is liquid water in the form of droplets that have condensed from
atmospheric water vapor and then becomes heavy enough to fall under
is a major component of the water cycle and is
responsible for depositing most of the fresh water on the Earth. It
provides suitable conditions for many types of ecosystems, as well as
water for hydroelectric power plants and crop irrigation.
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 is maximized within
windward sides of the terrain at elevation which forces moist air to
condense and fall out as rainfall along the sides of mountains. On the
leeward side of mountains, desert climates can exist due to the dry
air caused by downslope flow which causes heating and drying of the
air mass. The movement of the monsoon trough, or intertropical
convergence zone, brings rainy seasons to savannah climes.
The urban heat island effect leads to increased rainfall, both in
amounts and intensity, downwind of cities.
causing changes in the precipitation pattern globally, including
wetter conditions across eastern
and drier conditions in
the tropics.
is the driest continent. The
globally averaged annual precipitation over land is 715 mm
(28.1 in), but over the whole Earth it is much higher at
990 mm (39 in).
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.
is also known or suspected on other planets, where it may be
composed of methane, neon, sulfuric acid, or even iron rather than
1.1 Water-saturated air
1.2 Coalescence and fragmentation
Droplet size distribution
1.4 Raindrop impacts
2.1 Frontal activity
2.3 Orographic effects
2.4 Within the tropics
2.5 Human influence
3.3 Köppen climate classification
4.2 Remote sensing
4.4 Return period
6.1 Effect on agriculture
6.2 In culture and religion
7 Global climatology
7.2 Polar desert
7.5 Impact of the Westerlies
7.6 Wettest known locations
8 Outside Earth
9 See also
12 External links
Rain falling on a field, in southern Estonia
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, 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, 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
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 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
The shape of rain drops depending upon their size
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 0.1 to 9 mm (0.0039 to
0.3543 in) mean diameter, above which they tend to break up.
Smaller drops are called cloud droplets, and their shape is spherical.
As a raindrop increases in size, its shape becomes more oblate, with
its largest cross-section facing the oncoming airflow. Large rain
drops become increasingly flattened on the bottom, like hamburger
buns; very large ones are shaped like parachutes. 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 10 mm
(0.39 in). The large size is explained by condensation on large
smoke particles or by collisions between drops in small regions with
particularly high content of liquid water.
Rain drops associated with melting hail tend to be larger than other
A raindrop on a 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.
The number of droplets with diameter between
per unit volume of space is
displaystyle n(d)=n_ 0 e^ -d/langle drangle 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
displaystyle langle drangle ^ -1 =41R^ -0.21
(d in centimeters and R in millimetres 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
Raindrops impact at their terminal velocity, which is greater for
larger drops due to their larger mass to drag ratio. At sea level and
without wind, 0.5 mm (0.020 in) drizzle impacts at
2 m/s (6.6 ft/s) or 7.2 km/h (4.5 mph), while
large 5 mm (0.20 in) drops impact at around 9 m/s
(30 ft/s) or 32 km/h (20 mph).
Rain falling on loosely packed material such as newly fallen ash can
produce dimples that can be fossilized. 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
The sound of raindrops hitting water is caused by bubbles of air
METAR code for rain is RA, while the coding for rain showers is
Main article: Virga
In certain conditions precipitation may fall from a cloud but then
evaporate or sublime before reaching the ground. This is termed virga
and is more often seen in hot and dry climates.
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 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 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 3,400 m (11,000 ft) above ground
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 and hail indicate
convection. In mid-latitudes, convective precipitation is
intermittent and often associated with baroclinic boundaries such as
cold fronts, squall lines, and warm fronts.
Main articles: Orographic lift,
Precipitation types (meteorology), and
United States rainfall climatology
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 leeward 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 is observed.
In Hawaii, Mount Waiʻaleʻale, on the island of Kauai, is notable for
its extreme rainfall, as it has the second highest average annual
rainfall on Earth, with 12,000 mm (460 in). Systems
known as Kona storms affect the state with heavy rains between October
and April. Local climates vary considerably on each island due to
their topography, divisible into windward (Koʻolau) and leeward
(Kona) regions based upon location relative to the higher mountains.
