In
meteorologyMeteorology is the interdisciplinary scientific study of the atmosphere that focuses on weather processes and forecasting . Studies in the field stretch back millennia, though significant progress in meteorology did not occur until the eighteenth century...
,
precipitation (also known as one of the classes of
hydrometeors, which are atmospheric water phenomena) is any product of the condensation of
atmosphericAn atmosphere is a layer of gases that may surround a material body of sufficient mass, by the gravity of the body, and are retained for a longer duration if gravity is high and the atmosphere's temperature is low...
water vaporWater vapor or water vapour , also aqueous vapor, is the gas phase of water. Water vapor is one state of the water cycle within the hydrosphere. Water vapor can be produced from the evaporation or boiling of liquid water or from the sublimation of ice...
that is deposited on the Earth's surface. The main forms of precipitation include rain, snow, ice pellets, and graupel. It occurs when the atmosphere, a large gaseous
solutionIn chemistry, a solution is a homogeneous mixture composed of two or more substances. In such a mixture, a solute is dissolved in another substance, known as a solvent. Gases may dissolve in liquids, for example, carbon dioxide or oxygen in water. Liquids may dissolve in other liquids. Gases can...
, becomes saturated with water vapour and the water condenses, falling out of solution (i.e.,
precipitatesPrecipitation is the formation of a solid in a solution during a chemical reaction. When the reaction occurs, the solid formed is called the precipitate, and the liquid remaining above the solid is called the supernate...
). Two processes, possibly acting together, can lead to air becoming saturated: cooling the air or adding water vapour to the air.
VirgaIn meteorology, virga is an observable streak or shaft of precipitation that falls from a cloud but evaporates before reaching the ground. At high altitudes the precipitation falls mainly as ice crystals before melting and finally evaporating; this is usually due to compressional heating, because...
is precipitation that begins falling to the earth but evaporates before reaching the surface; it is one of the ways air can become saturated. Precipitation forms via collision with other rain drops or ice crystals within a
cloudA cloud is a visible mass of droplets or frozen crystals suspended in the atmosphere above the surface of the Earth or another planetary body. A cloud is also a visible mass attracted by gravity, such as masses of material in space called interstellar clouds and nebulae...
.
Moisture overriding associated with weather fronts is an overall major method of precipitation production. If enough moisture and upward motion is present, precipitation falls from convective clouds such as cumulonimbus and can organize into narrow rainbands. 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
cyclonicIn meteorology, a cyclone is an area of closed, circular fluid motion rotating in the same direction as the Earth. This is usually characterized by inward spiraling winds that rotate counter clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere of the Earth.Large-scale...
flow around the backside of
extratropical cycloneExtratropical cyclones, sometimes called mid-latitude cyclones or wave cyclones, are a group of cyclones defined as synoptic scale low pressure weather systems that occur in the middle latitudes of the Earth having neither tropical nor polar characteristics, and are connected with fronts and...
s. Lake-effect snowfall can be locally heavy.
ThundersnowThundersnow, also known as a winter thunderstorm or a thunder snowstorm, is a rare thunderstorm with snow falling as the primary precipitation instead of rain. It commonly falls in regions of strong upward motion within the cold sector of extratropical cyclones between autumn and spring when...
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. The movement of the
monsoon troughThe monsoon trough or those portions of the Intertropical Convergence Zone within a monsoon region, is depicted by a line on a weather map showing the locations of minimum sea level pressure, and is a convergence zone between the wind patterns of the southern and northern hemispheres...
, or
intertropical convergence zoneThe Intertropical Convergence Zone is a term that is used to describe the North-East and South-East trade wind convergence. Convectively active portions of the ITCZ are known as the monsoon trough...
, brings
rainy seasonsThe wet season, 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...
to
savannahSavannah or savanna is a type of grassland.It can also mean:-Software:* GNU Savannah, an aggregation of software development projects affiliated with the GNU project-Places in the USA:...
climeThe seven climes was a notion of dividing the Earth into zones in Classical Antiquity....
s.
Rain drops range in size from oblate, pancake-like shapes for larger drops, to small spheres for smaller drops. Precipitation that reaches the surface of the earth can occur in many different forms, including
rainRain is liquid precipitation, as opposed to other kinds of precipitation such as snow, hail and sleet. On Earth, it is the condensation of atmospheric water vapor into drops heavy enough to fall, often making it to the surface...
,
freezing rainFreezing rain is the name given to rain that falls when surface temperatures are below freezing. The raindrops become supercooled while passing through a sub-freezing layer of air, many hundred feet , just above the surface, and then freeze upon impact with any object they encounter. The resulting...
,
drizzleDrizzle is a light rain precipitation consisting of liquid water drops smaller than those of rain, and generally smaller than 0.5 mm in diameter. Drizzle is normally produced by low stratiform clouds and stratocumulus clouds. Precipitation rates due to drizzle are on the order of a millimeter per...
,
ice needlesDiamond dust is a ground-level cloud composed of tiny ice crystals. This meteorological phenomenon is also referred to simply as ice crystals and is reported in the METAR code as IC. Diamond dust generally forms under otherwise clear or nearly clear skies, so it is sometimes referred to as...
,
snowSnow is a type of precipitation in the form of crystalline water ice, consisting of a multitude of snowflakes that fall from clouds. Since snow is composed of small ice particles, it is a granular material. It has an open and therefore soft structure, unless packed by external pressure. Snowflakes...
,
ice pelletsIce pellets are a form of precipitation consisting of small, translucent balls of ice. This form of precipitation is also known as sleet in the United States. Ice pellets are usually smaller than hailstones. They often bounce when they hit the ground, and generally do not freeze into a solid...
or sleet, graupel and
hailHail is a form of solid precipitation which consists of balls or irregular lumps of ice, that are individually called hail stones. Hail stones on Earth consist mostly of water ice and measure between 5 and 150 millimeters in diameter, with the larger stones coming from severe and dangerous...
