. Bergen is the second-largest city in Norway. The municipality covers and is on the peninsula o ...
, 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 The Great Lakes, also called the Great Lakes of North America, are a series of large interconnected freshwater lakes in the mid-east region of North America that connect to the Atlantic Ocean via the Saint Lawrence River. There are five 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 In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate variability and change, Climate change in a broader sense also includes ...
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 ...
, situated on the southern slopes of the
Eastern Himalaya ] The Eastern Himalayas extend from eastern Nepal across Northeast India, Bhutan, the Tibet Autonomous Region to Yunnan in China and northern Myanmar. The climate of this region is influenced by the monsoon of South Asia from June to September. I ...
in
Shillong Shillong () is a hill station and the capital of Meghalaya, a state in northeastern India, which means "The Abode of Clouds". It is the headquarters of the East Khasi Hills district. Shillong is the 330th most populous city in India with a p ...
, 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 Mawsynram () is a town in the East Khasi Hills district of Meghalaya state in Northeastern India, 69 kilometres from Shillong, the state capital. Mawsynram receives the highest rainfall in India. It is reportedly the wettest place on Earth, w ...
, 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' ...
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 has the highest average annual rainfall in the Hawaiian Islands, at . Mount Waiʻaleʻale 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 21st largest island ...
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 Carib ...
, 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 Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but slightly less than one-thousandth ...
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 Ear ...
, 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 neon in the deep atmospheres. On Titan,
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 Ear ...
's largest natural satellite, infrequent
methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on E ...
rain is thought to carve the moon's numerous surface channels. On
Venus Venus is the second planet from the Sun. It is sometimes called Earth's "sister" or "twin" planet as it is almost as large and has a similar composition. As an interior planet to Earth, Venus (like Mercury) appears in Earth's sky never far ...
,
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 form ...
virga In meteorology, a virga, also called a dry storm, is an observable streak or shaft of precipitation falling from a cloud that evaporates or sublimates before reaching the ground. A shaft of precipitation that does not evaporate before re ...
evaporates from the surface. Extrasolar planet
OGLE-TR-56b OGLE-TR-56b is an extrasolar planet located approximately 1500 parsecs away in the constellation of Sagittarius, orbiting the star OGLE-TR-56. This planet was the first known exoplanet to be discovered with the transit method. The object was disc ...
in the constellation Sagittarius is hypothesized to have
iron Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in fr ...
rain. Accordingly, research carried out by the European Southern Observatory shows that WASP-76b can produce showers of burning liquid iron droplets once temperature decreases during the planet's night hours. There is evidence from samples of
basalt Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90% of a ...
brought back by the Apollo missions that the
Moon The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...
has been subject to
lava Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a moon onto its surface. Lava may be erupted at a volcano or through a fracture in the crust, on land or u ...
rain.Taylor, G. Jeffrey
"Finding distant chips from distant maria"
pp. 8–9, ''Planetary Science Research Discoveries'', 30 April 2006.

* Atmospheric river *
Hydropower Hydropower (from el, ὕδωρ, "water"), also known as water power, is the use of falling or fast-running water to produce electricity or to power machines. This is achieved by converting the gravitational potential or kinetic energy of a w ...
*
Intensity-duration-frequency curve An intensity-duration-frequency curve (IDF curve) is a mathematical function that relates the rainfall intensity with its duration and frequency of occurrence. These curves are commonly used in hydrology for flood forecasting and civil engineering ...
* Precipitation types * Rain dust *
Rain sensor A rain sensor or ''rain switch'' is a switching device activated by rainfall. There are two main applications for rain sensors. The first is a water conservation device connected to an automatic irrigation system that causes the system to shut do ...
* Rain water harvesting * Rainbow * Raining animals * Red rain in Kerala *
Petrichor Petrichor () is the earthy scent produced when rain falls on dry soil. The word is constructed , the ethereal fluid that is the blood of the gods in Greek mythology. Origins Long before this phenomenon received its name in 1964, it had been ...
– the cause of the scent during and after rain * Sanitary sewer overflow * Sediment precipitation *
Water resources Water resources are natural resources of water that are potentially useful for humans, for example as a source of drinking water supply or irrigation water. 97% of the water on the Earth is salt water and only three percent is fresh water; slight ...
*
Weather Weather is the state of the atmosphere, describing for example the degree to which it is hot or cold, wet or dry, calm or stormy, clear or cloudy. On Earth, most weather phenomena occur in the lowest layer of the planet's atmosphere, the tr ...
*
Rainmaking Rainmaking, also known as artificial precipitation, artificial rainfall and pluviculture, is the act of attempting to artificially induce or increase precipitation, usually to stave off drought or the wider global warming. According to the cloud ...
*
Johad A johad, also known as a pokhar or a percolation pond, is a community-owned traditional harvested rainwater storage wetland principally used for effectively harnessing water resources in the states of Haryana, Rajasthan, Punjab, and weste ...

# 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 Quibdóis an estimated amount. * Approximate elevation. * Recognized as "The Wettest place on Earth" by the '' Guinness Book of World Records''. * 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.