meteorology Meteorology is a branch of the atmospheric sciences (which include atmospheric chemistry and physics) with a major focus on weather forecasting. The study of meteorology dates back millennia, though significant progress in meteorology did not ...

, a cloud is an

aerosol An aerosol is a suspension (chemistry), suspension of fine solid particles or liquid Drop (liquid), droplets in air or another gas. Aerosols can be natural or Human impact on the environment, anthropogenic. Examples of natural aerosols are fog o ...

consisting of a visible mass of miniature

liquid A liquid is a nearly incompressible fluid that conforms to the shape of its container but retains a (nearly) constant volume independent of pressure. As such, it is one of the four fundamental states of matter (the others being solid, gas, a ...

droplets 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 d ...

, frozen crystals, or other

particles In the physical sciences, a particle (or corpuscule in older texts) is a small localized object which can be described by several physical or chemical properties, such as volume, density, or mass. They vary greatly in size or quantity, from s ...

suspended in the

atmosphere An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A s ...

of a

planet A planet is a large, rounded astronomical body that is neither a star nor its remnant. The best available theory of planet formation is the nebular hypothesis, which posits that an interstellar cloud collapses out of a nebula to create a you ...

ary body or similar space.

Water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as a ...

or various other chemicals may compose the droplets and crystals. On

Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...

, clouds are formed as a result of saturation of the

air The atmosphere of Earth is the layer of gases, known collectively as air, retained by Earth's gravity that surrounds the planet and forms its planetary atmosphere. The atmosphere of Earth protects life on Earth by creating pressure allowing f ...

when it is cooled to its

dew point The dew point is the temperature to which air must be cooled to become saturated with water vapor, assuming constant air pressure and water content. When cooled below the dew point, moisture capacity is reduced and airborne water vapor will cond ...

, or when it gains sufficient

moisture Moisture is the presence of a liquid, especially water, often in trace amounts. Small amounts of water may be found, for example, in the air (humidity), in foods, and in some commercial products. Moisture also refers to the amount of water vapo ...

(usually in the form of

water vapor (99.9839 °C) , - , Boiling point , , - , specific gas constant , 461.5 J/( kg·K) , - , Heat of vaporization , 2.27 MJ/kg , - , Heat capacity , 1.864 kJ/(kg·K) Water vapor, water vapour or aqueous vapor is the gaseous pha ...

) from an adjacent source to raise the dew point to the ambient

temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied o ...

. They are seen in the Earth's

homosphere The homosphere is the layer of an atmosphere where the bulk gases are homogeneously mixed due to turbulent mixing or eddy diffusion. The bulk composition of the air is mostly uniform so the concentrations of molecules are the same throughout the ho ...

, which includes the

troposphere The troposphere is the first and lowest layer of the atmosphere of the Earth, and contains 75% of the total mass of the planetary atmosphere, 99% of the total mass of water vapour and aerosols, and is where most weather phenomena occur. From ...

stratosphere The stratosphere () is the second layer of the atmosphere of the Earth, located above the troposphere and below the mesosphere. The stratosphere is an atmospheric layer composed of stratified temperature layers, with the warm layers of air ...

, and

mesosphere The mesosphere (; ) is the third layer of the atmosphere, directly above the stratosphere and directly below the thermosphere. In the mesosphere, temperature decreases as altitude increases. This characteristic is used to define its limits: it ...

. Nephology is the science of clouds, which is undertaken in the

cloud physics Cloud physics is the study of the physical processes that lead to the formation, growth and precipitation of atmospheric clouds. These aerosols are found in the troposphere, stratosphere, and mesosphere, which collectively make up the greatest ...

branch of

. There are two methods of naming clouds in their respective layers of the homosphere,

Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through the power of the ...

and

common name In biology, a common name of a taxon or organism (also known as a vernacular name, English name, colloquial name, country name, popular name, or farmer's name) is a name that is based on the normal language of everyday life; and is often contrast ...

Genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...

types in the troposphere, the atmospheric layer closest to Earth's surface, have

names because of the universal adoption of

Luke Howard Luke Howard, (28 November 1772 – 21 March 1864) was a British manufacturing chemist and an amateur meteorologist with broad interests in science. His lasting contribution to science is a nomenclature system for clouds, which he proposed i ...

nomenclature Nomenclature (, ) is a system of names or terms, or the rules for forming these terms in a particular field of arts or sciences. The principles of naming vary from the relatively informal naming conventions, conventions of everyday speech to the i ...

that was formally proposed in 1802. It became the basis of a modern international system that divides clouds into five physical ''forms'' which can be further divided or classified into

altitude Altitude or height (also sometimes known as depth) is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The exact definition and reference datum varies according to the context ...

''levels'' to derive ten basic ''genera''. The main representative cloud types for each of these forms are stratiform,

cumuliform Cumulus clouds are clouds which have flat bases and are often described as "puffy", "cotton-like" or "fluffy" in appearance. Their name derives from the Latin ''cumulo-'', meaning ''heap'' or ''pile''. Cumulus clouds are low-level clouds, gener ...

, stratocumuliform, cumulonimbiform, and cirriform. ''Low-level'' clouds do not have any altitude-related prefixes. However ''mid-level'' stratiform and stratocumuliform types are given the prefix ''alto-'' while ''high-level'' variants of these same two forms carry the prefix ''cirro-''. In both cases, ''strato-'' is dropped from the latter form to avoid double-prefixing. Genus types with sufficient vertical extent to occupy more than one level do not carry any altitude related prefixes. They are classified formally as low- or mid-level depending on the altitude at which each initially forms, and are also more informally characterized as ''multi-level'' or ''vertical''. Most of the ten genera derived by this method of

classification Classification is a process related to categorization, the process in which ideas and objects are recognized, differentiated and understood. Classification is the grouping of related facts into classes. It may also refer to: Business, organizat ...

can be subdivided into ''species'' and further subdivided into ''varieties''. Very low stratiform clouds that extend down to the Earth's surface are given the common names ''fog'' and ''mist'', but have no Latin names. In the stratosphere and mesosphere, clouds have common names for their main types. They may have the appearance of stratiform veils or sheets, cirriform wisps, or stratocumuliform bands or ripples. They are seen infrequently, mostly in the polar regions of Earth. Clouds have been observed in the atmospheres of other

s and

moons A natural satellite is, in the most common usage, an astronomical body that orbits a planet, dwarf planet, or small Solar System body (or sometimes another natural satellite). Natural satellites are often colloquially referred to as ''moons'' ...

in the

Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar S ...

and beyond. However, due to their different temperature characteristics, they are often composed of other substances such as

methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Eart ...

ammonia Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous was ...

, and

sulfuric acid Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid ( Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen and hydrogen, with the molecular formu ...

, as well as water. Tropospheric clouds can have a direct effect on

climate change In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to E ...

on Earth. They may reflect incoming rays from the sun which can contribute to a cooling effect where and when these clouds occur, or trap longer wave radiation that reflects back up from the Earth's surface which can cause a warming effect. The altitude, form, and thickness of the clouds are the main factors that affect the local heating or cooling of Earth and the atmosphere. Clouds that form above the troposphere are too scarce and too thin to have any influence on climate change. Clouds are the main uncertainty in

climate sensitivity Climate sensitivity is a measure of how much Earth's surface will cool or warm after a specified factor causes a change in its climate system, such as how much it will warm for a doubling in the atmospheric carbon dioxide () concentration. In te ...

Etymology

The origin of the term "cloud" can be found in the

Old English Old English (, ), or Anglo-Saxon, is the earliest recorded form of the English language, spoken in England and southern and eastern Scotland in the early Middle Ages. It was brought to Great Britain by Anglo-Saxon settlement of Britain, Anglo ...

words ''clud'' or ''clod'', meaning a hill or a mass of stone. Around the beginning of the 13th century, the word came to be used as a metaphor for rain clouds, because of the similarity in appearance between a mass of rock and cumulus heap cloud. Over time, the metaphoric usage of the word supplanted the Old English ', which had been the literal term for clouds in general.

