Cloud

Cloud

Overview
A cloud is a visible mass
Mass
Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...

 of liquid
Liquid
Liquid is one of the three classical states of matter . Like a gas, a liquid is able to flow and take the shape of a container. Some liquids resist compression, while others can be compressed. Unlike a gas, a liquid does not disperse to fill every space of a container, and maintains a fairly...

 droplets
Drop (liquid)
A drop or droplet is a small column of liquid, bounded completely or almost completely by free surfaces. A drop may form when liquid accumulates at the lower end of a tube or other surface boundary, producing a hanging drop called a pendant drop...

 or frozen crystal
Crystal
A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...

s made of water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

 and/or various chemicals suspended in the atmosphere
Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...

 above the surface of a planet
Planet
A 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,...

ary body. They are also known as aerosols. Clouds in Earth's atmosphere are studied in the cloud physics
Cloud physics
Cloud physics is the study of the physical processes that lead to the formation, growth and precipitation of clouds. Cloud formations are composed of microscopic droplets of liquid water , tiny crystals of ice , or both...

 branch of meteorology
Meteorology
Meteorology is the interdisciplinary scientific study of the atmosphere. Studies in the field stretch back millennia, though significant progress in meteorology did not occur until the 18th century. The 19th century saw breakthroughs occur after observing networks developed across several countries...

. Two processes, possibly acting together, can lead to air's becoming saturated: cooling the air or adding water vapor
Water vapor
Water vapor or water vapour , also aqueous vapor, is the gas phase of water. It is one state of water within the hydrosphere. Water vapor can be produced from the evaporation or boiling of liquid water or from the sublimation of ice. Under typical atmospheric conditions, water vapor is continuously...

 to the air. In general, precipitation
Precipitation (meteorology)
In meteorology, precipitation In meteorology, precipitation In meteorology, precipitation (also known as one of the classes of hydrometeors, which are atmospheric water phenomena is any product of the condensation of atmospheric water vapor that falls under gravity. The main forms of precipitation...

 will fall to the surface; an exception is virga
Virga
In 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...

, which evaporates before reaching the surface.

Clouds can show convective development like cumulus
Cumulus
Cumulus is a type of cloud with the appearance of a lump of cotton wool.Cumulus may also refer to:*Cumulus Media, a radio broadcasting company*Cumulus , digital asset management software developed by Canto Software*Reinhard Cumulus, glider...

, appear in layered sheets such as stratus
Stratus cloud
A 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...

, or take the form of thin fibrous wisps, as in the case of cirrus
Cirrus cloud
Cirrus clouds are atmospheric clouds generally characterized by thin, wispy strands, giving them their name from the Latin word cirrus meaning a ringlet or curling lock of hair...

.
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Encyclopedia
A cloud is a visible mass
Mass
Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...

 of liquid
Liquid
Liquid is one of the three classical states of matter . Like a gas, a liquid is able to flow and take the shape of a container. Some liquids resist compression, while others can be compressed. Unlike a gas, a liquid does not disperse to fill every space of a container, and maintains a fairly...

 droplets
Drop (liquid)
A drop or droplet is a small column of liquid, bounded completely or almost completely by free surfaces. A drop may form when liquid accumulates at the lower end of a tube or other surface boundary, producing a hanging drop called a pendant drop...

 or frozen crystal
Crystal
A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...

s made of water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

 and/or various chemicals suspended in the atmosphere
Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...

 above the surface of a planet
Planet
A 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,...

ary body. They are also known as aerosols. Clouds in Earth's atmosphere are studied in the cloud physics
Cloud physics
Cloud physics is the study of the physical processes that lead to the formation, growth and precipitation of clouds. Cloud formations are composed of microscopic droplets of liquid water , tiny crystals of ice , or both...

 branch of meteorology
Meteorology
Meteorology is the interdisciplinary scientific study of the atmosphere. Studies in the field stretch back millennia, though significant progress in meteorology did not occur until the 18th century. The 19th century saw breakthroughs occur after observing networks developed across several countries...

. Two processes, possibly acting together, can lead to air's becoming saturated: cooling the air or adding water vapor
Water vapor
Water vapor or water vapour , also aqueous vapor, is the gas phase of water. It is one state of water within the hydrosphere. Water vapor can be produced from the evaporation or boiling of liquid water or from the sublimation of ice. Under typical atmospheric conditions, water vapor is continuously...

 to the air. In general, precipitation
Precipitation (meteorology)
In meteorology, precipitation In meteorology, precipitation In meteorology, precipitation (also known as one of the classes of hydrometeors, which are atmospheric water phenomena is any product of the condensation of atmospheric water vapor that falls under gravity. The main forms of precipitation...

 will fall to the surface; an exception is virga
Virga
In 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...

, which evaporates before reaching the surface.

Clouds can show convective development like cumulus
Cumulus
Cumulus is a type of cloud with the appearance of a lump of cotton wool.Cumulus may also refer to:*Cumulus Media, a radio broadcasting company*Cumulus , digital asset management software developed by Canto Software*Reinhard Cumulus, glider...

, appear in layered sheets such as stratus
Stratus cloud
A 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...

, or take the form of thin fibrous wisps, as in the case of cirrus
Cirrus cloud
Cirrus clouds are atmospheric clouds generally characterized by thin, wispy strands, giving them their name from the Latin word cirrus meaning a ringlet or curling lock of hair...

. Prefixes are used in connection with clouds: strato for low cumuliform-category clouds that show some stratiform characteristics, nimbo for thick stratiform clouds that can produce moderate to heavy precipitation, alto for middle clouds, and cirro for high clouds. Whether or not a cloud is low, middle, or high, level depends on how far above the ground its base forms.

Cloud types with significant vertical extent can form in the low or middle ranges depending on the moisture content of the air. Clouds in the troposphere
Troposphere
The troposphere is the lowest portion of Earth's atmosphere. It contains approximately 80% of the atmosphere's mass and 99% of its water vapor and aerosols....

 have Latin
Latin
Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...

 names due to the popular adaptation of Luke Howard
Luke Howard
Luke Howard FRS was a British manufacturing chemist and an amateur meteorologist with broad interests in science...

's cloud categorization system, which began to spread in popularity during December 1802. Synoptic surface weather observation
Surface weather observation
Surface weather observations are the fundamental data used for safety as well as climatological reasons to forecast weather and issue warnings worldwide. They can be taken manually, by a weather observer, by computer through the use of automated weather stations, or in a hybrid scheme using...

s use code numbers for the types of tropospheric cloud visible at each scheduled observation time based on the height and physical appearance of the clouds.

While a majority of clouds form in Earth's troposphere, there are occasions where clouds in the stratosphere
Stratosphere
The stratosphere is the second major layer of Earth's atmosphere, just above the troposphere, and below the mesosphere. It is stratified in temperature, with warmer layers higher up and cooler layers farther down. This is in contrast to the troposphere near the Earth's surface, which is cooler...

 and mesosphere
Mesosphere
The mesosphere is the layer of the Earth's atmosphere that is directly above the stratosphere and directly below the thermosphere. In the mesosphere temperature decreases with increasing height. The upper boundary of the mesosphere is the mesopause, which can be the coldest naturally occurring...

 are observed. Clouds have been observed on other planet
Planet
A 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,...

s and moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...

s within the Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...

, but, due to their different temperature characteristics, they are composed of other substances such as methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...

, ammonia
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...

, and sulfuric acid
Sulfuric acid
Sulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...

.

Latin tropospheric nomenclature: historical background



Luke Howard, a methodical observer with a strong grounding in the Latin language, used his background to categorize the various tropospheric cloud types and forms during December 1802. He believed that the changing cloud forms in the sky could unlock the key to weather forecasting
Weather forecasting
Weather forecasting is the application of science and technology to predict the state of the atmosphere for a given location. Human beings have attempted to predict the weather informally for millennia, and formally since the nineteenth century...