. Hail is formed within cumulonimbus clouds when strong updrafts of air cause the stones to cycle back and forth through the cloud, causing the hailstone to form in layers until it becomes heavy enough to fall from the cloud. Unlike raindrops, snowflakes grow in a variety of different shapes and patterns, determined by the
temperatureIn physics, temperature is a physical property of a system that underlies the common notions of hot and cold; something that feels hotter generally has the higher temperature. Temperature is one of the principal parameters of thermodynamics...
and
humidityHumidity is the amount of water vapour in the air. In daily language the term "humidity" is normally taken to mean relative humidity. Relative humidity is defined as the ratio of the partial pressure of water vapour in a parcel of air to the saturated vapour pressure of water vapour at a prescribed...
characteristics of the air the snowflake moves through on its way to the ground. While snow and ice pellets require temperatures close to the ground to be near or below freezing, hail can occur during much warmer temperature regimes due to the process of its formation. Precipitation may occur on other celestial bodies, e.g. when it gets cold,
MarsMars is the fourth planet from the Sun in the Solar System. The planet is named after Mars, the Roman god of war. It is also referred to as the "Red Planet" because of its reddish appearance, due to iron oxide prevalent on its surface....
has precipitation which most likely takes the form of ice needles, rather than rain or snow.
The
urban heat islandAn urban heat island is a metropolitan area which is significantly warmer than its surrounding rural areas. The phenomenon was first investigated and described, though not by name, by Luke Howard FRS in the 1810s. The temperature difference usually is larger at night than during the day, and is...
effect leads to increased rainfall, both in amounts and intensity, downwind of cities.
Global warmingGlobal warming is the increase in the average temperature of the Earth's near-surface air and oceans since the mid-20th century and its projected continuation. Global surface temperature increased 0.74 ± 0.18 °C during the last century...
is also causing changes in the precipitation pattern globally, including wetter conditions across eastern
North AmericaNorth America is the northern continent of the Americas, situated in the Earth's northern hemisphere and in the western hemisphere. It is bordered on the north by the Arctic Ocean, on the east by the North Atlantic Ocean, on the southeast by the Caribbean Sea, and on the west by the North Pacific...
and drier conditions in the tropics. Precipitation is a major component of the
water cycleThe water cycle, also known as the hydrologic cycle, describes the continuous movement of water on, above and below the surface of the Earth. Since the water cycle is truly a "cycle," there is no beginning or end. Water can change states among liquid, vapor, and ice at various places in the water...
, and is responsible for depositing most of the
fresh waterFresh Water is the debut album by Australian rock and blues singer Alison McCallum, released in 1972. Rare for an Australian artist at the time, it came in a gatefold sleeve...
on the
planetA planet , is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...
. Approximately of water falls as precipitation each year; of it over the
oceanAn ocean is a large body of saline water, and a principal component of the hydrosphere. Approximately 75% of the Earth's surface is covered by ocean, a continuous body of water that is customarily divided into several principal oceans and smaller seas.More than half of this area is over 3,000...
s. Given the
EarthEarth is the third planet from the Sun. It is the fifth largest of the eight planets in the solar system, and the largest of the terrestrial planets in the Solar System in terms of diameter, mass and density...
's surface area, that means the globally-averaged annual precipitation is .
ClimateClimate encompasses the statistics of temperature, humidity, atmospheric pressure, wind, rainfall, atmospheric particle count and numerous other meteorological elements in a given region over long periods of time...
classification systems such as the
Köppen climate classificationThe Köppen climate classification is one of the most widely used climate classification systems. It was first published by the Russian climatologist Wladimir Köppen in 1884, with several later modifications by Köppen himself notably in 1918 and 1936...
system use average annual rainfall to help differentiate between differing climate regimes.
Hydrometeor
The term meteor describes an object from
outer spaceOuter space comprises the relatively empty regions of the universe outside the atmospheres of celestial bodies. Outer space is used to distinguish it from airspace and terrestrial locations....
which has entered the Earth's atmosphere and produces a light phenomenon. In contrast, any phenomenon which was at some point produced due to condensation or precipitation of moisture within the Earth's atmosphere is known as a hydrometeor. Particles composed of fallen precipitation which fell onto the Earth's surface can become hydrometeors if blown off the landscape by wind. Formations due to condensation such as clouds,
hazeHaze is traditionally an atmospheric phenomenon where dust, smoke and other dry particles obscure the clarity of the sky. The WMO manual of codes includes a classification of horizontal obscuration into categories of fog, ice fog, steam fog, mist, haze, smoke, volcanic ash, dust, sand and snow...
,
fogFog is a cloud that is in contact with the ground. A cloud may be considered partly fog; for example, the part of a cloud that is suspended in the air above the ground is not considered fog, whereas the part of the cloud that comes in contact with higher ground is considered fog...
, and
mistMist is a phenomenon of small droplets suspended in air. It can occur as part of natural weather or volcanic activity, and is common in cold air above warmer water, in exhaled air in the cold, and in a steam room of a sauna. It can also be created artificially with aerosol canisters if the...
are composed of hydrometeors. All precipitation types are hydrometeors by definition, including virga, which is precipitation which evaporates before reaching the ground. Particles removed from the Earth's surface by wind such as blowing snow and blowing sea spray are also hydrometeors.
Types
Precipitation is a major component of the
water cycleThe water cycle, also known as the hydrologic cycle, describes the continuous movement of water on, above and below the surface of the Earth. Since the water cycle is truly a "cycle," there is no beginning or end. Water can change states among liquid, vapor, and ice at various places in the water...
, and is responsible for depositing most of the
fresh waterFresh Water is the debut album by Australian rock and blues singer Alison McCallum, released in 1972. Rare for an Australian artist at the time, it came in a gatefold sleeve...
on the
planetA planet , is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...
. Approximately 505,000 km
3 (121,000 cu mi) of water falls as precipitation each year, 398,000 km
3 (95,000 cu mi) of it over the
oceanAn ocean is a large body of saline water, and a principal component of the hydrosphere. Approximately 75% of the Earth's surface is covered by ocean, a continuous body of water that is customarily divided into several principal oceans and smaller seas.More than half of this area is over 3,000...
s. Given the
EarthEarth is the third planet from the Sun. It is the fifth largest of the eight planets in the solar system, and the largest of the terrestrial planets in the Solar System in terms of diameter, mass and density...
's surface area, that means the globally-averaged annual precipitation is .
