is liquid precipitation
, as opposed to
other kinds of precipitation such as snow
. On Earth, it is the condensation
of atmospheric water vapor
heavy enough to fall, often making it to the surface. Rain is the
primary source of fresh water for most areas of the world,
providing suitable conditions for diverse ecosystems
, as well as water for hydroelectric power plants
. However, not all rain reaches
the surface; some evaporates while falling through dry air
. This is called virga
phenomenon often seen in hot, dry desert
regions. The METAR
code for rain is
Rain is also known or suspected on other worlds. On Titan
moon, infrequent methane
rain is thought to
carve the moon's numerous surface channels. On Venus
, sulfuric acid
evaporates 25 km from the surface.
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.
Rain plays a role in the hydrologic
in which moisture
evaporates, condenses into drops,
precipitates (falls) from the sky, and eventually returns to the
ocean via rivers
to repeat the cycle again. The water vapor
from plant respiration also
contributes to the moisture in the atmosphere.
A scientific model known as the Bergeron process
explains how rain forms
and falls. More recent research points to the influence of Cloud condensation nuclei
as the result of biological processes.
The fine particulate matter produced by car exhaust and other human
sources of pollution forms cloud condensation nuclei
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
, the effect can be dramatic: there is a 22% higher
chance of rain on Saturdays than on Mondays.
Classifying the amount of rain
When classified according to the rate of precipitation, rain can be
- Very light rain — when the precipitation rate is
- Light rain — when the precipitation rate is between
0.25 mm/hour - 1.0 mm/hour
- Moderate rain — when the precipitation rate is between
1.0 mm/hour - 4.0 mm/hour
- Heavy rain — when the precipitation rate is between
4.0 mm/hour - 16.0 mm/hour
- Very heavy rain — when the precipitation rate is between
16.0 mm/hour - 50 mm/hour
- Extreme rain — when the precipitation rate is >
Precipitation is measured using a Rain
Falling raindrops are often depicted in popular culture as
"teardrop-shaped" — round at the bottom and narrowing towards
the top — but this is incorrect. Only drops of water dripping
from some sources are tear-shaped at the moment of formation. Small
raindrops are nearly spherical
. Larger ones
become increasingly flattened on the bottom, like hamburger
buns; very large ones are shaped like
. The shape of raindrops was
studied by Philipp Lenard
in 1898. He
found that small raindrops (less than about 2 mm diameter) are
approximately spherical. As they get larger (to about 5 mm
diameter) they become more doughnut shaped. Beyond about 5 mm
they become unstable and fragment. On average, raindrops are 1 to
2 mm in diameter. The biggest raindrops on Earth were recorded
over Brazil and the
Islands in 2004 — some of them were as large as
The large size is explained by condensation on
particles or by collisions between
drops in small regions with particularly high content of liquid
Raindrops impact at their terminal
, which is greater for larger drops due to their larger
mass. At sea level and without wind, 0.5 mm drizzle
impacts at about 2 m/s, while large
5 mm drops impact at around 9 m/s. The sound of raindrops
hitting water is caused by bubbles of air oscillating underwater.
See droplet's sound
Precipitation with low pH levels
, or acid rain
, is also a frequent risk produced by
lightning. Distilled water
contains no carbon dioxide
, has a
Liquids with a pH less than 7 are acidic, and those with a pH
greater than 7 are bases. “Clean” or unpolluted rain has a slightly
acidic pH of about 5.2, because carbon dioxide and water in the air
react together to form carbonic acid
a weak acid (pH 5.6 in distilled water), but unpolluted rain also
contains other chemicals. Nitrous
present during thunderstorm phenomena, caused by the
splitting of nitrogen molecules, can result in the production of
acid rain, if nitrous oxide forms compounds with the water
molecules in precipitation, thus creating acid rain. Acid rain can
damage infrastructures containing calcite, or other solid carbon
compounds. In ecosystems, acid rain can dissolve plant tissues of
vegetations and increase acidification process in bodies of water,
resulting in deaths of marine organisms.
Effect on agriculture
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
kill crops in massive numbers, while overly wet weather can cause
disease and harmful fungus
need varying amounts of rainfall to survive.
For example, cacti
need small amounts of
water while tropical plants may need up to hundreds of inches of
rain per year to survive.
Agriculture of all nations at least to some extent is dependent on
rain. Indian agriculture, for example, (which accounts for 25
percent of the GDP and employs 70 percent of the nation's
population) is heavily dependent on the rains, especially crops
like cotton, rice,
oilseeds and coarse grains.
A delay of
a few days in the arrival of the monsoon
can, and does, badly affect the economy, as evidenced in the
numerous droughts in India in the
Cultural attitudes towards rain differ across the world. In the
largely temperate Europe
, rain metaphorically has a sad and negative
connotation — reflected in children's rhymes like Rain Rain Go Away
— in contrast to
the bright and happy sun
. Though the traditional
notion of rain in the Western World is negative, rain can also
bring joy, as some consider it to be soothing or enjoy the
aesthetic appeal of it. In dry places, such as parts of Africa, Australia, India, the
Western United States, and the
Middle East, rain is greeted with
euphoria. (In Botswana, the
Setswana word for rain, "pula," is used as
the name of the national currency, in
recognition of the economic importance of rain in this desert
Several cultures have developed means of dealing with rain and have
developed numerous protection devices such as umbrellas
and diversion devices such as gutter
that lead rains to sewers.