. Jean-Baptiste Lamarck
Jean-Baptiste Lamarck
Jean-Baptiste Pierre Antoine de Monet, Chevalier de la Marck , often known simply as Lamarck, was a French naturalist...

 worked independently on cloud categorization and came up with a different naming scheme that failed to make an impression even in his home country of France
France
The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...

 because it used unusual French names for cloud types. Howard used universally accepted Latin, which caught on quickly. As a sign of the popularity of the naming scheme, the German dramatist and poet Johann Wolfgang von Goethe
Johann Wolfgang von Goethe
Johann Wolfgang von Goethe was a German writer, pictorial artist, biologist, theoretical physicist, and polymath. He is considered the supreme genius of modern German literature. His works span the fields of poetry, drama, prose, philosophy, and science. His Faust has been called the greatest long...

 composed four poems about clouds, dedicating them to Howard. Classification systems would be proposed by Heinrich Dove of Germany in 1828 and Elias Loomis of the United States in 1841, but neither became the international standard that Howard's system became. It was formally adopted by the International Meteorological Commission in 1929.

Howard's original system established three general cloud categories based on physical appearance and process of formation: cirriform (mainly detached and wispy), cumuliform or convective (mostly detached and heaped, rolled, or rippled), and stratiform (mainly continuous layers in sheets). These were cross-classified into lower and upper families. Cumuliform clouds forming in the lower level were given the genus name cumulus, and low stratiform clouds the genus name stratus. Physically similar clouds forming in the upper height range were given the genus names cirrocumulus and cirrostratus, respectively. Cirriform category clouds were identified as always upper level and given the genus name cirrus. To these, Howard added the genus Nimbus
Nimbus
-General meanings:* Nimbus cloud, a cloud that produces precipitation* Halo , light or mist from an object* Halo , the disk or ring around the head of a sacred figure-Specific meanings:* Nimbus , A video game...

 for all clouds producing significant precipitation.

Around 1840-41, German meteorologist Ludwig Kaemtz added stratocumulus as a mostly detached low-cloud genus with both cumuliform and stratiform characteristics, similar to upper level cirrocumulus. About fifteen years later, Emilien Renou, director of the Parc Saint-Maur and Montsouris observatories, began work on an elaboration of Howard's classifications that would lead to the introduction of altocumulus (physically more closely related to stratocumulus than to cumulus) and altostratus during the 1870s. These were cumuliform and stratiform cloud genera, respectively, of a newly defined middle height range above stratocumulus and stratus but below cirrocumulus and cirrostratus, with cumulus and nimbus occupying more than one altitude range as clouds with vertical extent. In 1880, Philip Weilbach, secretary and librarian at the Art Academy in Copenhagen, and like Luke Howard, an amateur meteorologist, proposed and had accepted by the International Meteorological Committee (IMC) the designation of a new vertical genus type, cumulonimbus, which would be distinct from cumulus and nimbus and identifiable by its appearance and ability to produce thunder. With this addition, a canon of ten cloud genera was established that came to be officially and universally accepted. At about the same time, several cloud specialists proposed variations that came to be accepted as species subdivisions and varieties determined by more specific variable aspects of the structure of each genus. One further modification of the genus classification system came when an IMC commission for the study of clouds put forward a refined and more restricted definition of the genus Nimbus renamed Nimbostratus.

Cooling air to its dew point


In general, clouds will form when rising air is cooled to its dew point
Dew point
The dew point is the temperature to which a given parcel of humid air must be cooled, at constant barometric pressure, for water vapor to condense into liquid water. The condensed water is called dew when it forms on a solid surface. The dew point is a saturation temperature.The dew point is...

, the temperature at which the air becomes saturated. Water vapour normally begins to condense on condensation nuclei
Cloud condensation nuclei
Cloud condensation nuclei or CCNs are small particles typically 0.2 µm, or 1/100 th the size of a cloud droplet ) 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...

 such as dust, ice, and salt in order to form clouds. Condensation at surface level results in the formation of fog. If sufficient condensation particles are not present, the air will become supersaturated and the formation of cloud or fog will be inhibited. There are four main mechanisms for cooling the air to its dew point: adiabatic cooling, which tends to produce cloud, and conductive, radiational, and evaporative cooling, which can result in the formation of fog. Adiabatic cooling occurs when air rises and expands. The air can rise due to convection
Convection
Convection is the movement of molecules within fluids and rheids. It cannot take place in solids, since neither bulk current flows nor significant diffusion can take place in solids....

, large-scale atmospheric lift along weather front
Weather front
A weather front is a boundary separating two masses of air of different densities, and is the principal cause of meteorological phenomena. In surface weather analyses, fronts are depicted using various colored lines and symbols, depending on the type of front...

s and around centres of low pressure, or as a result of being forced over a physical barrier such as a mountain
Mountain
Image:Himalaya_annotated.jpg|thumb|right|The Himalayan mountain range with Mount Everestrect 58 14 160 49 Chomo Lonzorect 200 28 335 52 Makalurect 378 24 566 45 Mount Everestrect 188 581 920 656 Tibetan Plateaurect 250 406 340 427 Rong River...

 (orographic lift
Orographic lift
Orographic 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 quickly cools down 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 radiation
Thermal radiation
Thermal radiation is electromagnetic radiation generated by the thermal motion of charged particles in matter. All matter with a temperature greater than absolute zero emits thermal 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
Wet-bulb temperature
The 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. Wet bulb temperature is the lowest temperature that can be reached by the evaporation of water only. It is the...

, or until it reaches saturation.

Adding moisture to the air


The main ways water vapour is added to the air are: wind convergence over water or moist ground 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, and cool or dry air moving over warmer water,

Physical categories


As established by Howard, cloud types or genera are grouped into three physical categories: cirriform, cumuliform or convective, and stratiform. These designations distinguish a cloud's physical structure and process of formation. All weather-related clouds form in the troposphere, the lowest major layer of the Earth's atmosphere.

Cumuliform-category clouds are the product of localized convective or orographic lift. If the airmass is only slightly unstable, clouds of limited convection
Convection
Convection is the movement of molecules within fluids and rheids. It cannot take place in solids, since neither bulk current flows nor significant diffusion can take place in solids....

 that show both cumuliform and stratiform characteristics will form. If a poorly organized weather system is present, weak intermittent precipitation may fall from these clouds. With greater airmass instability caused by a steeper temperature gradient
Gradient
In vector calculus, the gradient of a scalar field is a vector field that points in the direction of the greatest rate of increase of the scalar field, and whose magnitude is the greatest rate of change....

 from warm or hot at surface level to cold aloft, clouds of free convection will form and rise to greater heights, especially if associated with fast-moving unstable cold front
Cold front
A cold front is defined as the leading edge of a cooler mass of air, replacing a warmer mass of air.-Development of cold front:The cooler and denser air wedges under the less-dense warmer air, lifting it...

s. Large free-convective clouds can produce light to moderate showers if the airmass is sufficiently moist. The largest free-convective cumuliform types produce thunderstorm
Thunderstorm
A thunderstorm, also known as an electrical storm, a lightning storm, thundershower 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 a variety of types of lightning
Lightning
Lightning is an atmospheric electrostatic discharge accompanied by thunder, which typically occurs during thunderstorms, and sometimes during volcanic eruptions or dust storms...

 including cloud-to-ground that can cause wildfire
Wildfire
A wildfire is any uncontrolled fire in combustible vegetation that occurs in the countryside or a wilderness area. Other names such as brush fire, bushfire, forest fire, desert fire, grass fire, hill fire, squirrel fire, vegetation fire, veldfire, and wilkjjofire may be used to describe the same...

s. Other convective severe weather may or may not be associated with thunderstorms and include heavy rain
Rain
Rain is liquid precipitation, as opposed to non-liquid kinds of precipitation such as snow, hail and sleet. Rain requires the presence of a thick layer of the atmosphere to have temperatures above the melting point of water near and above the Earth's surface...

 or snow
Snow
Snow is a form of precipitation within the Earth's atmosphere 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...

 showers, hail
Hail
Hail is a form of solid precipitation. It consists of balls or irregular lumps of ice, each of which is referred to as a hail stone. Hail stones on Earth consist mostly of water ice and measure between and in diameter, with the larger stones coming from severe thunderstorms...