Cross classification

The table that follows is very broad in scope like the cloud template that follows it. There are some variations in styles of nomenclature between the classification scheme used for the troposphere (strict Latin except for surface based aerosols) and the higher levels of the homosphere (common terms, some informally derived from Latin). However, the schemes presented here share a cross-classification of physical forms and altitude levels to derive the 10 tropospheric genera, the fog and mist that forms at surface level, and several additional major types above the troposphere. The cumulus genus includes four species that indicate vertical size which can affect the altitude levels.

History of cloud science

Ancient cloud studies were not made in isolation, but were observed in combination with other

weather Weather is the state of the atmosphere, describing for example the degree to which it is hot or cold, wet or dry, calm or stormy, clear or cloudy. On Earth, most weather phenomena occur in the lowest layer of the planet's atmosphere, the ...

elements and even other natural sciences. Around 340 BC, Greek philosopher

Aristotle Aristotle (; grc-gre, Ἀριστοτέλης ''Aristotélēs'', ; 384–322 BC) was a Greek philosopher and polymath during the Classical period in Ancient Greece. Taught by Plato, he was the founder of the Peripatetic school of phil ...

wrote ''

Meteorologica ''Meteorology'' (Greek: ; Latin: ''Meteorologica'' or ''Meteora'') is a treatise by Aristotle. The text discusses what Aristotle believed to have been all the affections common to air and water, and the kinds and parts of the earth and the affect ...

'', a work which represented the sum of knowledge of the time about natural science, including weather and climate. For the first time, precipitation and the clouds from which precipitation fell were called meteors, which originate from the Greek word ''meteoros'', meaning 'high in the sky'. From that word came the modern term

, the study of clouds and weather. ''Meteorologica'' was based on intuition and simple observation, but not on what is now considered the scientific method. Nevertheless, it was the first known work that attempted to treat a broad range of meteorological topics in a systematic way, especially the

hydrological cycle The water cycle, also known as the hydrologic cycle or the hydrological cycle, is a biogeochemical cycle that describes the continuous movement of water on, above and below the surface of the Earth. The mass of water on Earth remains fairly cons ...

After centuries of speculative theories about the formation and behavior of clouds, the first truly scientific studies were undertaken by

in England and

Jean-Baptiste Lamarck Jean-Baptiste Pierre Antoine de Monet, chevalier de Lamarck (1 August 1744 – 18 December 1829), often known simply as Lamarck (; ), was a French naturalist, biologist, academic, and soldier. He was an early proponent of the idea that biologi ...

in France. Howard was a methodical observer with a strong grounding in the Latin language, and used his background to formally classify the various tropospheric cloud types during 1802. He believed that scientific observations of the changing cloud forms in the sky could unlock the key to weather forecasting. Lamarck had worked independently on cloud classification the same year and had come up with a different naming scheme that failed to make an impression even in his home country of

France France (), officially the French Republic ( ), is a country primarily located in Western Europe. It also comprises of Overseas France, overseas regions and territories in the Americas and the Atlantic Ocean, Atlantic, Pacific Ocean, Pac ...

because it used unusually descriptive and informal French names and phrases for cloud types. His system of nomenclature included 12 categories of clouds, with such names as (translated from French) hazy clouds, dappled clouds, and broom-like clouds. By contrast, Howard used universally accepted Latin, which caught on quickly after it was published in 1803. As a sign of the popularity of the naming scheme, German dramatist and poet

Johann Wolfgang von Goethe Johann Wolfgang von Goethe (28 August 1749 – 22 March 1832) was a German poet, playwright, novelist, scientist, statesman, theatre director, and critic. His works include plays, poetry, literature, and aesthetic criticism, as well as trea ...

composed four poems about clouds, dedicating them to Howard. An elaboration of Howard's system was eventually formally adopted by the International Meteorological Conference in 1891. This system covered only the tropospheric cloud types. However, the discovery of clouds above the troposphere during the late 19th century eventually led to the creation of separate classification schemes that reverted to the use of descriptive common names and phrases that somewhat recalled Lamarck's methods of classification. These very high clouds, although classified by these different methods, are nevertheless broadly similar to some cloud forms identified in the troposphere with Latin names.

Formation

Terrestrial clouds can be found throughout most of the homosphere, which includes the troposphere, stratosphere, and mesosphere. Within these layers of the

, air can become saturated as a result of being cooled to its

or by having moisture added from an adjacent source. In the latter case, saturation occurs when the dew point is raised to the ambient air temperature.

Adiabatic cooling

Adiabatic cooling In thermodynamics, an adiabatic process (Greek: ''adiábatos'', "impassable") is a type of thermodynamic process that occurs without transferring heat or mass between the thermodynamic system and its environment. Unlike an isothermal process, a ...

occurs when one or more of three possible lifting agents – convective, cyclonic/frontal, or orographic – cause a parcel of air containing invisible water vapor to rise and cool to its dew point, the temperature at which the air becomes saturated. The main mechanism behind this process is adiabatic cooling. As the air is cooled to its dew point and becomes saturated, water vapor normally condenses to form cloud drops. This condensation normally occurs on

cloud condensation nuclei Cloud condensation nuclei (CCNs), also known as cloud seeds, are small particles typically 0.2 µm, or one hundredth the size of a cloud droplet. CCNs are a unique subset of aerosols in the atmosphere on which water vapour condenses. This c ...

such as

salt Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of salts; salt in the form of a natural crystalline mineral is known as rock salt or halite. Salt is present in vast quantitie ...

or dust particles that are small enough to be held aloft by normal circulation of the air. One agent is the convective upward motion of air caused by daytime solar heating at surface level. Airmass instability allows for the formation of cumuliform clouds that can produce showers if the air is sufficiently moist. On moderately rare occasions, convective lift can be powerful enough to penetrate the tropopause and push the cloud top into the stratosphere. Frontal and

cyclonic In meteorology, a cyclone () is a large air mass that rotates around a strong center of low atmospheric pressure, counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere as viewed from above (opposite to an anti ...

lift occur when

stable A stable is a building in which livestock, especially horses, are kept. It most commonly means a building that is divided into separate stalls for individual animals and livestock. There are many different types of stables in use today; the ...

air is forced aloft at

weather fronts A weather front is a boundary separating air masses for which several characteristics differ, such as air density, wind, temperature, and humidity. Disturbed and unstable weather due to these differences often arises along the boundary. For in ...

and around centers of

low pressure In meteorology, a low-pressure area, low area or low is a region where the atmospheric pressure is lower than that of surrounding locations. Low-pressure areas are commonly associated with inclement weather (such as cloudy, windy, with possible ...

by a process called

convergence Convergence may refer to: Arts and media Literature *''Convergence'' (book series), edited by Ruth Nanda Anshen * "Convergence" (comics), two separate story lines published by DC Comics: **A four-part crossover storyline that united the four Wei ...

Warm front A warm front is a density discontinuity located at the leading edge of a homogeneous warm air mass, and is typically located on the equator-facing edge of an isotherm gradient. Warm fronts lie within broader troughs of low pressure than cold fro ...

s associated with extratropical cyclones tend to generate mostly cirriform and stratiform clouds over a wide area unless the approaching warm airmass is unstable, in which case cumulus congestus or cumulonimbus clouds are usually embedded in the main precipitating cloud layer.

Cold front A cold front is the leading edge of a cooler mass of air at ground level that replaces a warmer mass of air and lies within a pronounced surface trough of low pressure. It often forms behind an extratropical cyclone (to the west in the Norther ...

s are usually faster moving and generate a narrower line of clouds, which are mostly stratocumuliform, cumuliform, or cumulonimbiform depending on the stability of the warm airmass just ahead of the front. Dreamy Twilight

A third source of lift is wind circulation forcing air over a physical barrier such as a

mountain A mountain is an elevated portion of the Earth's crust, generally with steep sides that show significant exposed bedrock. Although definitions vary, a mountain may differ from a plateau in having a limited Summit (topography), summit area, and ...

(

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 cr ...

). If the air is generally stable, nothing more than

lenticular cap Lenticular clouds (Latin: ''Lenticularis'' lentil-shaped, from ''lenticula'' lentil) are stationary clouds that form mostly in the troposphere, typically in parallel alignment to the wind direction. They are often comparable in appearance to ...

clouds form. However, if the air becomes sufficiently moist and unstable, orographic showers or

thunderstorm A thunderstorm, also known as an electrical storm or a lightning storm, is a storm characterized by the presence of lightning and its acoustic effect on the Earth's atmosphere, known as thunder. Relatively weak thunderstorms are someti ...

s may appear.Pidwirny, M. (2006)
"Cloud Formation Processes"
, chapter 8 in ''Fundamentals of Physical Geography'', 2nd ed.

