In meteorology , PRECIPITATION is any product of the condensation of
atmospheric water vapor that falls under gravity . The main forms of
precipitation include drizzle , rain , sleet , snow , graupel and hail
Precipitation occurs when a portion of the atmosphere becomes
saturated with water vapor, so that the water condenses and
"precipitates". Thus, fog and mist are not precipitation but
suspensions, because the water vapor does not condense sufficiently to
precipitate. Two processes, possibly acting together, can lead to air
becoming saturated: cooling the air or adding water vapor to the air.
Precipitation forms as smaller droplets coalesce via collision with
other rain drops or ice crystals within a cloud . Short, intense
periods of rain in scattered locations are called "showers."
Moisture that is lifted or otherwise forced to rise over a layer of
sub-freezing air at the surface may be condensed into clouds and rain.
This process is typically active when freezing rain occurs. A
stationary front is often present near the area of freezing rain and
serves as the foci for forcing and rising air. Provided necessary and
sufficient atmospheric moisture content, the moisture within the
rising air will condense into clouds, namely stratus and cumulonimbus
. Eventually, the cloud droplets will grow large enough to form
raindrops and descend toward the
Earth where they will freeze on
contact with exposed objects. Where relatively warm water bodies are
present, for example due to water evaporation from lakes, lake-effect
snowfall becomes a concern downwind of the warm lakes within the cold
cyclonic flow around the backside of extratropical cyclones .
Lake-effect snowfall can be locally heavy.
Thundersnow is possible
within a cyclone's comma head and within lake effect precipitation
bands. In mountainous areas, heavy precipitation is possible where
upslope flow is maximized within windward sides of the terrain at
elevation. On the leeward side of mountains, desert climates can exist
due to the dry air caused by compressional heating. Most precipitation
occurs within the tropics and is caused by convection . The movement
of the monsoon trough , or intertropical convergence zone , brings
rainy seasons to savannah climes .
Precipitation is a major component of the water cycle , and is
responsible for depositing the fresh water on the planet .
Approximately 505,000 cubic kilometres (121,000 cu mi) of water falls
as precipitation each year; 398,000 cubic kilometres (95,000 cu mi) of
it over the oceans and 107,000 cubic kilometres (26,000 cu mi) over
land. Given the
Earth 's surface area, that means the globally
averaged annual precipitation is 990 millimetres (39 in), but over
land it is only 715 millimetres (28.1 in).
systems such as the
Köppen climate classification
Köppen climate classification system use average
annual rainfall to help differentiate between differing climate
Precipitation may occur on other celestial bodies, e.g. when it gets
Mars has precipitation which most likely takes the form of
frost, rather than rain or snow.
Part of the nature series
Tropical cyclone (Hurricane)
Freezing drizzle )
Ice pellets (
Diamond dust )
Freezing rain )
Rain and snow mixed
* 1 Types
* 2 How the air becomes saturated
* 2.1 Cooling air to its dew point
* 2.2 Adding moisture to the air
* 3 Formation
* 3.1 Raindrops
* 3.4 Snowflakes
* 4 Causes
* 4.1 Frontal activity
* 4.3 Orographic effects
* 4.5 Within the tropics
* 4.6 Large-scale geographical distribution
* 5 Measurement
* 5.1 Hydrometeor definition
* 5.2 Satellite estimates
* 5.3 Satellite data sets
* 7 Role in
Köppen climate classification
Köppen climate classification
* 8 Effect on agriculture
* 9 Changes due to global warming
* 10 Changes due to urban heat island
* 11 Forecasting
* 12 See also
* 13 References
* 14 External links
Precipitation types A thunderstorm with heavy
Precipitation is a major component of the water cycle , and is
responsible for depositing most of the fresh water on the planet .
Approximately 505,000 km3 (121,000 mi3) of water falls as
precipitation each year, 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 millimetres (39 in).