Windward sides face the east to northeast trade winds and receive much
more rainfall; leeward sides are drier and sunnier, with less rain and
less cloud cover.
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 range creates
the same effect in
North America forming the
Great Basin and Mojave
Within the tropics
Rainfall distribution by month in
Cairns showing the extent of the wet
season at that location
Main article: Wet season
The wet, or rainy, season is the time of year, covering one or more
months, when most of the average annual rainfall in a region
falls. 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 and subtropics.
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 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, rain
falls mainly during the late afternoon and early evening hours. The
wet season is a time when air quality improves, freshwater 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 cyclones can take an enormous toll
in lives and personal property, they may be important factors in the
precipitation regimes of places they impact, as they may bring
much-needed precipitation to otherwise dry regions. Areas in their
path can receive a year's worth of rainfall from a tropical cyclone
Atlanta, Georgia showing temperature distribution, with blue
showing cool temperatures, red warm, and hot areas appearing white.
Mean surface temperature anomalies during the period 1999 to 2008 with
respect to the average temperatures from 1940 to 1980
Global warming and Urban heat island
The fine particulate matter produced by car exhaust and other human
sources of pollution forms cloud condensation nuclei, leads to the
production of clouds and increases the likelihood of rain. As
commuters and commercial traffic cause pollution to build up over the
course of the week, the likelihood of rain increases: it peaks by
Saturday, after five days of weekday pollution has been built up. In
heavily populated areas that are near the coast, such as the United
States' Eastern Seaboard, the effect can be dramatic: there is a 22%
higher chance of rain on Saturdays than on Mondays. The urban heat
island effect warms cities 0.6 to 5.6 °C (1.1 to 10.1 °F)
above surrounding suburbs and rural areas. This extra heat leads to
greater upward motion, which can induce additional shower and
thunderstorm activity. Rainfall rates downwind of cities are increased
between 48% and 116%. Partly as a result of this warming, monthly
rainfall is about 28% greater between 32 to 64 km (20 to
40 mi) 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
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
become wetter. The Sahel, the Mediterranean, southern
Africa and parts
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 1 percent of all rain and snow days during the years
The most successful attempts at influencing weather involve cloud
seeding, which include techniques used to increase winter
precipitation over mountains and suppress hail.
Band of thunderstorms seen on a weather radar display
Main article: Rainband
Rainbands are cloud and precipitation areas which are significantly
Rainbands can be stratiform or convective, and are
generated by differences in temperature. When noted on weather radar
imagery, this precipitation elongation is referred to as banded
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, a trough of warm air aloft, or "trowal" for
short, will be caused by strong southerly winds on its eastern
periphery rotating aloft around its northeast, and ultimately
northwestern, periphery (also known as the warm conveyor belt),
forcing a surface trough to continue into the cold sector on a similar
curve to the occluded front. The trowal creates the portion of an
occluded cyclone known as its comma head, due to the comma-like shape
of the mid-tropospheric cloudiness that accompanies the feature. It
can also be the focus of locally heavy precipitation, with
thunderstorms possible if the atmosphere along the trowal is unstable
enough for convection. Banding within the comma head precipitation
pattern of an extratropical cyclone 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.
Sources of acid rain
See also: Acid rain
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
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
(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
Updated Köppen-Geiger climate map
Main article: Köppen climate classification
The Köppen classification depends on average monthly values of
temperature and precipitation. The most commonly used form of the
Köppen classification has five primary types labeled A through E.
Specifically, the primary types are A, tropical; B, dry; C, mild
mid-latitude; D, cold mid-latitude; and E, polar. The five primary
classifications can be further divided into secondary classifications
such as rain forest, monsoon, tropical savanna, humid subtropical,
humid continental, oceanic climate, Mediterranean climate, steppe,
subarctic climate, tundra, polar ice cap, and desert.
Rain forests are characterized by high rainfall, with definitions
setting minimum normal annual rainfall between 1,750 and 2,000 mm
(69 and 79 in). A tropical savanna is a grassland biome
located in semi-arid to semi-humid climate regions of subtropical and
tropical latitudes, with rainfall between 750 and 1,270 mm (30
and 50 in) 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, 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 and little
Standard rain gauge
Rain gauge, Disdrometer, and
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 25 mm
(0.98 in) of rain, with overflow flowing into the outer cylinder.