Mechanisms of producing precipitation include convective,
stratiformStratus may refer to:*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...
, and
orographicOrographic lift occurs when an air mass is forced from a low elevation to a higher elevation as it moves over rising terrain. As the air mass gains altitude it expands and cools adiabatically, which can raise the relative humidity to 100% and create clouds and, under the right conditions,...
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. 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 drizzle. Rain or drizzle that freezes on contact within a subfreezing
air massIn meteorology, an air mass is a large volume of air defined by its temperature and water vapor content. Air masses cover many hundreds or thousands of square miles, and adopt the characteristics of the surface below them. They are classified according to latitude and their continental or...
is called "freezing rain" or "freezing drizzle". Frozen forms of precipitation include
snowSnow is a type of precipitation in the form of crystalline water ice, consisting of a multitude of snowflakes that fall from clouds. Since snow is composed of small ice particles, it is a granular material. It has an open and therefore soft structure, unless packed by external pressure. Snowflakes...
,
ice needlesDiamond dust is a ground-level cloud composed of tiny ice crystals. This meteorological phenomenon is also referred to simply as ice crystals and is reported in the METAR code as IC. Diamond dust generally forms under otherwise clear or nearly clear skies, so it is sometimes referred to as...
, ice pellets,
hailHail is a form of solid precipitation which consists of balls or irregular lumps of ice, that are individually called hail stones. Hail stones on Earth consist mostly of water ice and measure between 5 and 150 millimeters in diameter, with the larger stones coming from severe and dangerous...
, and graupel.
Cooling air to its dew point
Air contains water vapour, measured in grams of water per kilogram of dry air (g/kg), but most commonly reported as a
relative humidityRelative humidity is a term used to describe the amount of water vapor that exists in a gaseous mixture of air and water vapor.- Definition :...
. How much water vapour a parcel of air can contain before it becomes saturated (100% relative humidity) depends on its temperature. Warmer air can contain more water vapour than cooler air before becoming saturated. Therefore, one way to saturate a parcel of air is to cool it. The
dew pointThe dew point is the temperature to which a given parcel of air must be cooled, at constant barometric pressure, for water vapor to condense into water. The condensed water is called dew. The dew point is a saturation point...
is the temperature to which a parcel must be cooled in order to become saturated. Water vapour normally begins to condense on
condensation nucleiCloud condensation nuclei or CCNs are small particles about which cloud droplets coalesce. Water requires a non-gaseous surface to make the transition from a vapour to a liquid. In the atmosphere, this surface presents itself as tiny solid or liquid particles called CCNs...
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.
StratusA stratus cloud is a cloud belonging to a class characterized by horizontal layering with a uniform base, as opposed to convective clouds that are as tall or taller than wide . More specifically, the term stratus is used to describe flat, hazy, featureless clouds of low altitude varying in color...
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 cooling, and evaporative cooling. Adiabatic cooling occurs when air rises and expands. The air can rise due to
convectionConvection is the movement of molecules within fluids . Convection is one of the major modes of heat transfer and mass transfer...
, large-scale atmospheric motions, or a physical barrier such as a mountain (
orographic liftOrographic lift occurs when an air mass is forced from a low elevation to a higher elevation as it moves over rising terrain. As the air mass gains altitude it expands and cools adiabatically, which can raise the relative humidity to 100% and create clouds and, under the right conditions,...
). 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 radiationThermal radiation is electromagnetic radiation emitted from the surface of an object which is due to the object's temperature. An example of thermal radiation is the infrared radiation emitted by a common household radiator or electric heater. A person near a raging bonfire will feel the radiated...
, 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 temperatureThe wet-bulb temperature is a type of temperature measurement that reflects the physical properties of a system with a mixture of a gas and a vapor, usually air and water vapor...
, or until it reaches saturation.
Adding moisture to the air
The main ways water vapour 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.
Raindrops
CoalescenceCoalescence is the process by which two or more droplets or particles merge during contact to form a single daughter droplet . It can take place in many processes, ranging from meteorology to astrophysics. For example, it is both involved in the formation of raindrops as well as planetary and star...
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 processThe Bergeron–Findeisen process , also known as the cold rain or ice crystal process, is the formation of precipitation in the cold clouds of the mid and upper latitudes by ice crystal growth. The equilibrium vapor pressure over water is greater than the saturation vapor pressure over ice, at the...
. 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. 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 to 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
Ice pelletsIce pellets are a form of precipitation consisting of small, translucent balls of ice. This form of precipitation is also known as sleet in the United States. Ice pellets are usually smaller than hailstones. They often bounce when they hit the ground, and generally do not freeze into a solid...
are a form of precipitation consisting of small, translucent balls of ice. This form of precipitation is also known as
sleetSleet may refer to:*Rain and snow mixed, particularly in countries where British English is spoken*Ice pellets, mainly within the United States...
in the
United StatesThe United States of America is a federal constitutional republic comprising fifty states and a federal district...
. Ice pellets are usually (but not always) smaller than
hailHail is a form of solid precipitation which consists of balls or irregular lumps of ice, that are individually called hail stones. Hail stones on Earth consist mostly of water ice and measure between 5 and 150 millimeters in diameter, with the larger stones coming from severe and dangerous...
stones. They often bounce when they hit the ground, and generally do not freeze into a solid mass unless mixed with
freezing rainFreezing rain is the name given to rain that falls when surface temperatures are below freezing. The raindrops become supercooled while passing through a sub-freezing layer of air, many hundred feet , just above the surface, and then freeze upon impact with any object they encounter. The resulting...
. The
METARMETAR is a format for reporting weather information. A METAR weather report is predominantly used by pilots in fulfillment of a part of a pre-flight weather briefing, and by meteorologists, who use aggregated METAR information to assist in weather forecasting....
code for ice pellets is
PL.
Ice pellets form when a layer of above-freezing air is located between and above the ground, with sub-freezing air both above and below it. 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 rainFreezing rain is the name given to rain that falls when surface temperatures are below freezing. The raindrops become supercooled while passing through a sub-freezing layer of air, many hundred feet , just above the surface, and then freeze upon impact with any object they encounter. The resulting...