Non-adiabatic cooling

Along with adiabatic cooling that requires a lifting agent, three major nonadiabatic mechanisms exist for lowering the temperature of the air to its dew point. Conductive, radiational, and evaporative cooling require no lifting mechanism and can cause condensation at surface level resulting in the formation of

fog Fog is a visible aerosol consisting of tiny water droplets or ice crystals suspended in the air at or near the Earth's surface. Reprint from Fog can be considered a type of low-lying cloud usually resembling stratus, and is heavily influ ...

Ackerman Ackerman may refer to: Surname *Ackerman (surname), people with the surname Ackerman *Ackerman is a family name for singer Barlin Ackerman Places *Ackerman, Mississippi, town in Choctaw County, Mississippi, US *Ackerman, West Virginia, former un ...

, p. 109

Adding moisture to the air

Several main sources of water vapor can be added to the air as a way of achieving saturation without any cooling process:

evaporation Evaporation is a type of vaporization that occurs on the surface of a liquid as it changes into the gas phase. High concentration of the evaporating substance in the surrounding gas significantly slows down evaporation, such as when humidi ...

from surface water or moist ground, precipitation or

virga In meteorology, a virga, also called a dry storm, is an observable :wikt:fallstreak, streak or Precipitation shaft, shaft of precipitation (meteorology), precipitation falling from a cloud that Evaporation, evaporates or Sublimation (phase t ...

, and

transpiration Transpiration is the process of water movement through a plant and its evaporation from aerial parts, such as leaves, stems and flowers. Water is necessary for plants but only a small amount of water taken up by the roots is used for growth a ...

from plants.

Classification

Tropospheric classification is based on a hierarchy of categories with physical forms and altitude levels at the top. These are cross-classified into a total of ten genus types, most of which can be divided into species and further subdivided into varieties which are at the bottom of the hierarchy.

Physical forms

Clouds in the troposphere assume five physical forms based on structure and process of formation. These forms are commonly used for the purpose of satellite analysis. They are given below in approximate ascending order of instability or

convective Convection is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the convect ...

activity. * Nonconvective stratiform clouds appear in stable airmass conditions and, in general, have flat, sheet-like structures that can form at any altitude in the troposphere. The stratiform group is divided by altitude range into the genera

cirrostratus Cirrostratus is a high-level, very thin, generally uniform ''stratiform'' genus-type of cloud. It is made out of ice-crystals, which are pieces of frozen water. It is difficult to detect and it can make halos. These are made when the cloud takes ...

(high-level),

altostratus Altostratus is a middle-altitude cloud genus made up of water droplets, ice crystals, or a mixture of the two. Altostratus clouds are formed when large masses of warm, moist air rise, causing water vapor to condense. Altostratus clouds are usuall ...

(mid-level),

stratus Stratus may refer to: Weather *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 Mus ...

(low-level), and

nimbostratus A nimbostratus cloud is a multi-level, amorphous, nearly uniform and often dark grey cloud that usually produces continuous rain, snow or sleet but no lightning or thunder. Fog is commonly considered a surface-based cloud layer. The fog may form at surface level in clear air or it may be the result of a very low stratus cloud subsiding to ground or sea level. Conversely, low stratiform clouds result when

advection fog In the field of physics, engineering, and earth sciences, advection is the transport of a substance or quantity by bulk motion of a fluid. The properties of that substance are carried with it. Generally the majority of the advected substance is al ...

is lifted above surface level during breezy conditions. * Cirriform clouds in the troposphere are of the genus cirrus and have the appearance of detached or semimerged filaments. They form at high tropospheric altitudes in air that is mostly stable with little or no convective activity, although denser patches may occasionally show buildups caused by limited high-level

convection Convection is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the convec ...

where the air is partly unstable. Clouds resembling cirrus, cirrostratus, and cirrocumulus can be found above the troposphere but are classified separately using common names. * Stratocumuliform clouds both cumuliform and stratiform characteristics in the form of rolls, ripples, or elements. They generally form as a result of limited convection in an otherwise mostly stable airmass topped by an inversion layer. If the inversion layer is absent or higher in the troposphere, increased airmass instability may cause the cloud layers to develop tops in the form of turrets consisting of embedded cumuliform buildups. The stratocumuliform group is divided into

cirrocumulus Cirrocumulus is one of the three main genus-types of high-altitude tropospheric clouds, the other two being cirrus and cirrostratus. They usually occur at an altitude of . Like lower-altitude cumuliform and stratocumuliform clouds, cirrocumulus s ...

(high-level, strato- prefix dropped),

altocumulus Altocumulus (From Latin ''Altus'', "high", ''cumulus'', "heaped") is a middle-altitude cloud genus that belongs mainly to the ''stratocumuliform'' physical category characterized by globular masses or rolls in layers or patches, the individual ele ...

(mid-level, strato- prefix dropped), and

stratocumulus A stratocumulus cloud, occasionally called a cumulostratus, belongs to a genus-type of clouds characterized by large dark, rounded masses, usually in groups, lines, or waves, the individual elements being larger than those in altocumulus, and th ...

(low-level). * Cumuliform clouds generally appear in isolated heaps or tufts. They are the product of localized but generally free-convective lift where no inversion layers are in the troposphere to limit vertical growth. In general, small cumuliform clouds tend to indicate comparatively weak instability. Larger cumuliform types are a sign of greater atmospheric instability and convective activity. Depending on their vertical size, clouds of the

cumulus Cumulus clouds are clouds which have flat bases and are often described as "puffy", "cotton-like" or "fluffy" in appearance. Their name derives from the Latin ''cumulo-'', meaning ''heap'' or ''pile''. Cumulus clouds are low-level clouds, gener ...

genus type may be low-level or multi-level with moderate to towering vertical extent. *

Cumulonimbus clouds Cumulonimbus (from Latin ''cumulus'', "heaped" and ''nimbus'', "rainstorm") is a dense, towering vertical cloud, typically forming from water vapor condensing in the lower troposphere that builds upward carried by powerful Buoyancy, buoyant air ...

are largest free-convective clouds, which has a towering vertical extent. They occur in highly unstable air and often have fuzzy outlines at the upper parts of the clouds that sometimes include anvil tops. These clouds are the product of very strong convection that can penetrate the lower stratosphere.

Levels and genera

Tropospheric clouds form in any of three levels (formerly called ''étages'') based on altitude range above the Earth's surface. The grouping of clouds into levels is commonly done for the purposes of

cloud atlas A cloud atlas is a pictorial key (or an atlas) to the nomenclature of clouds. Early cloud atlases were an important element in the training of meteorologists and in weather forecasting, and the author of a 1923 atlas stated that "increasing use ...

es,

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 automate ...

s, and

weather map A weather map, also known as synoptic weather chart, displays various meteorological features across a particular area at a particular point in time and has various symbols which all have specific meanings. Such maps have been in use since the m ...

s.JetStream (2008)
How to read weather maps.

National Weather Service The National Weather Service (NWS) is an Government agency, agency of the Federal government of the United States, United States federal government that is tasked with providing weather forecasts, warnings of hazardous weather, and other weathe ...

. Retrieved on 16 May 2007. The base-height range for each level varies depending on the latitudinal

geographical zone The five main latitude regions of Earth's surface comprise geographical zones, divided by the major circles of latitude. The differences between them relate to climate. They are as follows: # The North Frigid Zone, between the North Pole at 9 ...

. Each altitude level comprises two or three genus-types differentiated mainly by physical form. The standard levels and genus-types are summarised below in approximate descending order of the altitude at which each is normally based. Multi-level clouds with significant vertical extent are separately listed and summarized in approximate ascending order of instability or convective activity.

High-level

High clouds form at altitudes of in the

polar region The polar regions, also called the frigid geographical zone, zones or polar zones, of Earth are the regions of the planet that surround its geographical poles (the North Pole, North and South Poles), lying within the polar circles. These high l ...

s, in the

temperate regions In geography, the temperate climates of Earth occur in the middle latitudes (23.5° to 66.5° N/S of Equator), which span between the tropics and the polar regions of Earth. These zones generally have wider temperature ranges throughout t ...