Mechanisms of producing precipitation include convective, stratiform
, and orographic rainfall. Convective processes involve strong
vertical motions that can cause the overturning of the atmosphere in
that location within an hour and cause heavy precipitation, while
stratiform processes involve weaker upward motions and less intense
Precipitation can be divided into three categories,
based on whether it falls as liquid water, liquid water that freezes
on contact with the surface, or ice. Mixtures of different types of
precipitation, including types in different categories, can fall
simultaneously. Liquid forms of precipitation include rain and
Rain or drizzle that freezes on contact within a subfreezing
air mass is called "freezing rain" or "freezing drizzle". Frozen forms
of precipitation include snow , ice needles , ice pellets , hail , and
HOW THE AIR BECOMES SATURATED
COOLING AIR TO ITS DEW POINT
Late-summer rainstorm in
Denmark Lenticular cloud
forming due to mountains over Wyoming
The dew point is the temperature to which a parcel must be cooled in
order to become saturated, and (unless super-saturation occurs)
condenses to water.
Water vapor normally begins to condense on
condensation nuclei such as dust, ice, and salt in order to form
clouds. An elevated portion of a frontal zone forces broad areas of
lift, 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.
There are four main mechanisms for cooling the air to its dew point:
adiabatic cooling, conductive cooling, radiational coolin g, 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
ADDING MOISTURE TO THE AIR
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.
Condensation and coalescence are
important parts of the water cycle .
Coalescence occurs when water droplets fuse to create larger water
droplets, or when water droplets freeze onto an ice crystal, which is
known as the
Bergeron process . The fall rate of very small droplets
is negligible, hence clouds do not fall out of the sky; precipitation
will only occur when these coalesce into larger drops. 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.
Raindrops have sizes ranging from 0.1 millimetres (0.0039 in) to 9
millimetres (0.35 in) mean diameter, above which they tend to break
up. Smaller drops are called cloud droplets, and their shape is
spherical. As a raindrop increases in size, its shape becomes more
oblate, with its largest cross-section facing the oncoming airflow.
Contrary to the cartoon pictures of raindrops, their shape does not
resemble a teardrop. 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.
Rain drops associated with melting hail tend to be larger than other
rain drops. The
METAR code for rain is RA, while the coding for rain
showers is SHRA.
Ice pellets An accumulation of ice pellets
Ice pellets or sleet are a form of precipitation consisting of small,
translucent balls of ice.
Ice pellets are usually (but not always)
smaller than hailstones . They often bounce when they hit the ground,
and generally do not freeze into a solid mass unless mixed with
freezing rain . The
METAR code for ice pellets is PL.
Ice pellets form when a layer of above-freezing air exists with
sub-freezing air both above and below. This causes the partial or
complete melting of any snowflakes falling through the warm layer. As
they fall back into the sub-freezing layer closer to the surface, they
re-freeze into ice pellets. However, if the sub-freezing layer beneath
the warm layer is too small, the precipitation will not have time to
re-freeze, and freezing rain will be the result at the surface. A
temperature profile showing a warm layer above the ground is most
likely to be found in advance of a warm front during the cold season,
but can occasionally be found behind a passing cold front .
Hail A large hailstone, about 6 centimetres (2.4
in) in diameter
Like other precipitation, hail forms in storm clouds when supercooled
water droplets freeze on contact with condensation nuclei , such as
dust or dirt . The storm's updraft blows the hailstones to the upper
part of the cloud. The updraft dissipates and the hailstones fall
down, back into the updraft, and are lifted again.
Hail has a diameter
of 5 millimetres (0.20 in) or more. Within
METAR code, GR is used to
indicate larger hail, of a diameter of at least 6.4 millimetres (0.25
in). GR is derived from the French word grêle. Smaller-sized hail, as
well as snow pellets, use the coding of GS, which is short for the
French word grésil. Stones just larger than golf ball -sized are one
of the most frequently reported hail sizes. Hailstones can grow to 15
centimetres (6 in) and weigh more than 500 grams (1 lb). In large
hailstones, latent heat released by further freezing may melt the
outer shell of the hailstone. The hailstone then may undergo 'wet
growth', where the liquid outer shell collects other smaller
hailstones. The hailstone gains an ice layer and grows increasingly
larger with each ascent. Once a hailstone becomes too heavy to be
supported by the storm's updraft, it falls from the cloud.
Snowflake viewed in an optical
Snow crystals form when tiny supercooled cloud droplets (about 10 μm
in diameter) freeze . Once a droplet has frozen, it grows in the
supersaturated environment. Because water droplets are more numerous
than the ice crystals the crystals are able to grow to hundreds of
micrometers in size at the expense of the water droplets. This process
is known as the
Wegener–Bergeron–Findeisen process . The
corresponding depletion of water vapour causes the droplets to
evaporate, meaning that the ice crystals grow at the droplets'
expense. These large crystals are an efficient source of
precipitation, since they fall through the atmosphere due to their
mass, and may collide and stick together in clusters, or aggregates.