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 frontA warm front is defined as the leading edge of an advancing mass of warm air; it separates warm air from the colder air ahead.-Development of warm fronts:...
during the cold season , but can occasionally be found behind a passing
cold frontA cold front is defined as the leading edge of a cooler and drier mass of air, replacing a warmer mass of air.-Development of cold front:...
.
Hail
Like other precipitation, hail forms in storm
cloudA cloud is a visible mass of droplets or frozen crystals suspended in the atmosphere above the surface of the Earth or another planetary body. A cloud is also a visible mass attracted by gravity, such as masses of material in space called interstellar clouds and nebulae...
s when supercooled
waterWater is an ubiquitous chemical substance that is composed of hydrogen and oxygen and is essential for all known forms of life.In typical usage, water refers only to its liquid form or state, but the substance also has a solid state, ice, and a gaseous state, water vapor or steam. Water covers 71%...
droplets freeze on contact with condensation nuclei, such as
dustDust is a general name for minute solid particles with diameters less than 20 thou . Particles in the atmosphere arise from various sources such as soil dust lifted up by wind, volcanic eruptions, and pollution...
or
dirtDirt is unclean matter, especially when in contact with a person's clothes, skin or possessions when they are said to become dirty. Common types of dirt include soil and dust.-Cleaning:...
. 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 up again. Hail has a diameter of or more. Within METAR code, GR is used to indicate larger hail, of a diameter of at least . 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 and weigh more than . In large hailstones,
latent heatThe expression latent heat refers to the amount of energy released or absorbed by a chemical substance during a change of state that occurs without changing its temperature, meaning a phase transition such as the melting of ice or the boiling of water...
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.
Hail forms in strong
thunderstormA thunderstorm, also known as an electrical storm, a lightning storm, a hailstorm, or simply a storm is a form of weather characterized by the presence of lightning and its acoustic effect on the Earth's atmosphere known as thunder. The meteorologically-assigned cloud type associated with the...
clouds, particularly those with intense updrafts, high liquid water content, great vertical extent, large water droplets, and where a good portion of the cloud layer is below freezing . Hail-producing clouds are often identifiable by their green coloration. The growth rate is maximized at about , and becomes vanishingly small much below as supercooled water droplets become rare. For this reason, hail is most common within continental interiors of the mid-latitudes, as hail formation is considerably more likely when the freezing level is below the altitude of .
EntrainmentSee entrainment for other types.Entrainment is a phenomenon of the atmosphere which occurs when a turbulent flow captures a non-turbulent flow...
of dry air into strong thunderstorms over continents can increase the frequency of hail by promoting evaporational cooling which lowers the freezing level of thunderstorm clouds giving hail a larger volume to grow in. Accordingly, hail is actually less common in the tropics despite a much higher frequency of thunderstorms than in the mid-latitudes because the atmosphere over the tropics tends to be warmer over a much greater depth. Hail in the tropics occurs mainly at higher elevations.
Snowflakes
Snow crystals form when tiny supercooled cloud droplets (about 10 μm in diameter)
freezeIn physical science, freezing or solidification is the process in which a liquid turns into a solid when cold enough. The freezing point is the temperature at which this happens. Melting, the process of turning a solid to a liquid, is almost the exact opposite of freezing...
. These droplets are able to remain liquid at temperatures lower than , because to freeze, a few molecules in the droplet need to get together by chance to form an arrangement similar to that in an ice lattice; then the droplet freezes around this "nucleus." Experiments show that this "homogeneous" nucleation of cloud droplets only occurs at temperatures lower than . In warmer clouds an aerosol particle or "ice nucleus" must be present in (or in contact with) the droplet to act as a nucleus. Our understanding of what particles make efficient ice nuclei is poor — what we do know is they are very rare compared to that cloud condensation nuclei on which liquid droplets form. Clays, desert dust and biological particles may be effective, although to what extent is unclear. Artificial nuclei include particles of
silver iodideSilver iodide is an inorganic compound. This yellow photosensitive solid is used in photography, as an antiseptic in medicine, and in rainmaking or cloud seeding.Silver iodide is highly insoluble in water....
and
dry iceDry ice, sometimes referred to as "Cardice" or as "card ice" is the solid form of carbon dioxide. It is commonly used as a versatile cooling agent.- Properties :...
, and these are used to stimulate precipitation in
cloud seedingCloud seeding, a form of weather modification, is the attempt to change the amount or type of precipitation that falls from clouds, by dispersing substances into the air that serve as cloud condensation or ice nuclei, which alter the microphysical processes within the cloud...
.
Once a droplet has frozen, it grows in the supersaturated environment, which is one where air is saturated with respect to ice when the temperature is below the freezing point. The droplet then grows by diffusion of water molecules in the air (vapor) onto the ice crystal surface where they are collected. Because water droplets are so much more numerous than the ice crystals due to their sheer abundance, the crystals are able to grow to hundreds of
micrometerA micrometer , sometimes known as a micrometer screw gauge, is a device used widely in mechanical engineering and machining for precision measurement, along with other metrological instruments such as dial calipers and vernier calipers...
s or millimeters in size at the expense of the water droplets. This process is known as the Wegner-Bergeron-Findeison process. The corresponding depletion of water vapor 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 controversial. Possibilities include mechanical interlocking,
sinteringSintering is a method for making objects from powder, by heating the material in a sintering furnace below its melting point until its particles adhere to each other...
, electrostatic attraction as well as the existence of a "sticky" liquid-like layer on the crystal surface. The individual ice crystals often have hexagonal symmetry. 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 reflectionDiffuse reflection is the reflection of light from an uneven or granular surface such that an incident ray is seemingly reflected at a number of angles. It is the complement to specular reflection...
of the whole
spectrumA spectrum is a condition that is not limited to a specific set of values but can vary infinitely within a continuum. The word saw its first scientific use within the field of optics to describe the rainbow of colors in visible light when separated using a prism; it has since been applied by...
of
lightLight is electromagnetic radiation, particularly radiation of a wavelength that is visible to the human eye ....
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 , 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 due to the 10,000,000,000,000,000,000 water molecules which make up a snowflake, which grow at different rates and in different patterns depending on the changing temperature and humidity within the atmosphere that the snowflake falls through on its way to the ground. The METAR code for snow is SN, while snow showers are coded SHSN.