, and in the

tropics The tropics are the regions of Earth surrounding the Equator. They are defined in latitude by the Tropic of Cancer in the Northern Hemisphere at N and the Tropic of Capricorn in the Southern Hemisphere at S. The tropics are also referred to ...

. All cirriform clouds are classified as high, thus constitute a single genus ''cirrus'' (Ci). Stratocumuliform and stratiform clouds in the high altitude range carry the prefix ''cirro-'', yielding the respective genus names ''cirrocumulus'' (Cc) and ''cirrostratus'' (Cs). If limited-resolution satellite images of high clouds are analysed without supporting data from direct human observations, distinguishing between individual forms or genus types becomes impossible, and they are collectively identified as ''high-type'' (or informally as ''cirrus-type'', though not all high clouds are of the cirrus form or genus). * Genus

cirrus Cirrus may refer to: Science *Cirrus (biology), any of various thin, thread-like structures on the body of an animal *Cirrus (botany), a tendril *Infrared cirrus, in astronomy, filamentary structures seen in infrared light *Cirrus cloud, a type ...

(Ci) – these are mostly fibrous wisps of delicate, white, cirriform, ice crystal clouds that show up clearly against the blue sky. Cirrus are generally non-convective except castellanus and floccus subtypes which show limited convection. They often form along a high altitude

jetstream Jet streams are fast flowing, narrow, meandering air currents in the atmospheres of some planets, including Earth. On Earth, the main jet streams are located near the altitude of the tropopause and are westerly winds (flowing west to east) ...

and at the very leading edge of a frontal or low-pressure disturbance where they may merge into cirrostratus. This high-level cloud genus does not produce precipitation. * Genus

(Cc) – this is a pure white high stratocumuliform layer of limited convection. It is composed of ice crystals or supercooled water droplets appearing as small unshaded round masses or flakes in groups or lines with ripples like sand on a beach. Cirrocumulus occasionally forms alongside cirrus and may be accompanied or replaced by cirrostratus clouds near the leading edge of an active weather system. This genus-type occasionally produces virga, precipitation that evaporates below the base of the cloud. * Genus

(Cs) – cirrostratus is a thin nonconvective stratiform 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 does not produce precipitation, but often thickens into altostratus ahead of a warm front or low-pressure area, which sometimes does.

Mid-level

Nonvertical clouds in the middle level are prefixed by ''alto-'', yielding the genus names ''altocumulus'' (Ac) for stratocumuliform types and ''altostratus'' (As) for stratiform types. These clouds can form as low as above surface at any latitude, but may be based as high as near the poles, at midlatitudes, and in the tropics. As with high clouds, the main genus types are easily identified by the human eye, but distinguishing between them using satellite photography alone is not possible. When the supporting data of human observations are not available, these clouds are usually collectively identified as ''middle-type'' on satellite images. * Genus

(Ac) – This is a midlevel cloud layer of limited convection that is usually appears in the form of irregular patches or more extensive sheets arranged in groups, lines, or waves. Altocumulus may occasionally resemble cirrocumulus, but is usually thicker and composed of a mix of water droplets and ice crystals, so the bases show at least some light-grey shading. Altocumulus can produce virga, very light precipitation that evaporates before reaching the ground. * Genus

(As) – Altostratus is a midlevel opaque or translucent nonconvective veil of grey/blue-grey cloud that often forms along warm fronts and around low-pressure areas. 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.

Low-level

Low clouds are found from near the surface up to . Genus types in this level either have no prefix or carry one that refers to a characteristic other than altitude. Clouds that form in the low level of the troposphere are generally of larger structure than those that form in the middle and high levels, so they can usually be identified by their forms and genus types using satellite photography alone. * Genus

(Sc) – This genus type is a stratocumuliform cloud layer of limited convection, usually in the form of irregular patches or more extensive sheets similar to altocumulus but having larger elements with deeper-gray shading. Stratocumulus is often present during wet weather originating from other rain clouds, but can only produce very light precipitation on its own. * Species

cumulus humilis Cumulus humilis are cumuliform clouds with little vertical extent, common in the summer, that are often referred to as "fair weather cumulus". If they develop into cumulus mediocris or cumulus congestus, thunderstorms could form later in the day. ...

– These are small detached fair-weather cumuliform clouds that have nearly horizontal bases and flattened tops, and do not produce rain showers. * Genus

(St) – This is a flat or sometimes ragged nonconvective stratiform type that sometimes resembles elevated fog. Only very weak precipitation can fall from this cloud, usually drizzle or snow grains. When a very low stratus cloud subsides to surface level, it loses its Latin terminology and is given the common name fog if the prevailing surface visibility is less than 1 km. If the visibility is 1 km or higher, the visible condensation is termed

mist Mist is a phenomenon caused by small droplets of water suspended in the cold air, usually by condensation. Physically, it is an example of a dispersion. It is most commonly seen where water vapor in warm, moist air meets sudden cooling, such a ...

Multi-level or moderate vertical

These clouds have low- to mid-level bases that form anywhere from near the surface to about and tops that can extend into the mid-altitude range and sometimes higher in the case of nimbostratus. * Genus

nimbostratus A nimbostratus cloud is a multi-level, amorphous, nearly uniform and often dark grey cloud that usually produces continuous rain, snow or sleet but no lightning or thunder. Nimbostratus normally forms from mid-level altostratus, and develops at least moderate vertical extent when the base subsides into the low level during precipitation that can reach moderate to heavy intensity. It achieves even greater vertical development when it simultaneously grows upward into the high level due to large-scale frontal or cyclonic lift. The ''nimbo-'' prefix refers to its ability to produce continuous rain or snow over a wide area, especially ahead of a warm front.

, p. 118 This thick cloud layer lacks any towering structure of its own, but may be accompanied by embedded towering cumuliform or cumulonimbiform types. Meteorologists affiliated with the

World Meteorological Organization The World Meteorological Organization (WMO) is a specialized agency of the United Nations responsible for promoting international cooperation on atmospheric science, climatology, hydrology and geophysics. The WMO originated from the Internati ...

(WMO) officially classify nimbostratus as mid-level for synoptic purposes while informally characterizing it as multi-level. Independent meteorologists and educators appear split between those who largely follow the WMO model and those who classify nimbostratus as low-level, despite its considerable vertical extent and its usual initial formation in the middle altitude range. * Species

cumulus mediocris Cumulus mediocris is a low to middle level cloud with some vertical extent (Family D1) of the genus cumulus, larger in vertical development than Cumulus humilis. It also may exhibit small protuberances from the top and may show the cauliflower fo ...

– T hese cumuliform clouds of free convection have clear-cut, medium-grey, flat bases and white, domed tops in the form of small sproutings and generally do not produce precipitation. They usually form in the low level of the troposphere except during conditions of very low relative humidity, when the clouds bases can rise into the middle-altitude range. Cumulus mediocris is officially classified as low-level and more informally characterized as having moderate vertical extent that can involve more than one altitude level.

Towering vertical

These very large cumuliform and cumulonimbiform types have cloud bases in the same low- to mid-level range as the multi-level and moderate vertical types, but the tops nearly always extend into the high levels. Unlike less vertically developed clouds, they are required to be identified by their standard names or abbreviations in all aviation observations (METARS) and forecasts (TAFS) to warn pilots of possible severe weather and turbulence. * Species

cumulus congestus Cumulus congestus clouds, also known as towering cumulus, are a form of cumulus that can be based in the low or middle height ranges. They achieve considerable vertical development in areas of deep, moist convection. They are an intermediate stage ...

– Increasing airmass instability can cause free-convective cumulus to grow very tall to the extent that the vertical height from base to top is greater than the base-width of the cloud. The cloud base takes on a darker grey coloration and the top commonly resembles a cauliflower. This cloud type can produce moderate to heavy showers and is designated ''Towering cumulus'' (Tcu) by the

International Civil Aviation Organization (ICAO) The International Civil Aviation Organization (ICAO, ) is a specialized agency of the United Nations that coordinates the principles and techniques of international air navigation, and fosters the planning and development of international a ...