Many people also prefer to stay inside on rainy days, especially in
tropical climates where rain is usually accompanied by thunderstorms
or is extremely heavy (as in a
). Rain may be harvested
, though rainwater is rarely
pure (as acid rain
occurs naturally), or
used as greywater
. Excessive rain,
particularly after a dry period that has hardened the soil so that
it cannot absorb water, can cause floods
people find the scent during and immediately after rain especially
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. Light or heavy rain is sometimes seen as romantic.
Kingdom most rain is driven into the country by the
south-western trade winds following the warm gulf stream currents.
Areas along the
western coasts can receive between 1000 mm (40 in, at
sea-level) and 2500 mm (100 in, on the mountains) of rain per
Bergen in Norway is one of the more famous European
rain-cities with its yearly precipitation of 2250 mm (88 in)
that is known for rain is Seattle, Washington.
Rain is common in the winter, but mostly
the climate is cloudy with little rain. Seattle's average rainfall
is 942 mm (37.1 in) per year, less than New York City's
1173 mm (46.2 in), but Seattle has 201 cloudy days per year,
compared to 152 in New York. Seattle's neighbor to the south, Portland,
Oregon, gets more rain with an average of 1143 mm (45
in) a year. However, it should be noted that Seattle
lies in the rain shadow of the nearby
Mountains, with some locations on the windward sides of the mountains receiving close to
3300 mm (about 130 in) per year. The wettest city in
the 48 contiguous United States is Mobile, Alabama, which average 1702 mm (67 in) of rainfall per
year. Ketchikan and other locations in the temperate rainforest of
southeast Alaska get an average of 4064 mm (160 in) of rain a
year, sometimes receiving over 5000 mm (about 200 inches) in a
[[File:Raindrops sizes.svg|thumb|A. Raindrops are not tear-shaped,
as most people think.
B. Very small raindrops are almost spherical in shape.
C. Larger raindrops become flattened at the bottom, like that of a
hamburger bun, due to air resistance.
D. Large raindrops have a large amount of air resistance, which
makes them begin to become unstable.
E. Very large raindrops split into smaller raindrops due to air
Cherrapunji, situated on the southern slopes of the Eastern Himalaya in Shillong, India is one of
the wettest places on Earth, with an average annual rainfall of
11,430 mm (450 in).
The highest recorded rainfall in a
single year was 22,987 mm (904.9 in) in 1861.
38-year average at Mawsynram, Meghalaya, India is
11,873 mm (467.4 in).
Like in Cherrapunji, Mawsynram's
rainfall is concentrated in the monsoon season, while the rain at
other famously humid places, like Tutunendo and Waialeale is more
evenly distributed through the year. However, as there has been no
meteorological department office, but a peon of the Meghalaya
Public Works Department posted there in the area who takes the
figure without supervision, Cherrapunji has been presented
frequently as the wettest place in the world for statistical
purposes. Doubts have been expressed by the knowledgeable about the
rainfall data. In 1850, Joseph Dalton Hooker, a Royal Navy doctor
turned naturalist, who spent the monsoon months at Cherrapunji,
recorded such variations. "He was puzzled by the curiously
localised patterns of rain; move your gauge a few hundred yards and
it registered only half as much as before", writes Alexander Frater
in his book, Chasing the Monsoon
. This shows that
recording the rainfall at the most humid places on earth has always
been a difficult task for weathermen.
||Cherrapunji Rainfall (mm)
||Mawsynram Rainfall (mm)
Lloró, a town situated in Chocó, Colombia, is probably the place with the largest measured
rainfall in the world, averaging 13,300 mm per year (523.6
In fact, the whole Department of Chocó is
extraordinarily humid. Tutunendo
, a small
town situated in the same department, is one of the wettest places
on earth, averaging 11,394 mm per year (448 in); in 1974 the
town received 26,303 mm (86 ft 3½ in), the largest annual
rainfall measured in Colombia. Unlike Cherrapunji, which receives
most of its rainfall between April and September, Tutunendo
receives rain almost uniformly distributed throughout the year. The
months of January and February have somewhat less frequent storms.
On average, Tutunendo has 280 days with rainfall per year. Over ⅔
of the rain (68%) falls during the night. The average relative
humidity is 90% and the average temperature is 26.4 °C.
Quibdó, the capital of Chocó, receives the most rain in
the world among cities with over 100,000 inhabitants: 9000 mm
(29 ft 6 in) per year.
Storms in Chocó can drop 500 mm
(19.7 in) of rainfall in a day. This amount is more than falls in
many cities in a year's time.
Australia is the world's driest continent, Mount Bellenden Ker in the north-east of the country records an average
of 8000 mm (315 in) per year, with over 12,000 mm (472
in) of rain recorded in the year 2000.
Melbourne has a similar reputation to Seattle's.
popular imagination it is thought of as being much rainier than
Sydney; however, Sydney receives an average of
1213 mm (47.8 in) of rain per year compared to Melbourne's
648 mm (25.5 in).
Sydney, meanwhile, experiences 46
fewer overcast days per year than Melbourne, with 133 cloudy days,
compared to Melbourne's 179.
Waialeale on the
island of Kauaʻi in the
Hawaiian Islands averages more than
11,680 mm (38 ft 4 in or 460 in) of rain over the last 32
years, with a record 17,340 mm (683 in) in 1982.
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.
- Cerveny, R. S., and R. C. Balling. Weekly cycles of air
pollutants, precipitation and tropical cyclones in the coastal NW
Atlantic region. Nature. 394, 561-563.