These aggregates are snowflakes, and are usually the type of ice
particle that falls to the ground. Guinness World Records list the
world's largest snowflakes as those of January 1887 at Fort Keogh,
Montana; allegedly one measured 38 cm (15 inches) wide. The exact
details of the sticking mechanism remain a subject of research.
Although the ice is clear, scattering of light by the crystal facets
and hollows/imperfections mean that the crystals often appear white in
color due to diffuse reflection of the whole spectrum of light by the
small ice particles. The shape of the snowflake is determined broadly
by the temperature and humidity at which it is formed. Rarely, at a
temperature of around −2 °C (28 °F), snowflakes can form in
threefold symmetry—triangular snowflakes. The most common snow
particles are visibly irregular, although near-perfect snowflakes may
be more common in pictures because they are more visually appealing.
No two snowflakes are alike, as they grow at different rates and in
different patterns depending on the changing temperature and humidity
within the atmosphere through which they fall on their way to the
METAR code for snow is SN, while snow showers are coded
Diamond dust, also known as ice needles or ice crystals, forms at
temperatures approaching −40 °C (−40 °F) due to air with
slightly higher moisture from aloft mixing with colder, surface based
air. They are made of simple ice crystals that are hexagonal in
METAR identifier for diamond dust within international
hourly weather reports is IC.
Stratiform or dynamic precipitation occurs as a consequence of slow
ascent of air in synoptic systems (on the order of cm/s), such as over
surface cold fronts , and over and ahead of warm fronts . Similar
ascent is seen around tropical cyclones outside of 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
Precipitation may occur on celestial bodies other
than Earth. When it gets cold,
Mars has precipitation that most likely
takes the form of ice needles, rather than rain or snow.
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.
Orographic lift and
Precipitation types Orographic
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.
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 millimetres (460 in).
affect the state with heavy rains between October and March. 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
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
Deserts . Similarly, in Asia, the
Himalaya mountains create an
obstacle to monsoons which leads to extremely high precipitation on
the southern side and lower precipitation levels on the northern side.
Lake-effect snow bands near the Korean
Peninsula in early-December 2008.
Extratropical cyclones can bring cold and dangerous conditions with
heavy rain and snow with winds exceeding 119 km/h (74 mph),
(sometimes referred to as windstorms in Europe). The band of
precipitation that is associated with their warm front is often
extensive, forced by weak upward vertical motion of air over the
frontal boundary which condenses as it cools and produces
precipitation within an elongated band, which is wide and stratiform
, meaning falling out of nimbostratus clouds. When moist air tries to
dislodge an arctic air mass, overrunning snow can result within the
poleward side of the elongated precipitation band . In the Northern
Hemisphere , poleward is towards the
North Pole , or north. Within the
Southern Hemisphere , poleward is towards the
South Pole , or south.
Southwest of extratropical cyclones, curved cyclonic flow bringing
cold air across the relatively warm water bodies can lead to narrow
lake-effect snow bands. Those bands bring strong localized snowfall
which can be understood as follows: Large water bodies such as lakes
efficiently store heat that results in significant temperature
differences (larger than 13 °C or 23 °F) between the water surface
and the air above. Because of this temperature difference, warmth and
moisture are transported upward, condensing into vertically oriented
clouds (see satellite picture) which produce snow showers. The
temperature decrease with height and cloud depth are directly affected
by both the water temperature and the large-scale environment. The
stronger the temperature decrease with height, the deeper the clouds
get, and the greater the precipitation rate becomes.
In mountainous areas, heavy snowfall accumulates when air is forced
to ascend the mountains and squeeze out precipitation along their
windward slopes, which in cold conditions, falls in the form of snow.
Because of the ruggedness of terrain, forecasting the location of
heavy snowfall remains a significant challenge.
WITHIN THE TROPICS
Wet season See also:
Rainfall distribution by month in
Cairns showing the extent of the
wet season at that location
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 .
Savanna climates and areas with monsoon
regimes have wet summers and dry winters.
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.
Soil nutrients diminish and erosion
increases. 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.
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 counterclockwise
(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
LARGE-SCALE GEOGRAPHICAL DISTRIBUTION
Earth rainfall climatology
On the large scale, the highest precipitation amounts outside
topography fall in the tropics, closely tied to the Intertropical
Convergence Zone , itself the ascending branch of the
Hadley cell .