. * Genus

cumulonimbus Cumulonimbus (from Latin ''cumulus'', "heaped" and ''nimbus'', "rainstorm") is a dense, towering vertical cloud, typically forming from water vapor condensing in the lower troposphere that builds upward carried by powerful Buoyancy, buoyant air ...

(Cb) – This genus type is a heavy, towering, cumulonimbiform mass of free-convective cloud with a dark-grey to nearly black base and a very high top in the form of a mountain or huge tower. Cumulonimbus can produce

s, local very heavy downpours of

rain Rain is water droplets that have condensed from atmospheric water vapor and then fall under gravity. Rain is a major component of the water cycle and is responsible for depositing most of the fresh water on the Earth. It provides water f ...

that may cause

flash flood A flash flood is a rapid flooding of low-lying areas: washes, rivers, dry lakes and depressions. It may be caused by heavy rain associated with a severe thunderstorm, hurricane, or tropical storm, or by meltwater from ice or snow flowing o ...

s, and a variety of types of

lightning Lightning is a naturally occurring electrostatic discharge during which two electric charge, electrically charged regions, both in the atmosphere or with one on the land, ground, temporarily neutralize themselves, causing the instantaneous ...

including cloud-to-ground that can cause

wildfire A wildfire, forest fire, bushfire, wildland fire or rural fire is an unplanned, uncontrolled and unpredictable fire in an area of Combustibility and flammability, combustible vegetation. Depending on the type of vegetation present, a wildfire ...

s. Other convective severe weather may or may not be associated with thunderstorms and include heavy

snow Snow comprises individual ice crystals that grow while suspended in the atmosphere—usually within clouds—and then fall, accumulating on the ground where they undergo further changes. It consists of frozen crystalline water throughout ...

showers,

hail Hail is a form of solid precipitation. It is distinct from ice pellets (American English "sleet"), though the two are often confused. It consists of balls or irregular lumps of ice, each of which is called a hailstone. Ice pellets generally fal ...

, strong

wind shear Wind shear (or windshear), sometimes referred to as wind gradient, is a difference in wind speed and/or direction over a relatively short distance in the atmosphere. Atmospheric wind shear is normally described as either vertical or horizontal ...

downburst In meteorology, a downburst is a strong downward and outward gushing wind system that emanates from a point source above and blows radially, that is, in straight lines in all directions from the area of impact at surface level. Capable of pro ...

s, and

tornado A tornado is a violently 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. It is often referred to as a twister, whirlwind or cyclone, altho ...

es. Of all these possible cumulonimbus-related events, lightning is the only one of these that requires a thunderstorm to be taking place since it is the lightning that creates the thunder. Cumulonimbus clouds can form in unstable airmass conditions, but tend to be more concentrated and intense when they are associated with unstable

cold front A cold front is the leading edge of a cooler mass of air at ground level that replaces a warmer mass of air and lies within a pronounced surface trough of low pressure. It often forms behind an extratropical cyclone (to the west in the Norther ...

Species

Genus types are commonly divided into subtypes called ''species'' that indicate specific structural details which can vary according to the stability and windshear characteristics of the atmosphere at any given time and location. Despite this hierarchy, a particular species may be a subtype of more than one genus, especially if the genera are of the same physical form and are differentiated from each other mainly by altitude or level. There are a few species, each of which can be associated with genera of more than one physical form. The species types are grouped below according to the physical forms and genera with which each is normally associated. The forms, genera, and species are listed from left to right in approximate ascending order of instability or convective activity.

Stable or mostly stable

Of the non-convective stratiform group, high-level cirrostratus comprises two species. Cirrostratus ''nebulosus'' has a rather diffuse appearance lacking in structural detail. Cirrostratus ''fibratus'' is a species made of semi-merged filaments that are transitional to or from cirrus. Mid-level altostratus and multi-level nimbostratus always have a flat or diffuse appearance and are therefore not subdivided into species. Low stratus is of the species nebulosus except when broken up into ragged sheets of stratus

fractus Fractus clouds (''scuds'') also known as Fractostratus or Fracto-Cumulus 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 ...

(see below). Cirriform clouds have three non-convective species that can form in ''stable'' airmass conditions. Cirrus fibratus comprise filaments that may be straight, wavy, or occasionally twisted by wind shear. The species ''uncinus'' is similar but has upturned hooks at the ends. Cirrus ''spissatus'' appear as opaque patches that can show light grey shading. Stratocumuliform genus-types (cirrocumulus, altocumulus, and stratocumulus) that appear in mostly stable air with limited convection have two species each. The ''stratiformis'' species normally occur in extensive sheets or in smaller patches where there is only minimal convective activity. Clouds of the ''lenticularis'' species tend to have lens-like shapes tapered at the ends. They are most commonly seen as orographic mountain-

wave cloud A wave cloud is a cloud form created by atmospheric internal waves. Formation The atmospheric internal waves that form wave clouds are created as stable air flows over a raised land feature such as a mountain range, and can form either direc ...

s, but can occur anywhere in the troposphere where there is strong wind shear combined with sufficient airmass stability to maintain a generally flat cloud structure. These two species can be found in the high, middle, or low levels of the troposphere depending on the stratocumuliform genus or genera present at any given time.

Ragged

The species ''fractus'' shows ''variable'' instability because it can be a subdivision of genus-types of different physical forms that have different stability characteristics. This subtype can be in the form of ragged but mostly ''stable'' stratiform sheets (stratus fractus) or small ragged cumuliform heaps with somewhat greater instability (cumulus fractus). When clouds of this species are associated with precipitating cloud systems of considerable vertical and sometimes horizontal extent, they are also classified as ''accessory clouds'' under the name ''pannus'' (see section on supplementary features).

Partly unstable

These species are subdivisions of genus types that can occur in partly unstable air with limited

. The species ''castellanus'' appears when a mostly stable stratocumuliform or cirriform layer becomes disturbed by localized areas of airmass instability, usually in the morning or afternoon. This results in the formation of embedded cumuliform buildups arising from a common stratiform base. Castellanus resembles the turrets of a castle when viewed from the side, and can be found with stratocumuliform genera at any tropospheric altitude level and with limited-convective patches of high-level cirrus. Tufted clouds of the more detached ''floccus'' species are subdivisions of genus-types which may be cirriform or stratocumuliform in overall structure. They are sometimes seen with cirrus, cirrocumulus, altocumulus, and stratocumulus. A newly recognized species of stratocumulus or altocumulus has been given the name ''volutus'', a roll cloud that can occur ahead of a cumulonimbus formation. There are some volutus clouds that form as a consequence of interactions with specific geographical features rather than with a parent cloud. Perhaps the strangest geographically specific cloud of this type is the

Morning Glory Morning glory (also written as morning-glory) is the common name for over 1,000 species of flowering plants in the family Convolvulaceae, whose current taxonomy and systematics are in flux. Morning glory species belong to many genera, some of ...

, a rolling cylindrical cloud that appears unpredictably over the

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 eastern Arafura Sea (the body of water that lies between Australia and New Guinea). The northern boundary is ...

Northern Australia The unofficial geographic term Northern Australia includes those parts of Queensland and Western Australia north of latitude 26° and all of the Northern Territory. Those local government areas of Western Australia and Queensland that lie p ...

. Associated with a powerful "ripple" in the atmosphere, the cloud may be "surfed" in

glider Glider may refer to: Aircraft and transport Aircraft * Glider (aircraft), heavier-than-air aircraft primarily intended for unpowered flight ** Glider (sailplane), a rigid-winged glider aircraft with an undercarriage, used in the sport of glidin ...

aircraft.

Unstable or mostly unstable

More general airmass instability in the troposphere tends to produce clouds of the more freely convective cumulus genus type, whose species are mainly indicators of degrees of atmospheric instability and resultant vertical development of the clouds. A cumulus cloud initially forms in the low level of the troposphere as a cloudlet of the species ''humilis'' that shows only slight vertical development. If the air becomes more unstable, the cloud tends to grow vertically into the species ''mediocris'', then strongly convective ''congestus'', the tallest cumulus species which is the same type that the International Civil Aviation Organization refers to as 'towering cumulus'. Cumulus mediocris atmospheric instability 01

Cumulus mediocris atmospheric instability 01

With highly unstable atmospheric conditions, large cumulus may continue to grow into even more strongly convective cumulonimbus ''calvus'' (essentially a very tall congestus cloud that produces thunder), then ultimately into the species ''capillatus'' when supercooled water droplets at the top of the cloud turn into ice crystals giving it a cirriform appearance.