Frontal activity
Stratiform or dynamic precipitation occurs as a consequence of slow ascent of air in
synoptic systemsThe synoptic scale in meteorology is a horizontal length scale of the order of 1000 kilometres or more. This corresponds to a horizontal scale typical of mid-latitude depressions...
(on the order of cm/s), such as over surface
cold frontA cold front is defined as the leading edge of a cooler and drier mass of air, replacing a warmer mass of air.-Development of cold front:...
s, and over and ahead of
warm frontA warm front is defined as the leading edge of an advancing mass of warm air; it separates warm air from the colder air ahead.-Development of warm fronts:...
s. Similar ascent is seen around
tropical cycloneA tropical cyclone is a storm system characterized by a large low-pressure center and numerous thunderstorms that produce strong winds and heavy rain. Tropical cyclones feed on heat released when moist air rises, resulting in condensation of water vapor contained in the moist air...
s outside of the
eyewallThe eye is a region of mostly calm weather found at the center of strong tropical cyclones. The eye of a storm is a roughly circular area and typically 30–65 km in diameter. It is surrounded by the eyewall, a ring of towering thunderstorms where the most severe weather of a cyclone occurs...
, and in comma-head precipitation patterns around
mid-latitude cycloneCyclogenesis is the development or strengthening of cyclonic circulation in the atmosphere . Cyclogenesis is an umbrella term for several different processes, all of which result in the development of some sort of cyclone. It can occur at various scales, from the microscale to the synoptic scale...
s. A wide variety of weather can be found along an occluded front, with thunderstorms possible, but usually their passage is associated with a drying of the air mass. Occluded fronts usually form around mature low-pressure areas. Precipitation may occur on celestial bodies other than Earth. When it gets cold,
MarsMars is the fourth planet from the Sun in the Solar System. The planet is named after Mars, the Roman god of war. It is also referred to as the "Red Planet" because of its reddish appearance, due to iron oxide prevalent on its surface....
has precipitation that most likely takes the form of ice needles, rather than rain or snow.
Convection
Convective rain, or showery precipitation, occurs from convective clouds, e.g., cumulonimbus or cumulus congestus. It falls as showers with rapidly changing intensity. Convective precipitation falls over a certain area for a relatively short time, as convective clouds have limited horizontal extent. Most precipitation in the
tropicsThe tropics,the tropics are very hot. hi everyone! peace out!seated in the equatorial regions of the world, are limited in latitude by the Tropic of Cancer in the northern hemisphere at approximately 23°26' N latitude and the Tropic of Capricorn in the southern hemisphere at 23°26' S latitude...
appears to be convective; however, it has been suggested that stratiform precipitation also occurs. Graupel and
hailHail is a form of solid precipitation which consists of balls or irregular lumps of ice, that are individually called hail stones. Hail stones on Earth consist mostly of water ice and measure between 5 and 150 millimeters in diameter, with the larger stones coming from severe and dangerous...
indicate convection. In mid-latitudes, convective precipitation is intermittent and often associated with baroclinic boundaries such as
cold frontA cold front is defined as the leading edge of a cooler and drier mass of air, replacing a warmer mass of air.-Development of cold front:...
s,
squall lineA squall line is a line of severe thunderstorms that can form along and/or ahead of a cold front. In the early 20th century, the term was used as a synonym for cold front. It contains heavy precipitation, hail, frequent lightning, strong straight line winds, and possibly tornadoes and waterspouts...
s, and warm fronts.
Orographic effects
Orographic precipitation occurs on the windward side of mountains and is caused by the rising air motion of a large-scale flow of moist air across the mountain ridge, resulting in adiabatic cooling and condensation. In mountainous parts of the world subjected to relatively consistent winds (for example, the
trade windThe trade winds are the prevailing pattern of easterly surface winds found in the tropics near the Earth's equator. The trade winds blow predominantly from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere...
s), a more moist
climateClimate encompasses the statistics of temperature, humidity, atmospheric pressure, wind, rainfall, atmospheric particle count and numerous other meteorological elements in a given region over long periods of time...
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 windA katabatic wind, from the Greek word katabatikos meaning "going downhill", is the technical name for 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 sometimes also called fall winds.Not all downslope winds are...
) on the descending and generally warming, leeward side where a
rain shadowA rain shadow is an area of land that has suffered desertification from proximity to mountain ranges. The mountains block the passage of rain-producing weather systems, casting a "shadow" of dryness behind them.-Description:...
is observed.
In
HawaiiHawaii is the newest of the 50 U.S. states, and is the only state made up entirely of islands. It is located on an archipelago in the central Pacific Ocean, southwest of the continental United States, southeast of Japan, and northeast of Australia. The state was admitted to the Union on August...
, 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 . Storm systems affect the state with heavy rains between October and March. Local climates vary considerably on each island due to their topography, divisible into windward (
Koolau) and leeward (
Kona) regions based upon location relative to the higher mountains. Windward sides face the east to northeast trade winds and receive much more rainfall; leeward sides are drier and sunnier, with less rain and less cloud cover.
In South America, the
AndesThe Andes are the world's longest exposed mountain range. They lie as a continuous chain of highland along the western coast of South America...
mountain range blocks
PacificThe Pacific Ocean is the largest of the Earth's oceanic divisions. Its name is derived from the Latin name Tepre Pacificum, "peaceful sea", bestowed upon it by the Portuguese explorer Ferdinand Magellan. It extends from the Arctic in the north to Antarctica in the south, bounded by Asia and...
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 BasinThe Great Basin is a large, arid region of the western United States. Its boundaries vary depending on how it is defined, but it is most commonly defined as the contiguous endorheic basin roughly between the Wasatch Mountains and the Sierra Nevada mountains. Culturally, the Great Basin is home to...
and
Mojave DesertThe Mojave Desert , , locally referred to as the High Desert, occupies a significant portion of southeastern California and smaller parts of central California, southern Nevada, southwestern Utah and northwestern Arizona, in the United States...
s.