, strong wind shear
Wind shear
Wind shear, sometimes referred to as windshear or wind gradient, is a difference in wind speed and direction over a relatively short distance in the atmosphere...

, downburst
Downburst
A downburst is created by an area of significantly rain-cooled air that, after reaching ground level, spreads out in all directions producing strong winds. Unlike winds in a tornado, winds in a downburst are directed outwards from the point where it hits land or water...

s, and tornado
Tornado
A tornado is a violent, dangerous, rotating column of air that is in contact with both the surface of the earth and a cumulonimbus cloud or, in rare cases, the base of a cumulus cloud. They are often referred to as a twister or a cyclone, although the word cyclone is used in meteorology in a wider...

es.

In general, stratiform-category clouds form as the result of non-convective lift of relatively stable air, especially along slow-moving warm fronts, around areas of low pressure, and sometimes along stable slow moving cold fronts. In general, precipitation is steady and widespread, with intensity varying from light to heavy according to the thickness of the stratiform layer as determined by moisture content of the air and the intensity of the weather system creating the clouds and weather. Low stratiform clouds can also form in precipitation below the main frontal cloud deck where the colder air is trapped under the warmer airmass being forced above by the front. Non-frontal low stratiform cloud can form when advection fog is lifted above surface level during breezy conditions.

Cirriform-category clouds form mostly at high altitudes along the very leading edges of a frontal and/or low-pressure weather disturbance and often along the fringes of its other borders. In general, they are non-convective but occasionally acquire a tufted or turreted appearance caused by small scale high-altitude convection. These high clouds do not produce precipitation as such but can merge and thicken into lower stratiform layers that do.

Families and cross-classification into genera



The individual genus types result from the physical categories being cross-classified by height range family within the troposphere. These include family A (high), family B (middle), family C (low), family D1 (moderate vertical with low to middle bases), and family D2 (towering vertical with low to middle bases). The family designation for a particular genus is determined by the base height of the cloud and its vertical extent. The base height range for each family varies depending on the latitudinal geographical zone
Geographical zone
The five main latitude regions of the Earth's surface comprise geographical zones, divided by the major circles of latitude. The differences between them relate to climate, and the behaviour of the Sun...

.

Families A and B: All Cirriform-category clouds are classified as high range family A and thus constitute a single genus cirrus
Cirrus cloud
Cirrus clouds are atmospheric clouds generally characterized by thin, wispy strands, giving them their name from the Latin word cirrus meaning a ringlet or curling lock of hair...

 (Ci). Cumuliform and stratiform-category clouds in the high-altitude family carry the prefix cirro, yielding the respective genus names cirrocumulus (Cc) and cirrostratus (Cs). Similar genera in the middle-range family B are prefixed by alto, yielding the genus names altocumulus (Ac) and altostratus (As).

Families C and D1: Any cumuliform or stratiform genus in these two families either has no prefix or carries one that refers to a characteristic other than altitude. The two non-prefixed genera are non-convective low stratus
Stratus cloud
A 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...

 (St: family C),and free-convective moderate vertical cumulus
Cumulus cloud
Cumulus clouds are a type of cloud with noticeable vertical development and clearly defined edges. Cumulus means "heap" or "pile" in Latin. They are often described as "puffy" or "cotton-like" in appearance. Cumulus clouds may appear alone, in lines, or in clusters...

 (Cu: family D1). One prefixed cloud in this group is stratocumulus (Sc), a limited convection genus of the low-altitude family C that has some stratiform characteristics (as do the middle- and high-based genera altocumulus and cirrocumulus, the genus names of which exclude strato to avoid double-prefixing). The other prefixed cloud is nimbostratus (Ns), a non-convective genus of the moderate vertical family D1 that has some vertical extent and whose prefix refers to its ability to produce significant precipitation.

Family D2: This family comprises large towering free-convective clouds that typically occupy all altitude ranges and, therefore, also carry no height related prefixes. They comprise the genus cumulonimbus (Cb) and the cumulus species cumulus congestus (Cu con), which is also known informally as towering cumulus
Towering cumulus
Towering cumulus clouds can be based in the low or middle height ranges and achieve considerable vertical development in areas of deep, moist convection. They are an intermediate stage between cumulus mediocris and cumulonimbus...

 (Tcu). Under conditions of very low humidity, free-convective clouds may form above the low-altitude range and, therefore, be found only at middle- and high-tropospheric altitudes. In the modern system of cloud nomenclature, cumulonimbus is something of an anomaly. The cumuliform-category designation appears in the prefix rather than the root, which refers instead to the cloud's ability to produce storms and heavy precipitation. This apparent reversal of prefix and root is a carry-over from the nineteenth century, when nimbus was the root word for all precipitating clouds.

Major precipitation clouds: Although they do not comprise a family as such, cloud genera with nimbo or nimbus in their names are the principal bearers of precipitation. Although nimbostratus initially forms in the middle height range, it can be classified as moderate vertical because it achieves considerable thickness despite not being a convective cloud like cumulonimbus, the other main precipitating cloud genus. Frontal lift can push the top of a nimbostratus deck into the high-altitude range while precipitation drags the base down to low altitudes. The World Meteorological Organization
World Meteorological Organization
The World Meteorological Organization is an intergovernmental organization with a membership of 189 Member States and Territories. It originated from the International Meteorological Organization , which was founded in 1873...

 (WMO) classifies nimbostratus as a middle cloud whose base typically thickens down into the low altitude range during precipitation.

Species


Genus types are subdivided into species that indicate specific structural details. However, because these latter types are not always restricted by height range, some species can be common to several genera that are differentiated mainly by altitude. The best examples of these are the species stratiformis, lenticularis, and castellanus, which are common to cumuliform genera of limited convection in the high-, middle-, and low-height ranges (cirrocumulus, altocumulus, and stratocumulus, respectively). Stratiformis species normally occur in extensive sheets or in smaller patches with only minimal convective activity. Lenticularis species tend to have lens-like shapes tapered at the ends. They are most commonly seen as orographic mountain-wave clouds, but can occur anywhere in the troposphere where there is strong wind shear. Castellanus structures, which resemble the turrets of a castle when viewed from the side, can also be found in convective patches of cirrus, as can the more detached tufted floccus species, which are common to cirrus, cirrocumulus, and altocumulus. However, floccus is not associated with stratocumulus in the lower levels where local airmass instability tends to produce clouds of the more freely convective cumulus and cumulonimbus genera whose species are mainly indicators of degrees of vertical development.

Cirrus clouds have several additional species unique to the wispy structures of this genus, which include uncinus, filaments with upturned hooks, and spissatus, filaments that merge into dense patches. One exception is the species fibratus, which also occurs with cirrostratus that is transitional to or from cirrus. Cirrostratus at its most characteristic tends to be mostly of the species nebulosus, which creates a rather diffuse appearance lacking in structural detail. All altostratus and nimbostratus clouds share this physical appearance without significant variation or deviation and, therefore, do not need to be subdivided into species. Low continuous stratus is also of the species nebulosus except when broken up into ragged sheets of stratus fractus. This latter fractus species also occurs with ragged cumulus.

Varieties


Genus and species types are further subdivided into varieties; some of which are determined by the opacities of particular low and middle cloud structures (translucidus, opacus, and perlucidus; the last of which is opaque with translucent breaks). By implication rather than formal designation, all family A high clouds are translucidus. On the converse, all clouds with at least some significant vertical extent, including moderate vertical family D1 nimbostratus and cumulus, are opacus, as are the towering vertical family D2 clouds, cumulonimbus and cumulus congestus.

Other varieties are determined by the arrangements of the cloud structures into particular patterns that are discernable by a surface-based observer (cloud fields usually being visible only from a significant altitude above the formations). The variety undulatus (having a wavy undulating base) is common to all high, middle, and low genera except those with significant vertical extent. Another common variety, duplicatus (closely spaced layers of the same genus, one above the other), is found with all the same genera except cirrocumulus. The variety radiatus is associated with cloud rows of a particular genus that appear to converge at the horizon and is seen mostly with cirrus, altocumulus, altostratus, stratocumulus, and cumulus.