Mountainous locales near the equator in Colombia are amongst the
wettest places on Earth. North and south of this are regions of
descending air that form subtropical ridges where precipitation is
low; the land surface underneath these ridges is usually arid, and
these regions make up most of the Earth's deserts. An exception to
this rule is in Hawaii, where upslope flow due to the trade winds lead
to one of the wettest locations on Earth. Otherwise, the flow of the
Westerlies into the Rocky Mountains lead to the wettest, and at
elevation snowiest, locations within
North America . In
the wet season, the flow of moist air into the
Himalayas leads to some
of the greatest rainfall amounts measured on
Earth in northeast India
Rain gauge ,
Disdrometer , and
Snow gauge Standard
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 (1 in) of
rain, with overflow flowing into the outer cylinder. Plastic gauges
have markings on the inner cylinder down to 0.25 mm (0.01 in)
resolution, while metal gauges require use of a stick designed with
the appropriate 0.25 mm (0.01 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. These gauges are used in the winter by removing the
funnel and inner cylinder and allowing snow and freezing rain to
collect inside the outer cylinder. Some add anti-freeze to their gauge
so they do not have to melt the snow or ice that falls into the gauge.
Once the snowfall/ice is finished accumulating, or as 300 mm (12 in)
is approached, one can either bring it inside to melt, or use lukewarm
water to fill the inner cylinder with in order to melt the frozen
precipitation in the outer cylinder, keeping track of the warm fluid
added, which is subsequently subtracted from the overall total once
all the ice/snow is melted.
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. The wedge and tipping bucket gauges will have
problems with snow. Attempts to compensate for snow/ice by warming the
tipping bucket meet with limited success, since snow may sublimate if
the gauge is kept much above freezing. Weighing gauges with antifreeze
should do fine with snow, but again, the funnel needs to be removed
before the event begins. 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 office will likely be interested in the measurement.
A concept used in precipitation measurement is the hydrometeor. Bits
of liquid or solid water in the atmosphere are known as hydrometeors.
Formations due to condensation, such as clouds, haze , fog, and mist,
are composed of hydrometeors. All precipitation types are made up of
hydrometeors by definition, including virga, which is precipitation
which evaporates before reaching the ground. Particles blown from the
Earth's surface by wind, such as blowing snow and blowing sea spray,
are also hydrometeors.
Although surface precipitation gauges are considered the standard for
measuring precipitation, there are many areas in which their use is
not feasible. This includes the vast expanses of ocean and remote land
areas. In other cases, social, technical or administrative issues
prevent the dissemination of gauge observations. As a result, the
modern global record of precipitation largely depends on satellite
Satellite sensors work by remotely sensing precipitation—recording
various parts of the electromagnetic spectrum that theory and practice
show are related to the occurrence and intensity of precipitation. The
sensors are almost exclusively passive, recording what they see,
similar to a camera, in contrast to active sensors (radar , lidar )
that send out a signal and detect its impact on the area being
Satellite sensors now in practical use for precipitation fall into
two categories. Thermal infrared (IR ) sensors record a channel around
11 micron wavelength and primarily give information about cloud tops.
Due to the typical structure of the atmosphere, cloud-top temperatures
are approximately inversely related to cloud-top heights, meaning
colder clouds almost always occur at higher altitudes. Further, cloud
tops with a lot of small-scale variation are likely to be more
vigorous than smooth-topped clouds. Various mathematical schemes, or
algorithms, use these and other properties to estimate precipitation
from the IR data.
The second category of sensor channels is in the microwave part of
the electromagnetic spectrum. The frequencies in use range from about
10 gigahertz to a few hundred GHz. Channels up to about 37 GHz
primarily provide information on the liquid hydrometeors (rain and
drizzle) in the lower parts of clouds, with larger amounts of liquid
emitting higher amounts of microwave radiant energy. Channels above 37
GHz display emission signals, but are dominated by the action of solid
hydrometeors (snow, graupel, etc.) to scatter microwave radiant
energy. Satellites such as the
Rainfall Measuring Mission
(TRMM) and the
Global Precipitation Measurement (GPM) mission employ
microwave sensors to form precipitation estimates.