Snow
Extratropical cycloneExtratropical cyclones, sometimes called mid-latitude cyclones or wave cyclones, are a group of cyclones defined as synoptic scale low pressure weather systems that occur in the middle latitudes of the Earth having neither tropical nor polar characteristics, and are connected with fronts and...
s can bring cold and dangerous conditions with heavy rain and snow with winds exceeding 119 km/h (74 mph), (sometimes referred to as
windstormsA European windstorm is a severe cyclonic windstorm associated with areas of low pressure that track across the North Atlantic towards northwestern Europe. They are most common in the winter months. Deep low pressure areas are relatively common over the North Atlantic and frequently track past the...
in Europe). The band of precipitation that is associated with their
warm frontA warm front is defined as the leading edge of an advancing mass of warm air; it separates warm air from the colder air ahead.-Development of warm fronts:...
is often extensive, forced by weak upward vertical motion of air over the frontal boundary which
condensesCondensation is the change of the physical state of aggregation of matter from gaseous phase into liquid phase and the reverse of evaporation. When the transition happens from the gaseous phase into the solid phase directly, bypassing the liquid phase, the change is called deposition...
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 bandA 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 imagery, this precipitation elongation is referred to as...
. In the
Northern HemisphereThe Northern Hemisphere is the half of a planet that is north of the equator—the word hemisphere literally means 'half sphere'. It is also that half of the celestial sphere north of the celestial equator...
, poleward is towards the
North PoleThe North Pole, also known as the Geographic North Pole or Terrestrial North Pole, is, subject to the caveats explained below, defined as the point in the northern hemisphere where the Earth's axis of rotation meets the Earth's surface...
, or north. Within the
Southern HemisphereThe Southern Hemisphere is the half of a planet that is south of the equator—the word hemisphere literally means 'half ball'...
, poleward is towards the
South PoleThe South Pole, also known as the Geographic South Pole or Terrestrial South Pole, is one of the two points where the Earth's axis of rotation intersects the surface. It is the southernmost point on the surface of the Earth and lies on the opposite side of the Earth from the North Pole...
, or south.
Within the
cold sector, poleward and west of the cyclone center,
small scaleMicroscale meteorology is the study of short-lived atmospheric phenomena smaller than mesoscale, about 1 km or less. These two branches of meteorology are sometimes grouped together as "mesoscale and microscale meteorology" and together study all phenomena smaller than synoptic scale; that is they...
or
mesoscaleMesoscale meteorology is the study of weather systems smaller than synoptic scale systems but larger than microscale and storm-scale cumulus systems. Horizontal dimensions generally range from around 5 kilometers to several hundred kilometers...
bands of heavy snow can occur within a cyclone's comma-head pattern. This pattern is a comma-shaped area of clouds and precipitation found around mature extratropical cyclones. These snow bands typically have a width of to . The bands in the comma head are associated with areas of
frontogenesisFrontogenesis, in meteorology, refers to the formation or strengthening of an atmospheric front....
, or zones of strengthening temperature contrast.
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
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
euphemismA euphemism is a substitution of an agreeable or less offensive expression in place of one that may offend or suggest something unpleasant to the listener, or in the case of Doublespeak, to make it less troublesome for the speaker...
by tourist authorities. Areas with wet seasons are dispersed across portions of the
tropicsThe tropics,the tropics are very hot. hi everyone! peace out!seated in the equatorial regions of the world, are limited in latitude by the Tropic of Cancer in the northern hemisphere at approximately 23°26' N latitude and the Tropic of Capricorn in the southern hemisphere at 23°26' S latitude...
and
subtropicsThe subtropics are the geographical zone of the Earth immediately north and south of the tropical zone, which is bounded by the Tropic of Cancer and the Tropic of Capricorn, at latitudes 23.5°N and 23.5°S...
.
SavannaA savanna, or savannah, is a grassland ecosystem characterized by the trees being sufficiently small or widely spaced so that the canopy does not close...
climates and areas with
monsoonA pennis is traditionally defined as a seasonal reversing wind accompanied by seasonal changes in precipitation, but now is used to describe seasonal changes atmospheric circulation and precipitation The major monsoon systems of the world consist of the African and Asia-Australian monsoons...
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 zoneThe Intertropical Convergence Zone is a term that is used to describe the North-East and South-East trade wind convergence. Convectively active portions of the ITCZ are known as the monsoon trough...
or
monsoon troughThe monsoon trough or those portions of the Intertropical Convergence Zone within a monsoon region, is depicted by a line on a weather map showing the locations of minimum sea level pressure, and is a convergence zone between the wind patterns of the southern and northern hemispheres...
move poleward of their location during the middle of the warm season. When the wet season occurs during the warm season, or
summerSummer is one of the four temperate seasons, marked by the time of year with the longest days, and lies between spring and autumn. The seasons are popularly considered to start on different dates in different cultures based on astronomy and regional meteorology. When it is summer in the southern...
, 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.
SoilSoil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics. It is composed of particles of broken rock that have been altered by chemical and...
nutrients diminish and erosion increases. Animals have adaptation and survival strategies for the wetter regime. Unfortunately, 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
cycloneIn meteorology, a cyclone is an area of closed, circular fluid motion rotating in the same direction as the Earth. This is usually characterized by inward spiraling winds that rotate counter clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere of the Earth.Large-scale...
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.
Measurement
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 gages have markings on the inner cylinder down to 0.25 mm (0.01 in) resolution, while metal gages 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 gages are winterized 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 gage so they do not have to melt the snow or ice that falls into the gage. Once the snowfall/ice is finished accumulating, or as you approach 300 mm (12 in), one can either bring it inside to melt, or use luke warm 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 gages include the popular wedge gage (the cheapest rain gage and most fragile), the tipping bucket rain gage, and the weighing rain gage. The wedge and tipping bucket gages 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 gage is kept much above freezing. Weighing gages 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 gage if left out in the open, but its accuracy will depend on what ruler you use to measure the rain with. Any of the above rain gages 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
CoCoRAHSThe Community Collaborative Rain, Hail and Snow network, or CoCoRaHS, is a network of volunteer weather watchers who take daily readings of precipitation, and report them online or by phone.-History:...
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.
Return period
The likelihood or probability of an event with a specified intensity and duration, is called the
return periodA return period also known as a recurrence interval is an estimate of the interval of time between events like an earthquake, flood or river discharge flow of a certain intensity or size. It is a statistical measurement denoting the average recurrence interval over an extended period of time, and...