Varieties

Genus and species types are further subdivided into ''varieties'' whose names can appear after the species name to provide a fuller description of a cloud. Some cloud varieties are not restricted to a specific altitude level or form, and can therefore be common to more than one genus or species.

Opacity-based

All cloud varieties fall into one of two main groups. One group identifies the opacities of particular low and mid-level cloud structures and comprises the varieties ''translucidus'' (thin translucent), ''perlucidus'' (thick opaque with translucent or very small clear breaks), and ''opacus'' (thick opaque). These varieties are always identifiable for cloud genera and species with variable opacity. All three are associated with the stratiformis species of altocumulus and stratocumulus. However, only two varieties are seen with altostratus and stratus nebulosus whose uniform structures prevent the formation of a perlucidus variety. Opacity-based varieties are not applied to high clouds because they are always translucent, or in the case of cirrus spissatus, always opaque.

Pattern-based

A second group describes the occasional arrangements of cloud structures into particular patterns that are discernible by a surface-based observer (cloud fields usually being visible only from a significant altitude above the formations). These varieties are not always present with the genera and species with which they are otherwise associated, but only appear when atmospheric conditions favor their formation. ''Intortus'' and ''vertebratus'' varieties occur on occasion with cirrus fibratus. They are respectively filaments twisted into irregular shapes, and those that are arranged in fishbone patterns, usually by uneven wind currents that favor the formation of these varieties. The variety ''radiatus'' is associated with cloud rows of a particular type that appear to converge at the horizon. It is sometimes seen with the fibratus and uncinus species of cirrus, the stratiformis species of altocumulus and stratocumulus, the mediocris and sometimes humilis species of cumulus, and with the genus altostratus. Morning Sky 7

Another variety, ''duplicatus'' (closely spaced layers of the same type, one above the other), is sometimes found with cirrus of both the fibratus and uncinus species, and with altocumulus and stratocumulus of the species stratiformis and lenticularis. The variety ''undulatus'' (having a wavy undulating base) can occur with any clouds of the species stratiformis or lenticularis, and with altostratus. It is only rarely observed with stratus nebulosus. The variety ''lacunosus'' is caused by localized downdrafts that create circular holes in the form of a honeycomb or net. It is occasionally seen with cirrocumulus and altocumulus of the species stratiformis, castellanus, and floccus, and with stratocumulus of the species stratiformis and castellanus.

Combinations

It is possible for some species to show combined varieties at one time, especially if one variety is opacity-based and the other is pattern-based. An example of this would be a layer of altocumulus stratiformis arranged in seemingly converging rows separated by small breaks. The full technical name of a cloud in this configuration would be ''altocumulus stratiformis radiatus perlucidus'', which would identify respectively its genus, species, and two combined varieties.

Other types

Supplementary features and accessory clouds are not further subdivisions of cloud types below the species and variety level. Rather, they are either ''hydrometeors'' or special cloud types with their own Latin names that form in association with certain cloud genera, species, and varieties. Supplementary features, whether in the form of clouds or precipitation, are directly attached to the main genus-cloud. Accessory clouds, by contrast, are generally detached from the main cloud.

Precipitation-based supplementary features

One group of supplementary features are not actual cloud formations, but precipitation that falls when water droplets or ice crystals 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 altostratus opacus, which can produce widespread but usually light precipitation, and with thicker clouds that show significant vertical development. Of the latter, ''upward-growing'' cumulus mediocris produces only isolated light showers, while ''downward growing'' nimbostratus is capable of heavier, more extensive precipitation. Towering vertical clouds 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.

Cloud-based supplementary features

''Incus'' is the most type-specific supplementary feature, seen only with cumulonimbus of the species capillatus. A

cumulonimbus incus A cumulonimbus incus (Latin ''incus'', "anvil"), also known as an anvil cloud, is a cumulonimbus cloud which has reached the level of stratospheric stability and has formed the characteristic flat, anvil-top shape. It signifies the thunderstor ...

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 The tropopause is the atmospheric boundary that demarcates the troposphere from the stratosphere; which are two of the five layers of the atmosphere of Earth. The tropopause is a thermodynamic gradient-stratification layer, that marks the end of ...

where the air no longer continues to get colder with increasing altitude. The ''mamma'' feature forms on the bases of clouds as downward-facing bubble-like protuberances caused by localized downdrafts within the cloud. It is also sometimes called ''mammatus'', an earlier version of the term used before a standardization of Latin nomenclature brought about by the World Meteorological Organization during the 20th century. The best-known is

cumulonimbus with mammatus Mammatus (also called mamma or mammatocumulus, meaning "mammary cloud") is a cellular pattern of pouches hanging underneath the base of a cloud, typically a cumulonimbus raincloud, although they may be attached to other classes of parent cloud ...

, but the mamma feature is also seen occasionally with cirrus, cirrocumulus, altocumulus, altostratus, and stratocumulus. A ''tuba'' feature is a cloud column that may hang from the bottom of a cumulus or cumulonimbus. A newly formed or poorly organized column might be comparatively benign, but can quickly intensify into a funnel cloud or tornado. An '' arcus'' feature is a roll cloud with ragged edges attached to the lower front part of cumulus congestus or cumulonimbus that forms along the leading edge of a squall line or thunderstorm outflow. A large arcus formation can have the appearance of a dark menacing arch. Several new supplementary features have been formally recognized by the

(WMO). The feature ''fluctus'' can form under conditions of strong atmospheric wind shear when a stratocumulus, altocumulus, or cirrus cloud breaks into regularly spaced crests. This variant is sometimes known informally as a Kelvin–Helmholtz (wave) cloud. This phenomenon has also been observed in cloud formations over other planets and even in the sun's atmosphere. Another highly disturbed but more chaotic wave-like cloud feature associated with stratocumulus or altocumulus cloud has been given the Latin name ''asperitas''. The supplementary feature ''cavum'' is a circular fall-streak hole that occasionally forms in a thin layer of supercooled altocumulus or cirrocumulus. Fall streaks consisting of virga or wisps of cirrus are usually seen beneath the hole as ice crystals fall out to a lower altitude. This type of hole is usually larger than typical lacunosus holes. A ''murus'' feature is a cumulonimbus wall cloud with a lowering, rotating cloud base than can lead to the development of tornadoes. A ''cauda'' feature is a tail cloud that extends horizontally away from the murus cloud and is the result of air feeding into the storm.

Accessory clouds

Supplementary cloud formations detached from the main cloud are known as

accessory cloud An accessory cloud is a cloud which is dependent on a larger cloud system for its development and continuance. It is often an appendage but also can be adjacent to the parent cloud system. The arcus and roll clouds, shelf cloud, wall cloud, and ...

s. The heavier precipitating clouds, nimbostratus, towering cumulus (cumulus congestus), and cumulonimbus typically see the formation in precipitation of the ''pannus'' feature, low ragged clouds of the genera and species cumulus fractus or stratus fractus. A group of accessory clouds comprise formations that are associated mainly with upward-growing cumuliform and cumulonimbiform 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 sometimes forms like an apron around the middle or in front of the parent cloud. An accessory cloud recently officially recognized by the World meteorological Organization is the ''flumen'', also known more informally as the ''beaver's tail''. It is formed by the warm, humid

inflow Inflow may refer to: * Inflow (hydrology), the water entering a body of water * Inflow (meteorology) Inflow is the flow of a fluid into a large collection of that fluid. Within meteorology, inflow normally refers to the influx of warmth and moi ...

of a super-cell thunderstorm, and can be mistaken for a tornado. Although the flumen can indicate a tornado risk, it is similar in appearance to pannus or

scud A Scud missile is one of a series of tactical ballistic missiles developed by the Soviet Union during the Cold War. It was exported widely to both Second World, Second and Third World, Third World countries. The term comes from the NATO reporti ...

clouds and does not rotate.