Intortus and vertebratus varieties occur only with the genus cirrus and are, respectively, filaments twisted into irregular shapes and those that are arranged in fishbone patterns. Probably the most uncommonly seen is the variety lacunosus, caused by localized downdrafts that punch circular holes into high, middle, and/or low cumuliform cloud layers of limited convection.

Supplementary features



One group of supplementary features are not actual cloud formations but precipitation that falls when water droplets that make up visible clouds have grown too heavy to remain aloft. Virga is a feature seen with clouds producing precipitation that evaporates before reaching the ground, these being of the genera cirrocumulus, altocumulus, altostratus, nimbostratus, stratocumulus, cumulus, and cumulonimbus. When the precipitation reaches the ground without completely evaporating, it is designated as the feature praecipitatio. This normally occurs with family B altostratus opacus, which can produce widespread but usually light precipitation, and with clouds of the thicker moderate vertical family D1. Of the latter family, cumulus mediocris produces only isolated light showers, while nimbostratus is capable of heavier more extensive precipitation. Towering vertical clouds of the family D2 have the greatest ability to produce intense precipitation events, but these tend to be localized unless organized along fast-moving cold fronts. Showers of moderate to heavy intensity can fall from cumulus congestus clouds. Cumulonimbus, the largest of all cloud genera, has the capacity to produce very heavy showers. Low stratus clouds usually produce only light precipitation, but this always occurs as the feature praecipitatio due to the fact this cloud genus lies too close to the ground to allow for the formation of virga. The heavier precipitating clouds, nimbostratus, towering cumulus (cumulus congestus), and cumulonimbus, also typically see the formation in precipitation of the pannus feature, low ragged clouds of the genera and species cumulus fractus and/or stratus fractus.


Another group of supplementary features comprises cloud formations that are associated mainly with cumuliform clouds of free convection. Pileus is a cap cloud that can form over a cumulonimbus or large cumulus cloud, whereas a velum feature is a thin horizontal sheet that sometime forms around the middle or in front of the parent cloud. A tuba feature is a cloud column that may hang from the bottom of a cumulus or cumulonimbus. An arcus feature is a roll or shelf cloud that forms along the leading edge of a squall line or thunderstorm outflow. Some arcus clouds form as a consequence of interactions with specific geographical features. Perhaps the strangest geographically specific arcus cloud in the world is the Morning Glory
Morning glory cloud
The Morning Glory cloud is a rare meteorological phenomenon occasionally observed in different locations around the world. The southern part of Northern Australia's Gulf of Carpentaria is the only known location where it can be predicted and observed on a more or less regular basis. The settlement...

, a rolling
Arcus cloud
An arcus cloud is a low, horizontal cloud formation. Roll clouds and shelf clouds are the two types of arcus clouds. A shelf cloud is usually associated with the leading edge of thunderstorm outflow; roll clouds are usually formed by outflows of cold air from sea breezes or cold fronts in the...

 cylindrical cloud that appears unpredictably over the Gulf of Carpentaria
Gulf of Carpentaria
The Gulf of Carpentaria is a large, shallow sea enclosed on three sides by northern Australia and bounded on the north by the Arafura Sea...

 in Northern Australia
Northern Australia
The term northern Australia is generally known to include two State and Territories, being Queensland and the Northern Territory . The part of Western Australia north of latitude 26° south—a definition widely used in law and State government policy—is also usually included...

. Associated with a powerful "ripple" in the atmosphere, the cloud may be "surfed" in glider
Glider (sailplane)
A glider or sailplane is a type of glider aircraft used in the sport of gliding. Some gliders, known as motor gliders are used for gliding and soaring as well, but have engines which can, in some cases, be used for take-off or for extending a flight...

 aircraft. Mamma (sometimes known informally as mammatus) form on the bases of clouds as downward-facing bubble-like protuberances caused by localized downdrafts within the cloud. The best-known is cumulonimbus with mammatus, but the mamma feature is also seen occasionally with cirrus, cirrocumulus, altocumulus, altostratus, and stratocumulus. Incus is the most type-specific supplementary feature, seen only with cumulonimbus of the species capillatus. A cumulonimbus incus
Cumulonimbus incus
A cumulonimbus incus is a cumulonimbus cloud which has reached the level of stratospheric stability and has formed the characteristic flat, anvil-top shape...

 cloud top is one that has spread out into a clear anvil shape as a result of rising air currents hitting the stability layer at the tropopause
Tropopause
The tropopause is the atmospheric boundary between the troposphere and the stratosphere.-Definition:Going upward from the surface, it is the point where air ceases to cool with height, and becomes almost completely dry...

 where the air no longer continues to get colder with increasing altitude.

Stratocumulus fields


Stratocumulus clouds can be organized into 'fields' that take on certain specially classified shapes and characteristics. In general, these fields are more discernable from high altitudes than from ground level. They can often be found in the following forms:
  • Actinoform
    Actinoform cloud
    An actinoform or actiniform cloud is a collection of marine low clouds that takes a distinct shape. They are named after the Greek word for "ray" due to their radial structure. Actinoform clouds can spread out over across and thus cannot be easily seen with the naked eye...

    , which resembles a leaf or a spoked wheel.
  • Closed cell, which is cloudy in the center and clear on the edges, similar to a filled honeycomb
    Honeycomb
    A honeycomb is a mass of hexagonal waxcells built by honey bees in their nests to contain their larvae and stores of honey and pollen.Beekeepers may remove the entire honeycomb to harvest honey...

    .
  • Open cell, which resembles a honeycomb, with clouds around the edges and clear, open space in the middle.

High (Family A)



High clouds form between 10000 foot in the polar region
Polar region
Earth's polar regions are the areas of the globe surrounding the poles also known as frigid zones. The North Pole and South Pole being the centers, these regions are dominated by the polar ice caps, resting respectively on the Arctic Ocean and the continent of Antarctica...

s, 16500 foot in the temperate regions and 20000 foot in the tropical region
Tropics
The tropics is a region of the Earth surrounding the Equator. It is limited in latitude by the Tropic of Cancer in the northern hemisphere at approximately  N and the Tropic of Capricorn in the southern hemisphere at  S; these latitudes correspond to the axial tilt of the Earth...

. It is the only height range family that includes genera from all three physical categories.

Family A includes:
  • Genus Cirrus
    Cirrus cloud
    Cirrus clouds are atmospheric clouds generally characterized by thin, wispy strands, giving them their name from the Latin word cirrus meaning a ringlet or curling lock of hair...

     (Ci): Fibrous wisps of delicate white ice crystal cloud that show up clearly against the blue sky.

Cirrus clouds are generally non-convective except castellanus and floccus species. They often form along a high altitude jetstream
JetStream
JetStream and Mobile JetStream are two former brand names used by Telecom New Zealand to market its retail and resale ADSL-based fixed line and CDMA2000-based 3G wireless Internet access offerings respectively...

 and at the very leading edge of a frontal or low pressure disturbance where they may merge into cirrostratus.
    • Species Cirrus fibratus (Ci fib): Fibrous cirrus with no tufts or hooks.
    • Species Cirrus uncinus  (Ci unc): Hooked cirrus filaments.
    • Species Cirrus spissatus (Ci spi): Patchy dense cirrus.
    • Species Cirrus castellanus (Ci cas): Partly turreted cirrus.
    • Species Cirrus floccus (Ci flo): Partly tufted cirrus.
  • Genus Cirrocumulus
    Cirrocumulus cloud
    Cirrocumulus clouds are one of the three main types of high-altitude clouds, which also includes cirrus clouds and cirrostratus clouds. They usually occur at an altitude of to . Like other cumulus clouds, cirrocumulus clouds signify convection. Unlike other cirrus clouds, cirrocumulus include a...