Additional sensor channels and products have been demonstrated to
provide additional useful information including visible channels,
additional IR channels, water vapor channels and atmospheric sounding
retrievals. However, most precipitation data sets in current use do
not employ these data sources.
SATELLITE DATA SETS
The IR estimates have rather low skill at short time and space
scales, but are available very frequently (15 minutes or more often)
from satellites in geosynchronous
Earth orbit. IR works best in cases
of deep, vigorous convection—such as the tropics—and becomes
progressively less useful in areas where stratiform (layered)
precipitation dominates, especially in mid- and high-latitude regions.
The more-direct physical connection between hydrometeors and microwave
channels gives the microwave estimates greater skill on short time and
space scales than is true for IR. However, microwave sensors fly only
Earth orbit satellites, and there are few enough of them that
the average time between observations exceeds three hours. This
several-hour interval is insufficient to adequately document
precipitation because of the transient nature of most precipitation
systems as well as the inability of a single satellite to
appropriately capture the typical daily cycle of precipitation at a
Since the late 1990s, several algorithms have been developed to
combine precipitation data from multiple satellites' sensors, seeking
to emphasize the strengths and minimize the weaknesses of the
individual input data sets. The goal is to provide "best" estimates of
precipitation on a uniform time/space grid, usually for as much of the
globe as possible. In some cases the long-term homogeneity of the
dataset is emphasized, which is the
Climate Data Record standard.
In other cases, the goal is producing the best instantaneous
satellite estimate, which is the High Resolution
approach. In either case, of course, the less-emphasized goal is also
considered desirable. One key result of the multi-satellite studies is
that including even a small amount of surface gauge data is very
useful for controlling the biases that are endemic to satellite
estimates. The difficulties in using gauge data are that 1) their
availability is limited, as noted above, and 2) the best analyses of
gauge data take two months or more after the observation time to
undergo the necessary transmission, assembly, processing and quality
control. Thus, precipitation estimates that include gauge data tend to
be produced further after the observation time than the no-gauge
estimates. As a result, while estimates that include gauge data may
provide a more accurate depiction of the "true" precipitation, they
are generally not suited for real- or near-real-time applications.
The work described has resulted in a variety of datasets possessing
different formats, time/space grids, periods of record and regions of
coverage, input datasets, and analysis procedures, as well as many
different forms of dataset version designators. In many cases, one of
the modern multi-satellite data sets is the best choice for general
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 rare and is only likely
to occur once every 10 years, so it has a 10 percent likelihood any
given year. The rainfall will be greater and the flooding will be
worse than the worst storm expected in any single year. The term 1 in
100 year storm describes a rainfall event which is extremely rare and
which will occur with a likelihood of only once in a century, so has a
1 percent likelihood in any given year. The rainfall will be extreme
and flooding to be worse than a 1 in 10 year event. As with all
probability events, it is possible though unlikely to have two "1 in
100 Year Storms" in a single year.
ROLE IN KöPPEN CLIMATE CLASSIFICATION
Köppen climate classification
Köppen climate classification Updated
Köppen-Geiger climate map
Af Am Aw
BWh BWk BSh BSk
Cfa Cfb Cfc
Dsa Dsb Dsc Dsd
Dwa Dwb Dwc Dwd
Dfa Dfb Dfc Dfd
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 (and sometimes
snowfall ) 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 precipitation.
EFFECT ON AGRICULTURE
Rainfall estimates for southern Japan and the surrounding region
from July 20 to 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.
CHANGES DUE TO GLOBAL WARMING
Increasing temperatures tend to increase evaporation which leads to
Precipitation has generally increased over land
north of 30°N from 1900 to 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, more evaporation, or both). Over the
contiguous United States, total annual precipitation increased at an
average rate of 6.1% per century since 1900, with the greatest
increases within the East North Central climate region (11.6% per
century) and the South (11.1%).
Hawaii was the only region to show a
CHANGES DUE TO URBAN HEAT ISLAND
Urban heat island Image of
Atlanta, Georgia , showing
temperature distribution, with hot areas appearing white
The urban heat island 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
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 kilometres (20 to 40
mi) downwind of cities, compared with upwind. Some cities induce a
total precipitation increase of 51%.
Probability of precipitation and Quantitative
precipitation forecast Example of a five-day rainfall forecast
Hydrometeorological Prediction Center
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
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 six to seven 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
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Look up PRECIPITATION in Wiktionary, the free dictionary.