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 to have multiple "1 in 100 Year Storms" in a single year.
Role in 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,
monsoonA pennis is traditionally defined as a seasonal reversing wind accompanied by seasonal changes in precipitation, but now is used to describe seasonal changes atmospheric circulation and precipitation The major monsoon systems of the world consist of the African and Asia-Australian monsoons...
, tropical savanna, humid subtropical, humid continental,
oceanic climateAn oceanic climate is the climate typically found along the west coasts at the middle latitudes of all the world's continents, and in southeastern Australia...
,
Mediterranean climateA hi Mediterranean climate resembles the climate of the lands in the Mediterranean Basin, which includes most of the area with this climate type worldwide...
,
steppeIn physical geography, a steppe is a biome region characterised by grassland plain without trees . The prairie can be considered a steppe. It may be semi-desert, or covered with grass or shrubs or both, depending on the season and latitude...
,
subarctic climateRegions having a subarctic climate are characterized by long, usually very cold winters, and short, cool to mild summers. It is found on large landmasses, away from the moderating effects of an ocean, generally at latitudes from 50° to 70°N poleward of the humid continental climates...
,
tundraIn physical geography, tundra is a biome where the tree growth is hindered by low temperatures and short growing seasons. The term tundra comes from Kildin Sami tūndâr, which means "uplands, treeless mountain tract." There are two types of tundra: Arctic tundra and alpine tundra...
,
polar ice capA polar ice cap is a high latitude region of a 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 any geological requirement for it to be over land; only that it must be a body of...
, and
desertA desert is a landscape or region that receives almost no precipitation. 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. In the Köppen climate classification system,...
.
Rain forests are characterized by high rainfall, with definitions setting minimum normal annual rainfall between and . A tropical savanna is a
grasslandGrasslands are areas where the vegetation is dominated by grasses and other herbaceous plants . However, sedge and rush families can also be found. Grasslands occur naturally on all continents except Antarctica...
biomeBiome are climatically and geographically defined areas of ecologically similar climatic conditions such as communities of plants, animals, and soil organisms, and are often referred to as ecosystems...
located in
semi-aridA Semi-arid climate or steppe climate generally describes climatic regions that receive low annual rainfall . A more precise definition is given by the Köppen climate classification that treats steppe climates as intermediates between the desert climates and humid climates in ecological...
to semi-humid climate regions of subtropical and tropical latitudes, with rainfall between and a year. They are widespread on
AfricaAfrica is the world's second-largest and second most-populous continent, after Asia. At about 30.2 million km² including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area. With a billion people in 61 territories, it accounts for about 14.8% of the...
, and are also found in
IndiaIndia, officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country, and the most populous democracy in the world. Bounded by the Indian Ocean on the south, the Arabian Sea on the west, and the Bay of Bengal...
, the northern parts of
South AmericaSouth America is the southern continent of the Americas, situated entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere...
,
MalaysiaMalaysia is a country in Southeast Asia that consists of thirteen states and three Federal Territories, with a total landmass of . The capital city is Kuala Lumpur, while Putrajaya is the seat of the federal government. The population stands at over 28 million inhabitants...
, and
AustraliaAustralia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the continental mainland , the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans...
. The humid subtropical climate zone where winter rainfall (and sometimes snowfall) is associated with large
stormA storm is any disturbed state of an astronomical body's atmosphere, especially affecting its surface, and strongly implying severe weather...
s that the
westerliesThe Westerlies or the Prevailing Westerlies are the prevailing winds in the middle latitudes between 35 and 65 degrees latitude, blowing from the high pressure area in the horse latitudes towards the poles. These prevailing winds blow from the west to the east, and steer extratropical cyclones in...
steer from west to east. Most summer rainfall occurs during
thunderstormA thunderstorm, also known as an electrical storm, a lightning storm, a hailstorm, or simply a storm is a form of weather characterized by the presence of lightning and its acoustic effect on the Earth's atmosphere known as thunder. The meteorologically-assigned cloud type associated with the...
s and from occasional
tropical cycloneA tropical cyclone is a storm system characterized by a large low-pressure center and numerous thunderstorms that produce strong winds and heavy rain. Tropical cyclones feed on heat released when moist air rises, resulting in condensation of water vapor contained in the moist air...
s. Humid subtropical climates lie on the east side continents, roughly between
latitudeLatitude, usually denoted by the Greek letter phi gives the location of a place on Earth north or south of the equator. Lines of Latitude are the imaginary horizontal lines shown running east-to-west on maps that run either north or south of the equator...
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
AustraliaAustralia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the continental mainland , the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans...
, and is accompanied by plentiful precipitation year round. The Mediterranean climate regime resembles the climate of the lands in the
Mediterranean BasinThe Mediterranean Basin comprises the lands around and surrounded by the Mediterranean Sea. In biogeography, the Mediterranean Basin refers to the lands around the Mediterranean Sea that have a Mediterranean climate, with mild, rainy winters and hot, dry summers, which supports characteristic...
, parts of western
North AmericaNorth America is the northern continent of the Americas, situated in the Earth's northern hemisphere and in the western hemisphere. It is bordered on the north by the Arctic Ocean, on the east by the North Atlantic Ocean, on the southeast by the Caribbean Sea, and on the west by the North Pacific...
, parts of
WesternWestern Australia is a state of Australia, occupying the entire western third of the Australian continent. Australia's largest state and the second largest subnational entity in the world, it has 2.2 million inhabitants , 85% of whom live in the south-west corner of the state.The state's capital...
and
South AustraliaSouth Australia is a state of Australia in the southern central part of the country. It covers some of the most arid parts of the continent; with a total land area of , it is the fourth largest of Australia's six states and two territories....
, in southwestern
South AfricaThe Republic of South Africa is a country located at the southern tip of Africa, with a coastline on the Atlantic and Indian Oceans. To the north lie Namibia, Botswana and Zimbabwe, to the east are Mozambique and Swaziland, while Lesotho is an independent country surrounded by South Africa.Modern...
and in parts of central
ChileChile, officially the Republic of Chile , is a country in South America occupying a long, narrow coastal strip between the Andes mountains to the east and the Pacific Ocean to the west. It borders Peru to the north, Bolivia to the northeast, Argentina to the east, and the Drake Passage in the far...