     (Cc): A cloud layer of limited convection composed of ice crystals and/or supercooled water droplets appearing as small white rounded masses or flakes in groups or lines with ripples like sand on a beach. They occasionally form alongside cirrus and/or cirrostratus clouds at the very leading edge of an active weather system.
    • Species Cirrocumulus stratiformis (Cc str): Sheets or relatively flat patches of cirrocumulus.
    • Species Cirrocumulus lenticularis (Cc len): Lens-shaped cirrocumulus.
    • Species Cirrocumulus castellanus (Cc cas): Turreted cirrocumulus.
    • Species Cirrocumulus floccus (Cc flo): Tufted cirrocumulus.
  • Genus Cirrostratus
    Cirrostratus cloud
    Cirrostratus clouds are thin, generally uniform clouds, composed of ice-crystals. They are difficult to detect and if capable of forming halos the cloud takes the form of thin cirrostratus nebulosus. The cloud has a fibrous texture with no haloes if it is thicker cirrostratus fibratus. On the...

     (Cs): A thin non-convective ice crystal veil that typically gives rise to halos caused by refraction of the sun's rays. The sun and moon are visible in clear outline. Cirrostratus typically thickens into altostratus ahead of a warm front or low-pressure area.
    • Species Cirrostratus fibratus (Cs fib): Fibrous cirrostratus less detached than cirrus.
    • Species Cirrostratus nebulosus (Cs neb): A featureless veil of cirrostratus.

Middle (Family B)



Middle clouds tend to form at 6500 ft (1,981.2 m) but may form at heights up to 13000 ft (3,962.4 m), 23000 ft (7,010.4 m) or 25000 ft (7,620 m) depending on the latitudinal region. In general, the warmer the climate the higher the cloud base. Family B usually comprises one cumuliform and one stratiform-category genus.

Family B includes:
  • Genus Altocumulus
    Altocumulus cloud
    Altocumulus is a cloud belonging to a class characterized by globular masses or rolls in layers or patches, the individual elements being larger and darker than those of cirrocumulus and smaller than those of stratocumulus. Like other cumulus clouds, altocumulus signifies convection...

     (Ac): A cloud layer of limited convection usually in the form of irregular patches or rounded masses in groups, lines, or waves. High altocumulus may resemble cirrocumulus but is usually thicker and composed of water droplets so that the bases show at least some light-grey shading. Opaque altocumulus associated with a weak frontal or low-pressure disturbance can produce very light intermittent precipitation.
    • Species Altocumulus stratiformis (Ac str): Sheets or relatively flat patches of altocumulus.
    • Species Altocumulus lenticularis (Ac len): Lens-shaped altocumulus.
    • Species Altocumulus castellanus  (Ac cas): Turreted altocumulus.
    • Species Altocumulus floccus (Ac flo): Tufted altocumulus.
  • Genus Altostratus
    Altostratus cloud
    Altostratus is a cloud belonging to a class characterized by a generally uniform gray to bluish-gray sheet or layer, lighter in color than nimbostratus and darker than cirrostratus. The sun can be seen through thin altostratus, but thicker layers can be quite opaque...

     (As): An opaque or translucent non-convective veil of grey/blue-grey cloud that often forms along warm fronts and around low-pressure areas where it may thicken into nimbostratus. Altostratus is usually composed of water droplets but may be mixed with ice crystals at higher altitudes. Widespread opaque altostratus can produce light continuous or intermittent precipitation.
    • Altostratus is not subdivided into species.

Low (Family C)



Low clouds are found from near surface up to 6500 ft (1,981.2 m). Family C also typically includes one cumuliform and one stratiform-category genus. When low stratiform clouds contact the ground, they are called fog
Fog
Fog is a collection of water droplets or ice crystals suspended in the air at or near the Earth's surface. While fog is a type of stratus cloud, the term "fog" is typically distinguished from the more generic term "cloud" in that fog is low-lying, and the moisture in the fog is often generated...

, although radiation and advection types of fog do not form from stratus layers.

Family C includes:
  • Genus Stratocumulus
    Stratocumulus cloud
    A Stratocumulus cloud belongs to a class of clouds characterized by large dark, rounded masses, usually in groups, lines, or waves, the individual elements being larger than those in altocumuli, and the whole being at a lower altitude, usually below 2,400 m...

     (Sc): A cloud layer of limited convection usually in the form of irregular patches or rounded masses similar to altocumulus but having larger elements with deeper-gray shading. Opaque stratocumulus associated with a weak frontal or low-pressure distrubance can produce very light intermittent precipitation. This cloud often forms under a precipitating deck of altostratus or high-based nimbostratus associated with a well-developed warm front, slow-moving cold front, or low-pressure area. This can create the illusion of continuous precipitation of more than very light intensity falling from stratocumulus.
    • Species Stratocumulus stratiformis (Sc str): Sheets or relatively flat patches of stratocumulus.
    • Species Stratocumulus lenticularis (Sc len): Lens-shaped stratocumulus.
    • Species Stratocumulus castellanus (Sc cas): Turreted stratocumulus.
  • Genus Stratus
    Stratus cloud
    A 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...

     (St): A uniform layer of non-convective cloud resembling fog but not resting on the ground.
    • Species Stratus nebulosus (St neb): A featureless veil of stratus sometimes producing light drizzle.
    • Species Stratus fractus  (St fra): A ragged broken up sheet of stratus that often forms in precipitation falling from a higher cloud deck. This species may also result from a continuous sheet of stratus becoming broken up by the wind.

Moderate vertical (Family D1)



Family D1 clouds have low to middle bases anywhere from near surface to about 10000 ft (3,048 m) and, therefore, do not fit very well into the conventional height ranges of low, middle, and high. This group continues the pattern of comprising one cumuliform and one stratiform-category genus. Cumulus usually forms in the low-altitude range, but bases may rise into the lower part of the middle range during conditions of very low relative humidity. Nimbostratus normally forms from altostratus in the middle-altitude range and achieves vertical extent when the base subsides into the low range during precipitation. Some methods of cloud-height classification reserve the term vertical for upward-growing free-convective cumuliform clouds. Downward-growing nimbostratus is then classified as low or middle, even when it becomes very thick as a result of this process - which is often augmented by frontal lift causing non-convective upward growth as well. Some authorities do not use a vertical family designation at all, and therefore also include free-convective cumulus types with the family of low clouds.

Family D1 includes:
  • Genus Cumulus
    Cumulus cloud
    Cumulus clouds are a type of cloud with noticeable vertical development and clearly defined edges. Cumulus means "heap" or "pile" in Latin. They are often described as "puffy" or "cotton-like" in appearance. Cumulus clouds may appear alone, in lines, or in clusters...

     (Cu): Clouds of free convection with clear-cut flat bases and domed tops. Towering cumulus (cumulus congestus) are usually classified as family D2 clouds of considerable vertical development.
    • Species Cumulus fractus  (Cu fra): Cumulus clouds broken up into ragged and changing fragments.
    • Species Cumulus humilis
      Cumulus humilis cloud
      Cumulus humilis is a low to middle cloud with small vertical extent that is commonly referred to as "fair weather cumulus". In hot countries and over mountainous terrain these clouds occur at up to 6000 meters altitude, though elsewhere they are typically found lower.They are formed by rising warm...

       (Cu hum): Small cumulus clouds usually with just a light-grey shading underneath.
    • Species Cumulus mediocris
      Cumulus mediocris cloud
      Cumulus mediocris is a low to middle level cloud with some vertical extent of the genus cumulus, larger in vertical development than Cumulus humilis. It may or may not show the cauliflower form characteristic of cumulus clouds...

       (Cu med): Cumulus clouds of moderate size with medium-grey shading underneath. Cumulus mediocris can produce scattered showers of light intensity.
  • Genus Nimbostratus
    Nimbostratus cloud
    A Nimbostratus cloud is characterized by a formless cloud layer that is almost uniformly dark gray. "Nimbo" is from the Latin word "nimbus", which denotes precipitation. It is generally a stratiform cloud of moderate vertical development that produces precipitation, developing cloud bases between...

     (Ns): A diffuse dark-grey non-convective layer that looks feebly illuminated from the inside. It normally forms from altostratus along warm fronts and around low-pressure areas and produces widespread steady precipitation that can reach moderate or heavy intensity.
    • Nimbostratus is not subdivided into species.