Mother clouds

Clouds initially form in clear air or become clouds when fog rises above surface level. The genus of a newly formed cloud is determined mainly by air mass characteristics such as stability and moisture content. If these characteristics change over time, the genus tends to change accordingly. When this happens, the original genus is called a ''mother cloud''. If the mother cloud retains much of its original form after the appearance of the new genus, it is termed a ''genitus'' cloud. One example of this is ''stratocumulus cumulogenitus'', a stratocumulus cloud formed by the partial spreading of a cumulus type when there is a loss of convective lift. If the mother cloud undergoes a complete change in genus, it is considered to be a ''mutatus'' cloud. Sunset and Clouds

Other genitus and mutatus clouds

The genitus and mutatus categories have been expanded to include certain types that do not originate from pre-existing clouds. The term ''flammagenitus'' (Latin for 'fire-made') applies to cumulus congestus or cumulonimbus that are formed by large scale fires or volcanic eruptions. Smaller low-level "pyrocumulus" or "fumulus" clouds formed by contained industrial activity are now classified as cumulus ''homogenitus'' (Latin for 'man-made').

Contrail Contrails (; short for "condensation trails") or vapor trails are line-shaped clouds produced by aircraft engine exhaust or changes in air pressure, typically at aircraft cruising altitudes several miles above the Earth's surface. Contrails ar ...

s formed from the exhaust of aircraft flying in the upper level of the troposphere can persist and spread into formations resembling cirrus which are designated cirrus ''homogenitus''. If a cirrus homogenitus cloud changes fully to any of the high-level genera, they are termed cirrus, cirrostratus, or cirrocumulus ''homomutatus''. Stratus cataractagenitus (Latin for 'cataract-made') are generated by the spray from waterfalls. ''Silvagenitus'' (Latin for 'forest-made') is a stratus cloud that forms as water vapor is added to the air above a forest canopy.

Stratocumulus fields

Stratocumulus clouds can be organized into "fields" that take on certain specially classified shapes and characteristics. In general, these fields are more discernible from high altitudes than from ground level. They can often be found in the following forms: * Actinoform, 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 A honeycomb is a mass of Triangular prismatic honeycomb#Hexagonal prismatic honeycomb, hexagonal prismatic Beeswax, wax cells built by honey bees in their beehive, nests to contain their larvae and stores of honey and pollen. beekeeping, Beekee ...

. * Open cell, which resembles an empty honeycomb, with clouds around the edges and clear, open space in the middle.

Vortex streets

These patterns are formed from a phenomenon known as a

Kármán vortex Karman or Kármán is a Hungarian surname. Notable people with the surname include: * Harvey Karman (20th century), inventor of the Karman cannula * Janice Karman (born 1954), American film producer, record producer, singer, and voice artist * J� ...

which is named after the engineer and fluid dynamicist

Theodore von Kármán Theodore von Kármán ( hu, ( szőllőskislaki) Kármán Tódor ; born Tivadar Mihály Kármán; 11 May 18816 May 1963) was a Hungarian-American mathematician, aerospace engineer, and physicist who was active primarily in the fields of aeronaut ...

,. Wind driven clouds can form into parallel rows that follow the wind direction. When the wind and clouds encounter high elevation land features such as a vertically prominent islands, they can form eddies around the high land masses that give the clouds a twisted appearance.

Distribution

Convergence along low-pressure zones

Although the local distribution of clouds can be significantly influenced by topography, the global prevalence of cloud cover in the troposphere tends to vary more by

latitude In geography, latitude is a coordinate that specifies the north– south position of a point on the surface of the Earth or another celestial body. Latitude is given as an angle that ranges from –90° at the south pole to 90° at the north pol ...

. It is most prevalent in and along low pressure zones of surface tropospheric convergence which encircle the Earth close to the

equator The equator is a circle of latitude, about in circumference, that divides Earth into the Northern and Southern hemispheres. It is an imaginary line located at 0 degrees latitude, halfway between the North and South poles. The term can als ...

and near the 50th parallels of latitude in the northern and southern hemispheres. The adiabatic cooling processes that lead to the creation of clouds by way of lifting agents are all associated with convergence; a process that involves the horizontal inflow and accumulation of air at a given location, as well as the rate at which this happens. Near the equator, increased cloudiness is due to the presence of the low-pressure

Intertropical Convergence Zone The Intertropical Convergence Zone (ITCZ ), known by sailors as the doldrums or the calms because of its monotonous windless weather, is the area where the northeast and the southeast trade winds converge. It encircles Earth near the thermal e ...

(ITCZ) where very warm and unstable air promotes mostly cumuliform and cumulonimbiform clouds. Clouds of virtually any type can form along the mid-latitude convergence zones depending on the stability and moisture content of the air. These extratropical convergence zones are occupied by the

polar front In meteorology, the polar front is the weather front boundary between the polar cell and the Ferrel cell around the 60° latitude, near the polar regions, in both hemisphere. At this boundary a sharp gradient in temperature occurs between these ...

s where

air mass In meteorology, an air mass is a volume of air defined by its temperature and humidity. Air masses cover many hundreds or thousands of square miles, and adapt to the characteristics of the surface below them. They are classified according to la ...

es of polar origin meet and clash with those of tropical or subtropical origin. This leads to the formation of weather-making

extratropical cyclone Extratropical cyclones, sometimes called mid-latitude cyclones or wave cyclones, are low-pressure areas which, along with the anticyclones of high-pressure areas, drive the weather over much of the Earth. Extratropical cyclones are capable of ...

s composed of cloud systems that may be stable or unstable to varying degrees according to the stability characteristics of the various airmasses that are in conflict.

Divergence along high pressure zones

Divergence is the opposite of convergence. In the Earth's troposphere, it involves the horizontal outflow of air from the upper part of a rising column of air, or from the lower part of a subsiding column often associated with an area or ridge of high pressure. Cloudiness tends to be least prevalent near the poles and in the subtropics close to the 30th parallels, north and south. The latter are sometimes referred to as the

horse latitudes The horse latitudes are the latitudes about 30 degrees north and south of the Equator. They are characterized by sunny skies, calm winds, and very little precipitation. They are also known as subtropical ridges, or highs. It is a high-pressure ...

. The presence of a large-scale high-pressure

subtropical ridge The horse latitudes are the latitudes about 30 degrees north and south of the Equator. They are characterized by sunny skies, calm winds, and very little precipitation. They are also known as Subtropics, subtropical ridges, or highs. It is a h ...

on each side of the equator reduces cloudiness at these low latitudes. Similar patterns also occur at higher latitudes in both hemispheres.

Luminance, reflectivity, and coloration

The luminance or brightness of a cloud is determined by how light is reflected, scattered, and transmitted by the cloud's particles. Its brightness may also be affected by the presence of haze or photometeors such as halos and rainbows. In the troposphere, dense, deep clouds exhibit a high reflectance (70% to 95%) throughout the

visible spectrum The visible spectrum is the portion of the electromagnetic spectrum that is visual perception, visible to the human eye. Electromagnetic radiation in this range of wavelengths is called ''visible light'' or simply light. A typical human eye wil ...

. 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.Increasing Cloud Reflectivity
, Royal Geographical Society, 2010. Cloud droplets tend to scatter light efficiently, so that the intensity of the

solar radiation Solar irradiance is the power per unit area (surface power density) received from the Sun in the form of electromagnetic radiation in the wavelength range of the measuring instrument. Solar irradiance is measured in watts per square metre (W/m ...

decreases with depth into the gases. As a result, the

cloud base A cloud base (or the base of the cloud) is the lowest altitude of the visible portion of a cloud. It is traditionally expressed either in metres or feet above mean sea level or above a planetary surface, or as the pressure level corresponding to ...

can vary from a very light to very-dark-grey depending on the cloud's thickness and how much

light Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 tera ...

is being reflected or transmitted back to the observer. High thin tropospheric clouds reflect less light because of the comparatively low concentration of constituent ice crystals or supercooled water droplets which results in a slightly off-white appearance. However, a thick dense ice-crystal cloud appears brilliant white with pronounced grey shading because of its greater reflectivity. As a tropospheric cloud matures, the dense water droplets may combine to produce larger droplets. If the droplets become too large and heavy to be kept aloft by the air circulation, they will fall from the cloud as

. 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, a percentage of the light that enters the cloud is not reflected back out but is absorbed giving the cloud a darker look. 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 or reflexion may refer to: Science and technology * Reflection (physics), a common wave phenomenon ** Specular reflection, reflection from a smooth surface *** Mirror image, a reflection in a mirror or in water ** Signal reflection, in ...