. The climate is characterized by hot, dry summers and cool, wet winters. A steppe is a dry
grasslandGrasslands are areas where the vegetation is dominated by grasses and other herbaceous plants . However, sedge and rush families can also be found. Grasslands occur naturally on all continents except Antarctica...
. Subarctic climates are cold with continuous
permafrostIn geology, permafrost or permafrost soil is soil at or below the freezing point of water for two or more years. Ice is not always present, as may be in the case of nonporous bedrock, but it frequently occurs and it may be in amounts exceeding the potential hydraulic saturation of the ground...
and little precipitation.
Changes due to global warming
Increasing temperatures tend to increase evaporation which leads to more precipitation. As average global temperatures have risen, average global precipitation has also increased. 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 per century 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).
Changes due to urban heat island
The urban heat island warms cities 0.6 °C (1.1 °F) to 5.6 °C (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 to downwind of cities, compared with upwind. Some cities induce a total precipitation increase of 51%.
Forecasting
The Quantitative Precipitation Forecast (abbreviated QPF) is the expected amount of liquid precipitation accumulated over a specified time period over a specified area. A QPF will be specified when a measurable precipitation type reaching a minimum threshold is forecast for any hour during a QPF valid period. Precipitation forecasts tend to be bound by synoptic hours such as 0000, 0600, 1200 and 1800 GMT. Terrain is considered in QPFs by use of topography or based upon climatological precipitation patterns from observations with fine detail. Starting in the mid to late 1990s, QPFs were used within hydrologic forecast models to simulate impact to rivers throughout the United States.
Forecast modelsNumerical weather prediction uses current weather conditions as input into mathematical models of the atmosphere to predict the weather. While the first efforts to accomplish this were done in the 1920s, it wasn't until the advent of the computer that it was feasible to do in real-time...
show significant sensitivity to humidity levels within the
planetary boundary layerThe planetary boundary layer , also known as the atmospheric boundary layer , is the lowest part of the atmosphere and its behavior is directly influenced by its contact with a planetary surface. On Earth it usually responds to changes in surface forcing in an hour or less...
, 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
skillSkill in forecasting or is a scaled representation of forecast error that relates the forecast accuracy of a particular forecast model to some reference model....
than model forecasts within 6 to 7 hours of the time of the radar image. The forecasts can be verified through use of rain gage measurements,
weather radarA weather radar, or weather surveillance radar , is a type of radar used to locate precipitation, calculate its motion, estimate its type , and forecast its future position and intensity....
estimates, or a combination of both. Various skill scores can be determined to measure the value of the rainfall forecast.
See also
- List of meteorology topics
- Mango showers
Mango showers are the pre-monsoon showers in the Indian states of Karnataka and Kerala that help in the ripening of mangoes. Also known as April rains or Summer showers, they are a result of thunderstorms over the Bay of Bengal...
, pre-monsoonA pennis is traditionally defined as a seasonal reversing wind accompanied by seasonal changes in precipitation, but now is used to describe seasonal changes atmospheric circulation and precipitation The major monsoon systems of the world consist of the African and Asia-Australian monsoons...
showers in the IndiaIndia, officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country, and the most populous democracy in the world. Bounded by the Indian Ocean on the south, the Arabian Sea on the west, and the Bay of Bengal...
n states of KarnatakaKarnataka is a state in the southern part of India. It was created on November 1, 1956, with the passing of the States Reorganisation Act...
and KeralaKerala , is a state located in southwestern India. The state was created in 1956 on linguistc basis, bringing together those places where Malayalam formed the principal language...
that help in the ripening of mangoes.
- Steam
Steam is vaporized water. It is a transparent gas . At standard temperature and pressure, pure steam occupies about 1,600 times the volume of an equal mass of liquid water...
- Sunshower
A sunshower or sun shower is an unusual meteorological phenomenon in which rain falls while the sun is shining. These conditions often lead to the appearance of a rainbow, if the sun is at a low enough angle. The term "sunshower" is used in the United States, Canada, Australia, New Zealand, Ireland...
, an unusual meteorological phenomenonA meteorological phenomenon is a weather event which can be explained by the principles of meteorology.* Air mass* Anticyclone* Arctic cyclone* Clouds* Crow Instability* Derecho* Drought* Dust devil* Dust storm* Extratropical cyclone* Foehn wind...
in which rainRain is liquid precipitation, as opposed to other kinds of precipitation such as snow, hail and sleet. On Earth, it is the condensation of atmospheric water vapor into drops heavy enough to fall, often making it to the surface...
falls while the sun is shining.
- Umbrella
An umbrella or parasol is a canopy designed to protect against precipitation or sunlight. The term parasol usually refers to an item designed to protect from the sun, and umbrella refers to a device more suited to protect from rain...
- Wintry showers, an informal meteorological
Meteorology is the interdisciplinary scientific study of the atmosphere that focuses on weather processes and forecasting . Studies in the field stretch back millennia, though significant progress in meteorology did not occur until the eighteenth century...
term for various mixtures of rainRain is liquid precipitation, as opposed to other kinds of precipitation such as snow, hail and sleet. On Earth, it is the condensation of atmospheric water vapor into drops heavy enough to fall, often making it to the surface...
, freezing rainFreezing rain is the name given to rain that falls when surface temperatures are below freezing. The raindrops become supercooled while passing through a sub-freezing layer of air, many hundred feet , just above the surface, and then freeze upon impact with any object they encounter. The resulting...
, sleetSleet may refer to:*Rain and snow mixed, particularly in countries where British English is spoken*Ice pellets, mainly within the United States...
and snowSnow is a type of precipitation in the form of crystalline water ice, consisting of a multitude of snowflakes that fall from clouds. Since snow is composed of small ice particles, it is a granular material. It has an open and therefore soft structure, unless packed by external pressure. Snowflakes...
.
- Probability of Precipitation
A Probability of Precipitation or POP is a formal measure of the likelihood of precipitation that is often published from weather forecasting models....
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