Towering vertical (Family D2)



These clouds can have strong vertical currents and rise far above their bases, which form anywhere in the low to lower-middle altitude range from near surface to about 10000 ft (3,048 m). Like smaller cumuliform clouds in family D1, these towering giants usually form in the low-altitude range at first, but the bases can rise into the middle range when the moisture content of the air is very low. Unlike families A through D1 that each include a cumuliform and stratiform-category genus, the family of towering clouds has instead one distinct cumuliform-category genus, cumulonimbus, and one species of cumulus, a genus otherwise considered a cloud of moderate vertical development. By definition, even very thick stratiform clouds cannot have towering vertical extent or structure, although they may be accompanied by embedded towering cumuliform types. As with the moderate vertical clouds, some authorities do not recognize a separate family of towering vertical types, and, instead, classify them as low family C. Others designate vertical clouds separately from low, middle, and high, but consider moderate and towering vertical types to be a single family. However, the International Civil Aviation Organization
International Civil Aviation Organization
The International Civil Aviation Organization , pronounced , , is a specialized agency of the United Nations. It codifies the principles and techniques of international air navigation and fosters the planning and development of international air transport to ensure safe and orderly growth...

 (ICAO) distinguishes towering vertical clouds by specifying that these very large family D2 cumuliform types must be identified by genus names or standard abbreviations in all aviation observations (METARS) and forecasts (TAFS) to warn pilots of possible severe weather and turbulance.

Family D2 includes:
  • Genus Cumulonimbus
    Cumulonimbus cloud
    Cumulonimbus is a towering vertical cloud that is very tall, dense, and involved in thunderstorms and other inclement weather. Cumulonimbus originates from Latin: Cumulus "Heap" and nimbus "rain". It is a result of atmospheric instability. These clouds can form alone, in clusters, or along a cold...

     (Cb): Heavy towering masses of free convective cloud with dark-grey to nearly black bases that are associated with thunderstorms and showers. Thunderstorms can produce a range of severe weather that includes hail, tornadoes, a variety of other localized strong wind events, several types of lightning, and local very heavy downpours of rain that can cause flash flood
    Flash flood
    A flash flood is a rapid flooding of geomorphic low-lying areas—washes, rivers, dry lakes and basins. It may be caused by heavy rain associated with a storm, hurricane, or tropical storm or meltwater from ice or snow flowing over ice sheets or snowfields...

    s, although lightning is the only one of these that requires a thunderstorm to be taking place. In general, cumulonimbus require moisture, an unstable air mass, and a lifting force (heat) in order to form. Cumulonimbus typically go through three stages: the developing stage, the mature stage, and the dissipation stage. The average thunderstorm has a 24 km (14.9 mi) diameter. Depending on the conditions present in the atmosphere, these three stages take an average of 30 minutes to go through.
    • Species Cumulonimbus calvus
      Cumulonimbus calvus
      Cumulonimbus calvus is a moderately tall cumulonimbus cloud which is capable of precipitation, but has not yet reached the height where it forms into a cumulonimbus capillatus or cumulonimbus incus...

       (Cb cal): Cumulonimbus clouds with very high clear-cut domed tops similar to towering cumulus.
    • Species Cumulonimbus capillatus (Cb cap): Cumulonimbus clouds with very high tops that have become fibrous due to the presence of ice crystals.
  • Genus Cumulus
    Cumulus cloud
    Cumulus clouds are a type of cloud with noticeable vertical development and clearly defined edges. Cumulus means "heap" or "pile" in Latin. They are often described as "puffy" or "cotton-like" in appearance. Cumulus clouds may appear alone, in lines, or in clusters...

     (Cu)
    • Species Cumulus congestus (WMO: Cu Con/ICAO: TCu): Towering cumulus
      Towering cumulus
      Towering cumulus clouds can be based in the low or middle height ranges and achieve considerable vertical development in areas of deep, moist convection. They are an intermediate stage between cumulus mediocris and cumulonimbus...

       clouds of great vertical size, usually with dark-grey bases, and capable of producing severe turbulence and showers of moderate to heavy intensity.
      • Pyrocumulus
        Pyrocumulus cloud
        A pyrocumulus, or literally fire cloud, is a dense cumuliform cloud associated with fire or volcanic activity.A pyrocumulus is similar dynamically in some ways to a firestorm, and the two phenomena may occur in conjunction with each other...

         (No official abbreviation): Cumulus clouds associated with volcanic eruptions and large-scale fires. Pyrocumulus is not recognised by the WMO as a distinct genus or species, but is, in essence, cumulus congestus formed under special circumstances that can also cause severe turbulance.


Above the troposphere


A few relatively uncommon clouds can be found above the troposphere, where moisture is very scarce. These include polar mesospheric noctilucent
Noctilucent cloud
Night clouds or Noctilucent clouds are tenuous cloud-like phenomena that are the "ragged-edge" of a much brighter and pervasive polar cloud layer called polar mesospheric clouds in the upper atmosphere, visible in a deep twilight. They are made of crystals of water ice. The name means roughly night...

 clouds and nacreous polar stratospheric cloud
Polar stratospheric cloud
Polar stratospheric clouds or PSCs, also known as nacreous clouds , are clouds in the winter polar stratosphere at altitudes of 15,000–25,000 meters...

s. They are composed mostly of ice crystals and occur at high latitudes, mostly within 40 degrees of the poles in the mesosphere
Mesosphere
The mesosphere is the layer of the Earth's atmosphere that is directly above the stratosphere and directly below the thermosphere. In the mesosphere temperature decreases with increasing height. The upper boundary of the mesosphere is the mesopause, which can be the coldest naturally occurring...

 and stratosphere
Stratosphere
The stratosphere is the second major layer of Earth's atmosphere, just above the troposphere, and below the mesosphere. It is stratified in temperature, with warmer layers higher up and cooler layers farther down. This is in contrast to the troposphere near the Earth's surface, which is cooler...

, respectively. Most clouds above the troposphere have a wispy or fibrous appearance and can be mistakenly identified as high-tropospheric cirrus clouds.

Polar stratospheric


Nacreous clouds occur in the stratosphere most typically at altitudes of 15,000–25,000 m (50,000–80,000 ft) during the winter
Winter
Winter is the coldest season of the year in temperate climates, between autumn and spring. At the winter solstice, the days are shortest and the nights are longest, with days lengthening as the season progresses after the solstice.-Meteorology:...

 when that part of the atmosphere is coldest and has the best chance of triggering condensation. Also known as mother of pearl clouds, they are typically very thin with a cirriform appearance. Nacreous clouds are sub-classified alpha-numerically based on chemical makeup rather than variations in physical appearance.
  • Type 1: Nacreous containing supercooled nitric acid and water droplets.


Subtypes
    • 1A: Nacreous containing crystals of water and nitric acid.
    • 1B: Also contains supercooled sulfuric acid in ternary solution.

  • Type 2: Nacreous consisting of water crystals only.

Polar mesospheric


The polar air in the mesosphere is colder during the summer
Summer
Summer is the warmest of the four temperate seasons, between spring and autumn. At the summer solstice, the days are longest and the nights are shortest, with day-length decreasing as the season progresses after the solstice...

 so it is mostly at this time of year that noctilucent clouds are seen. They can occasionally be seen illuminated by the sun during deep twilight
Twilight
Twilight is the time between dawn and sunrise or between sunset and dusk, during which sunlight scattering in the upper atmosphere illuminates the lower atmosphere, and the surface of the earth is neither completely lit nor completely dark. The sun itself is not directly visible because it is below...

 at ground level. Noctilucent clouds are the highest in the atmosphere and occur mostly at altitudes of 80 to 85 km (49.7 to 52.8 mi), in the mesosphere. An alpha-numeric sub-classification is used to identify variations in physical appearance.
  • Type 1: Very tenuous noctilucent resembling cirrus fibratus.

  • Type 2: Bands; noctilucent in the form of long streaks, often in groups or interwoven at small angles, similar to cirrus intortus.