Plastic gauges have markings on the inner cylinder down to
0.25 mm (0.0098 in) resolution, while metal gauges require
use of a stick designed with the appropriate 0.25 mm
(0.0098 in) markings. After the inner cylinder is filled, the
amount inside it is discarded, then filled with the remaining rainfall
in the outer cylinder until all the fluid in the outer cylinder is
gone, adding to the overall total until the outer cylinder is
empty. 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
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
Twenty-four-hour rainfall accumulation on the Val d'Irène radar in
Eastern Canada. Zones without data in the east and southwest are
caused by beam blocking from mountains. (Source: Environment Canada)
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 compliment 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,
displaystyle Z=AR^ 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 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.
1988 rain in the U.S. The heaviest rain is seen in reds and yellows.
1993 rain in the U.S.
Heavy rain in Glenshaw, Pennsylvania
The sound of a heavy rain fall in suburban neighborhood
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Rainfall intensity is classified according to the rate of
precipitation, which depends on the considered time:
Light rain — when the precipitation rate is < 2.5 mm
(0.098 in) per hour
Moderate rain — when the precipitation rate is between 2.5 mm
(0.098 in) - 7.6 mm (0.30 in) or 10 mm
(0.39 in) per hour
Heavy rain — when the precipitation rate is > 7.6 mm
(0.30 in) per hour, or between 10 mm (0.39 in) and
50 mm (2.0 in) per hour
Violent rain — when the precipitation rate is > 50 mm
(2.0 in) 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.
See also: 100-year flood
The likelihood or probability of an event with a specified intensity
and duration, is called the return period or frequency. 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 term 1 in 10 year storm describes a rainfall event which is
unusual and has a 50% chance of occurring in any 10-year period. The
term 1 in 100 year storm describes a rainfall event which is rare
and which will occur with a 50% probability in any 100-year period. As
with all probability events, it is possible, though improbable, to
have multiple "1 in 100 Year Storms" in a single year.
Main article: Quantitative precipitation forecast
Example of a five-day rainfall forecast from the Hydrometeorological
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
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
Effect on agriculture
Rainfall estimates for southern
Japan and the surrounding region from
July 20–27, 2009.
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 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.
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 floods.
In culture and religion
Photograph of a rain dance being performed in Harar, Ethiopia
See also: List of rain deities
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.
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 drains 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.
Rain holds an important religious significance in many cultures.
The ancient Sumerians 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 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
Days of Prayer for Rain in the State of Texas in
See also: Earth rainfall climatology
Approximately 505,000 km3 (121,000 cu mi) of water
falls as precipitation each year across the globe with
398,000 km3 (95,000 cu mi) of it over the oceans.
Given the Earth's surface area, that means the globally averaged
annual precipitation is 990 mm (39 in). Deserts are defined
as areas with an average annual precipitation of less than 250 mm
(10 in) per year, or as areas where more water is lost
by evapotranspiration than falls as precipitation.
Main article: Desert
The northern half of
Africa is occupied by the world's most extensive
hot, dry region, the Sahara Desert. Some deserts are also occupying
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 in Mongolia west-southwest through
western Pakistan (Balochistan) and Iran into the Arabian
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
Desert overspreads the
Desert Southwest, the Great Basin
and central Wyoming.
Isolated towering vertical desert shower
Polar desert and Polar climate
Since rain only falls as liquid, in frozen temperatures, rain cannot
fall. As a result, 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
See also: Rainforest
Rainforests are areas of the world with very high rainfall. Both
tropical and temperate rainforests exist.
Tropical rainforests occupy
a large band of the planet mostly along the equator. Most temperate
rainforests are located on mountainous west coasts between 45 and 55
degrees latitude, but they are often found in other areas.
Around 40–75% of all biotic life is found in rainforests.
Rainforests are also responsible for 28% of the world's oxygen
The equatorial region near the
Intertropical Convergence Zone
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 into the region. The monsoon
trough can reach as far north as the 40th parallel in East
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
Gulf of Mexico
Gulf of Mexico moisture moving around the subtropical
ridge in the Atlantic
Ocean bring the promise of afternoon and evening
thunderstorms to the southern tier of the
United States as well as the
Great Plains. The eastern half of the contiguous United States
east of the 98th meridian, the mountains of the Pacific Northwest, and
the Sierra Nevada range are the wetter portions of the nation, with
average rainfall exceeding 760 mm (30 in) per year.