Subtypes
    • 2A: Noctilucent streaks with diffuse, blurred edges.
    • 2B: Streaks with sharply defined edges.

  • Type 3: Billows; noctilucent in the form of short streaks that are clearly spaced and roughly parallel.


Subtypes
    • 3A: Noctilucent in the form of short, straight, narrow streaks.
    • 3B: Wave-like streaks similar to cirrus undulatus.

  • Type 4: Whirls; noctilucent in the form of partial or rarely complete rings with dark centers.


Subtypes
    • 4A: Noctilucent whirls of small angular radius having a similar appearance to surface water ripples.
    • 4B: Simple curve of medium angular radius with one or more bands.
    • 4C: Whirls with large-scale ring structure.

Coloration




The first recorded coloured cloud was seen by Nathan Ingleton in 1651, he wrote the event in his diary but the records were destroyed on 1666, in the Great Fire of London. The color of a cloud, as seen from Earth, tells much about what is going on inside the cloud. Dense, deep tropospheric clouds exhibit a high reflectance (70% to 95%) throughout the visible spectrum
Visible spectrum
The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or simply light. A typical human eye will respond to wavelengths from about 390 to 750 nm. In terms of...

. Tiny particles of water are densely packed and sunlight cannot penetrate far into the cloud before it is reflected out, giving a cloud its characteristic white color, especially when viewed from the top. Cloud droplets tend to scatter
Scatter
In ordinary English, to scatter is to distribute randomly. Scatter also has the following meanings:*In physics, scattering is the study of collisions, especially of waves and particles...

 light efficiently, so that the intensity of the solar radiation decreases with depth into the gases. As a result, the cloud base
Cloud base
This article refers to meteorology. For the airborne base of the TV series Captain Scarlet and the Mysterons, see Cloudbase.The cloud base is the lowest altitude of the visible portion of the cloud...

 can vary from a very light to very-dark-grey depending on the cloud's thickness and how much light
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...

 is being reflected or transmitted back to the observer. Thin clouds may look white or appear to have acquired the color of their environment
Natural environment
The natural environment encompasses all living and non-living things occurring naturally on Earth or some region thereof. It is an environment that encompasses the interaction of all living species....

 or background. High tropospheric and non-tropospheric clouds appear mostly white if composed entirely of ice crystals and/or supercooled water droplets.

As a tropospheric cloud matures, the dense water droplets may combine to produce larger droplets, which may combine to form droplets large enough to fall as rain
Rain
Rain is liquid precipitation, as opposed to non-liquid kinds of precipitation such as snow, hail and sleet. Rain requires the presence of a thick layer of the atmosphere to have temperatures above the melting point of water near and above the Earth's surface...

. By this process of accumulation, the space between droplets becomes increasingly larger, permitting light to penetrate farther into the cloud. If the cloud is sufficiently large and the droplets within are spaced far enough apart, it may be that a percentage of the light that enters the cloud is not reflected back out before it is absorbed. A simple example of this is one's being able to see farther in heavy rain than in heavy fog. This process of reflection
Reflection (physics)
Reflection is the change in direction of a wavefront at an interface between two differentmedia so that the wavefront returns into the medium from which it originated. Common examples include the reflection of light, sound and water waves...

/absorption
Absorption (electromagnetic radiation)
In physics, absorption of electromagnetic radiation is the way by which the energy of a photon is taken up by matter, typically the electrons of an atom. Thus, the electromagnetic energy is transformed to other forms of energy for example, to heat. The absorption of light during wave propagation is...

 is what causes the range of cloud color from white to black.

Other colors occur naturally in clouds. Bluish-grey is the result of light scattering within the cloud. In the visible spectrum, blue and green are at the short end of light's visible wavelengths, whereas red and yellow are at the long end. The short rays are more easily scattered by water droplets, and the long rays are more likely to be absorbed. The bluish color is evidence that such scattering is being produced by rain-size droplets in the cloud. A greenish tinge to a cloud is produced when sunlight is scattered by ice. A cumulonimbus cloud emitting green is a sign that it is a severe thunderstorm, capable of heavy rain, hail
Hail
Hail is a form of solid precipitation. It consists of balls or irregular lumps of ice, each of which is referred to as a hail stone. Hail stones on Earth consist mostly of water ice and measure between and in diameter, with the larger stones coming from severe thunderstorms...

, strong wind
Wind
Wind is the flow of gases on a large scale. On Earth, wind consists of the bulk movement of air. In outer space, solar wind is the movement of gases or charged particles from the sun through space, while planetary wind is the outgassing of light chemical elements from a planet's atmosphere into space...

s, and possible tornado
Tornado
A tornado is a violent, dangerous, rotating column of air that is in contact with both the surface of the earth and a cumulonimbus cloud or, in rare cases, the base of a cumulus cloud. They are often referred to as a twister or a cyclone, although the word cyclone is used in meteorology in a wider...

es. Yellowish clouds may occur in the late spring through early fall months during forest fire
Wildfire
A wildfire is any uncontrolled fire in combustible vegetation that occurs in the countryside or a wilderness area. Other names such as brush fire, bushfire, forest fire, desert fire, grass fire, hill fire, squirrel fire, vegetation fire, veldfire, and wilkjjofire may be used to describe the same...

 season. The yellow color is due to the presence of pollutants in the smoke. Yellowish clouds caused by the presence of nitrogen dioxide are sometimes seen in urban areas with high air pollution levels.

Red, orange, and pink clouds occur almost entirely at sunrise/sunset and are the result of the scattering of sunlight by the atmosphere. When the angle between the sun and the horizon is less than 10 percent, as it is just after sunrise or just prior to sunset, sunlight becomes too red due to refraction for any colors other than those with a reddish hue to be seen. The clouds do not become that color; they are reflecting long and unscattered rays of sunlight, which are predominant at those hours. The effect is much like if one were to shine a red spotlight on a white sheet. In combination with large, mature thunderheads, this can produce blood-red clouds. Clouds look darker in the near-infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...

 because water absorbs solar radiation at those wavelengths.

Effects on climate


The role of clouds in regulating weather
Weather
Weather is the state of the atmosphere, to the degree that it is hot or cold, wet or dry, calm or stormy, clear or cloudy. Most weather phenomena occur in the troposphere, just below the stratosphere. Weather refers, generally, to day-to-day temperature and precipitation activity, whereas climate...

 and climate
Climate
Climate encompasses the statistics of temperature, humidity, atmospheric pressure, wind, rainfall, atmospheric particle count and other meteorological elemental measurements in a given region over long periods...

 remains a leading source of uncertainty in projections of global warming
Global warming
Global warming refers to the rising average temperature of Earth's atmosphere and oceans and its projected continuation. In the last 100 years, Earth's average surface temperature increased by about with about two thirds of the increase occurring over just the last three decades...

. This uncertainty arises because of the delicate balance of processes related to clouds, spanning scales from millimeters to planetary. Hence, interactions between the large-scale (synoptic meteorology) and clouds becomes difficult to represent in global models. The complexity and diversity of clouds, as outlined above, adds to the problem. On the one hand, white-colored cloud tops promote cooling of Earth's surface by reflecting shortwave radiation from the Sun. Most of the sunlight that reaches the ground is absorbed, warming the surface, which emits radiation upward at longer, infrared, wavelengths. At these wavelengths, however, water in the clouds acts as an efficient absorber. The water reacts by radiating, also in the infrared, both upward and downward, and the downward radiation results in a net warming at the surface. This is analogous to the greenhouse effect of greenhouse gases and water vapor.

High clouds, such as cirrus, particularly show this duality with both shortwave albedo cooling and longwave greenhouse warming effects that nearly cancel or slightly favor net warming with increasing cloud cover. The shortwave effect is dominant with middle and low clouds like altocumulus and stratocumulus, which results in a net cooling with almost no longwave effect. As a consequence, much research has focused on the response of low clouds to a changing climate. Leading global models can produce quite different results, however, with some showing increasing low-level clouds and other showing decreases.

Global brightening


New research indicates a global brightening trend. The details are not fully understood, but much of the global dimming
Global dimming
Global dimming is the gradual reduction in the amount of global direct irradiance at the Earth's surface that was observed for several decades after the start of systematic measurements in the 1950s. The effect varies by location, but worldwide it has been estimated to be of the order of a 4%...

 (and subsequent reversal) is thought to be a consequence of changes in aerosol
Aerosol
Technically, an aerosol is a suspension of fine solid particles or liquid droplets in a gas. Examples are clouds, and air pollution such as smog and smoke. In general conversation, aerosol usually refers to an aerosol spray can or the output of such a can...

 loading in the atmosphere, especially sulfur-based aerosol associated with biomass burning and urban pollution. Changes in aerosol burden can have indirect effects on clouds by changing the droplet size distribution or the lifetime and precipitation characteristics of clouds.

Rainmaking bacteria


Bioprecipitation
Bioprecipitation
Bioprecipitation is the concept of rain-making bacteria and was proposed by David Sands from Montana State University before 1983. The formation of ice in clouds is required for snow and most rainfall. Dust and soot particles can serve as ice nuclei, but biological ice nuclei are capable of...

, the concept of rain-making bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

, was proposed by David Sands from Montana State University
Montana State University - Bozeman
Montana State University – Bozeman is a public university located in Bozeman, Montana. It is the state's land-grant university and primary campus in the Montana State University System, which is part of the Montana University System...

. Such microbes - called ice nucleators
Ice nucleus
An ice nucleus is a particle which acts as the nucleus for the formation of an ice crystal in the atmosphere.The presence of ice nuclei increase the temperature that ice will form in the atmosphere from around −42°C to about −10°C...

 - are found in rain, snow, and hail throughout the world. These bacteria may be part of a constant feedback
Feedback
Feedback describes the situation when output from an event or phenomenon in the past will influence an occurrence or occurrences of the same Feedback describes the situation when output from (or information about the result of) an event or phenomenon in the past will influence an occurrence or...

 between terrestrial ecosystem
Ecosystem
An ecosystem is a biological environment consisting of all the organisms living in a particular area, as well as all the nonliving , physical components of the environment with which the organisms interact, such as air, soil, water and sunlight....

s and clouds and may even have evolved
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...

 the ability to promote rainstorms as a means of dispersal
Biological dispersal
Biological dispersal refers to species movement away from an existing population or away from the parent organism. Through simply moving from one habitat patch to another, the dispersal of an individual has consequences not only for individual fitness, but also for population dynamics, population...

. They may rely on the rainfall to spread to new habitat
Habitat
* Habitat , a place where a species lives and grows*Human habitat, a place where humans live, work or play** Space habitat, a space station intended as a permanent settlement...

s, much as some plants rely on windblown pollen
Pollen
Pollen is a fine to coarse powder containing the microgametophytes of seed plants, which produce the male gametes . Pollen grains have a hard coat that protects the sperm cells during the process of their movement from the stamens to the pistil of flowering plants or from the male cone to the...

 grains.

Extraterrestrial



Within our Solar System, any planet or moon with an atmosphere also has clouds. Venus
Venus
Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows...

's thick clouds are composed of sulfur dioxide
Sulfur dioxide
Sulfur dioxide is the chemical compound with the formula . It is released by volcanoes and in various industrial processes. Since coal and petroleum often contain sulfur compounds, their combustion generates sulfur dioxide unless the sulfur compounds are removed before burning the fuel...

. Mars
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...

 has high, thin clouds of water ice. Both Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...

 and Saturn
Saturn
Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...

 have an outer cloud deck composed of ammonia clouds, an intermediate deck of ammonium hydrosulfide clouds and an inner deck of water clouds. Saturn's moon Titan
Titan (moon)
Titan , or Saturn VI, is the largest moon of Saturn, the only natural satellite known to have a dense atmosphere, and the only object other than Earth for which clear evidence of stable bodies of surface liquid has been found....

 has clouds believed to be composed largely of methane. The Cassini–Huygens Saturn mission uncovered evidence of a fluid cycle on Titan, including lakes near the poles and fluvial channels on the surface of the moon. Uranus
Uranus
Uranus is the seventh planet from the Sun. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. It is named after the ancient Greek deity of the sky Uranus , the father of Cronus and grandfather of Zeus...

 and Neptune
Neptune
Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times...

 have cloudy atmospheres dominated by water vapor and methane gas.

See also


  • Atmospheric Radiation Measurement
    Atmospheric Radiation Measurement
    The United States Department of Energy's Atmospheric Radiation Measurement Program was created in 1989 to develop several highly instrumented ground stations to study cloud formation processes and their influence on radiative transfer...

     (ARM) (in the US)
  • Cloud albedo
    Cloud albedo
    Cloud albedo is a measure of the albedo of a cloud - higher values mean that the cloud reflects more solar radiation, or more radiation is transmitted....

  • Cloud Appreciation Society
    Cloud Appreciation Society
    The Cloud Appreciation Society is a society founded by Gavin Pretor-Pinney from the United Kingdom in January 2005. The society aims to foster understanding and appreciation of clouds, and has over 22,000 members worldwide from 83 different countries, as of July 2010.Yahoo named the society's...

  • Cloud forcing
    Cloud forcing
    Cloud forcing is, in meteorology, the difference between the radiation budget components for average cloud conditions and cloud-free conditions...

  • Cloud seeding
    Cloud seeding
    Cloud seeding, a form of intentional 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...

  • Cloudscape (art)
    Cloudscape (art)
    In art, a cloudscape is the depiction of a view of clouds or the sky. Usually, as in the examples seen here, the clouds are depicted as viewed from the earth, often including just enough of a landscape to suggest scale, orientation, weather conditions, and distance...

  • Cloudscape photography
  • Coalescence
    Coalescence (meteorology)
    Coalescence is the process by which two or more droplets, bubbles or particles merge during contact to form a single daughter droplet, bubble or particle. It can take place in many processes, ranging from meteorology to astrophysics. For example, it is both inve formation of raindrops as well as...

  • Extraterrestrial skies
    Extraterrestrial skies
    The 'sky' of a world refers to the view of outer space from its surface. This view varies from world to world for many reasons. The most important factor in the appearance of a world's sky is the world's atmosphere, or the lack thereof. Depending on the atmosphere's density and chemical...

  • Flight ceiling
  • Fractus cloud
    Fractus cloud
    Fractus clouds are small, ragged cloud fragments that are usually found under an ambient cloud base. They form or have broken off from a larger cloud, and are generally sheared by strong winds, giving them a jagged, shredded appearance. Fractus have irregular patterns, appearing much like torn...

  • List of cloud types
  • Mist
    Mist
    Mist 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...

  • Mushroom cloud
    Mushroom cloud
    A mushroom cloud is a distinctive pyrocumulus mushroom-shaped cloud of condensed water vapor or debris resulting from a very large explosion. They are most commonly associated with nuclear explosions, but any sufficiently large blast will produce the same sort of effect. They can be caused by...

  • Pileus (meteorology)
    Pileus (meteorology)
    A pileus , also called scarf cloud or cap cloud, is a small, horizontal cloud that can appear above a cumulus or cumulonimbus cloud, giving the parent cloud a characteristic "hoodlike" appearance. Pilei tend to change shape rapidly. They are formed by strong updrafts acting upon moist air at lower...

  • Undulatus asperatus
    Undulatus asperatus
    Undulatus asperatus is a cloud formation, proposed in 2009 as a separate cloud classification by the founder of the Cloud Appreciation Society. If successful it will be the first cloud formation added since cirrus intortus in 1951 to the International Cloud Atlas of the World Meteorological...

  • Weather lore
    Weather lore
    Weather lore is the body of informal folklore related to the prediction of the weather.It has been a human desire for millennia to make accurate weather predictions. Oral and written history is full of rhymes, anecdotes, and adages meant to guide the uncertain in determining whether the next day...

  • Cloud Computing
    Cloud computing
    Cloud computing is the delivery of computing as a service rather than a product, whereby shared resources, software, and information are provided to computers and other devices as a utility over a network ....


External links