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
Impact of the Westerlies
Long-term mean precipitation by month
See also: Westerlies
Westerly flow from the mild north Atlantic leads to wetness across
western Europe, in particular
Ireland and the United Kingdom, where
the western coasts can receive between 1,000 mm (39 in), at
sea-level and 2,500 mm (98 in), on the mountains of rain per
year. Bergen, Norway is one of the more famous European rain-cities
with its yearly precipitation of 2,250 mm (89 in) on
average. During the fall, winter, and spring, Pacific storm systems
bring most of
Hawaii and the western
United States much of their
precipitation. 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,
Great Lakes during the warm season, contributing up to
10% of the annual precipitation to the region.
El Niño-Southern Oscillation
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
Wettest known locations
Cherrapunji, situated on the southern slopes of the Eastern Himalaya
India is the confirmed wettest place on Earth, with an
average annual rainfall of 11,430 mm (450 in). The highest
recorded rainfall in a single year was 22,987 mm (905.0 in)
in 1861. The 38-year average at nearby Mawsynram, Meghalaya,
11,873 mm (467.4 in). The wettest spot in
Mount Bellenden Ker
Mount Bellenden Ker in the north-east of the country which records an
average of 8,000 mm (310 in) per year, with over
12,200 mm (480.3 in) of rain recorded during 2000.
Mount Waiʻaleʻale on the island of
Kauaʻi in the Hawaiian Islands
averages more than 12,000 mm (460 in) of rain per year
over the last 32 years, with a record 17,340 mm (683 in) in
1982. Its summit is considered one of the rainiest
spots on earth. It has been promoted in tourist literature for many
years as the wettest spot in the world.[not in citation given]
Lloró, a town situated in Chocó, Colombia, is probably the place
with the largest rainfall in the world, averaging 13,300 mm
(523.6 in) per year. The Department of Chocó is
extraordinarily humid. Tutunendaó, a small town situated in the same
department, is one of the wettest estimated places on Earth, averaging
11,394 mm (448.6 in) per year; in 1974 the town received
26,303 mm (86 ft 3.6 in), the largest
annual rainfall measured in Colombia. Unlike Cherrapunji, which
receives most of its rainfall between April and September, Tutunendaó
receives rain almost uniformly distributed throughout the year.
Quibdó, the capital of Chocó, receives the most rain in the world
among cities with over 100,000 inhabitants: 9,000 mm
(354 in) per year. Storms in Chocó can drop 500 mm
(20 in) of rainfall in a day. This amount is more than what falls
in many cities in a year's time.
Years of record
Mount Waiʻaleʻale, Kauai,
Mount Bellenden Ker, Queensland
Henderson Lake, British Columbia
Source (without conversions): Global Measured Extremes of Temperature
and Precipitation, National Climatic Data Center. August 9, 2004.
Highest average annual rainfall
Highest in one year
Highest in one calendar month
Highest in 24 hours
Foc Foc, La Reunion Island
Highest in 12 hours
Foc Foc, La Reunion Island
Highest in one minute
Unionville, Maryland, USA
Rainfalls of diamonds have been suggested to occur on the gas giant
Jupiter and Saturn, as well as on the ice giant planets,
Uranus and Neptune. 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's largest natural satellite, infrequent methane rain is
thought to carve the moon's numerous surface channels. On Venus,
sulfuric acid virga evaporates 25 km (16 mi) from the
surface. Extrasolar planet
OGLE-TR-56b in the constellation
Sagittarius is hypothesized to have iron rain.
Red rain in Kerala
Petrichor – the cause of the scent during and after rain
Sanitary sewer overflow
John Rainwater – pseudonymous mathematician
a b c The value given is continent's highest and possibly the world's
depending on measurement practices, procedures and period of record
^ The official greatest average annual precipitation for South America
is 900 cm (354 in) at Quibdó, Colombia. The 1,330 cm
(523.6 in) average at
Lloró [23 km (14 mi) SE and at a
higher elevation than Quibdó] is an estimated amount.
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