absorption Absorption may refer to: Chemistry and biology * Absorption (biology), digestion **Absorption (small intestine) *Absorption (chemistry), diffusion of particles of gas or liquid into liquid or solid materials *Absorption (skin), a route by which ...

is what causes the range of cloud color from white to black. Striking cloud colorations can be seen at any altitude, with the color of a cloud usually being the same as the incident light. During daytime when the sun is relatively high in the sky, tropospheric clouds generally appear bright white on top with varying shades of grey underneath. Thin clouds may look white or appear to have acquired the color of their

environment Environment most often refers to: __NOTOC__ * Natural environment, all living and non-living things occurring naturally * Biophysical environment, the physical and biological factors along with their chemical interactions that affect an organism or ...

or background. 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 sun is just below the horizon, low-level clouds are gray, middle clouds appear rose-colored, and high clouds are white or off-white. Clouds at night are black or dark grey in a moonless sky, or whitish when illuminated by the moon. They may also reflect the colors of large fires, city lights, or auroras that might be present. A cumulonimbus cloud that appears to have a greenish or bluish tint is a sign that it contains extremely high amounts of water; hail or rain which scatter light in a way that gives the cloud a blue color. A green colorization occurs mostly late in the day when the sun is comparatively low in the sky and the incident sunlight has a reddish tinge that appears green when illuminating a very tall bluish cloud. Supercell type storms are more likely to be characterized by this but any storm can appear this way. Coloration such as this does not directly indicate that it is a severe thunderstorm, it only confirms its potential. Since a green/blue tint signifies copious amounts of water, a strong updraft to support it, high winds from the storm raining out, and wet hail; all elements that improve the chance for it to become severe, can all be inferred from this. In addition, the stronger the updraft is, the more likely the storm is to undergo tornadogenesis and to produce large hail and high winds. Yellowish clouds may be seen in the troposphere in the late spring through early fall months during

forest fire A wildfire, forest fire, bushfire, wildland fire or rural fire is an unplanned, uncontrolled and unpredictable fire in an area of Combustibility and flammability, combustible vegetation. Depending on the type of vegetation present, a wildfire ...

season. The yellow color is due to the presence of pollutants in the smoke. Yellowish clouds are caused by the presence of nitrogen dioxide and are sometimes seen in urban areas with high air pollution levels. File:Sunrise In The Peak District.jpg, Stratocumulus stratiformis and small castellanus made orange by the sun rising File:Irid clouds1.jpg, An occurrence of

cloud iridescence Cloud iridescence or irisation is a colorful optical phenomenon that occurs in a cloud and appears in the general proximity of the Sun or Moon. The colors resemble those seen in soap bubbles and oil on a water surface. It is a type of photom ...

with altocumulus volutus and cirrocumulus stratiformis File:Red Color in Gray Clouds.JPG, Sunset reflecting shades of pink onto grey stratocumulus stratiformis translucidus (becoming perlucidus in the background) File:Sharp View.JPG, Stratocumulus stratiformis perlucidus before sunset.

Bangalore Bangalore (), officially Bengaluru (), is the capital and largest city of the Indian state of Karnataka. It has a population of more than and a metropolitan population of around , making it the third most populous city and fifth most ...

, India. File:Regnbyge.jpg, Late-summer

rainstorm Rain is liquid water in the form of droplets that have condensed from atmospheric water vapor and then become heavy enough to fall under gravity. Rain is a major component of the water cycle and is responsible for depositing most of the fr ...

Denmark ) , song = ( en, "King Christian stood by the lofty mast") , song_type = National and royal anthem , image_map = EU-Denmark.svg , map_caption = , subdivision_type = Sovereign state , subdivision_name = Danish Realm, Kingdom of Denmark ...

. Nearly black color of base indicates main cloud in foreground probably cumulonimbus. File:Burning Yellow Sunset.jpg, Particles in the

and the

sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...

's angle enhance colors of stratocumulus cumulogenitus at evening

twilight Twilight is light produced by sunlight scattering in the upper atmosphere, when the Sun is below the horizon, which illuminates the lower atmosphere and the Earth's surface. The word twilight can also refer to the periods of time when this il ...

Effects

Tropospheric clouds exert numerous influences on Earth's troposphere and climate. First and foremost, they are the source of precipitation, thereby greatly influencing the distribution and amount of precipitation. Because of their differential buoyancy relative to surrounding cloud-free air, clouds can be associated with vertical motions of the air that may be convective, frontal, or cyclonic. The motion is upward if the clouds are less dense because condensation of water vapor releases heat, warming the air and thereby decreasing its density. This can lead to downward motion because lifting of the air results in cooling that increases its density. All of these effects are subtly dependent on the vertical temperature and moisture structure of the atmosphere and result in major redistribution of heat that affect the Earth's climate. The complexity and diversity of clouds in the troposphere is a major reason for difficulty in quantifying the effects of clouds on climate and climate change. On the one hand, white cloud tops promote cooling of Earth's surface by reflecting shortwave radiation (visible and near infrared) from the sun, diminishing the amount of solar radiation that is absorbed at the surface, enhancing the Earth's

albedo Albedo (; ) is the measure of the diffuse reflection of sunlight, solar radiation out of the total solar radiation and measured on a scale from 0, corresponding to a black body that absorbs all incident radiation, to 1, corresponding to a body ...

. 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 longwave radiation results in increased warming at the surface. This is analogous to the

greenhouse effect The greenhouse effect is a process that occurs when energy from a planet's host star goes through the planet's atmosphere and heats the planet's surface, but greenhouse gases in the atmosphere prevent some of the heat from returning directly ...

greenhouse gases A greenhouse gas (GHG or GhG) is a gas that absorbs and emits radiant energy within the thermal infrared range, causing the greenhouse effect. The primary greenhouse gases in Earth's atmosphere are water vapor (), carbon dioxide (), methane ...

and

. High-level genus-types particularly show this duality with both short-wave

cooling and long-wave greenhouse warming effects. On the whole, ''ice-crystal'' clouds in the upper troposphere (cirrus) tend to favor net warming. However, the cooling effect is dominant with mid-level and low clouds, especially when they form in extensive sheets. Measurements by NASA indicate that on the whole, the effects of low and mid-level clouds that tend to promote cooling outweigh the warming effects of high layers and the variable outcomes associated with vertically developed clouds.

, p. 124 As difficult as it is to evaluate the influences of current clouds on current climate, it is even more problematic to predict changes in cloud patterns and properties in a future, warmer climate, and the resultant cloud influences on future climate. In a warmer climate more water would enter the atmosphere by evaporation at the surface; as clouds are formed from water vapor, cloudiness would be expected to increase. But in a warmer climate, higher temperatures would tend to evaporate clouds. Both of these statements are considered accurate, and both phenomena, known as cloud feedbacks, are found in climate model calculations. Broadly speaking, if clouds, especially low clouds, increase in a warmer climate, the resultant cooling effect leads to a negative feedback in climate response to increased greenhouse gases. But if low clouds decrease, or if high clouds increase, the feedback is positive. Differing amounts of these feedbacks are the principal reason for differences in climate sensitivities of current global climate models. As a consequence, much research has focused on the response of low and vertical clouds to a changing climate. Leading global models produce quite different results, however, with some showing increasing low clouds and others showing decreases. For these reasons the role of tropospheric clouds in regulating

and

climate Climate is the long-term weather pattern in an area, typically averaged over 30 years. More rigorously, it is the mean and variability of meteorological variables over a time spanning from months to millions of years. Some of the meteorologic ...

remains a leading source of uncertainty in

global warming In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to E ...

projections.

Polar stratospheric

Polar stratospheric cloud Polar stratospheric clouds (PSCs) are clouds in the winter polar stratosphere at altitudes of . They are best observed during civil twilight, when the Sun is between 1 and 6 degrees below the horizon, as well as in winter and in more northerly l ...

s (PSC's) form in the lowest part of the stratosphere during the

winter Winter is the coldest season of the year in polar and temperate climates. It occurs after autumn and before spring. The tilt of Earth's axis causes seasons; winter occurs when a hemisphere is oriented away from the Sun. Different cultures ...

, at the altitude and during the season that produces the coldest temperatures and therefore the best chances of triggering condensation caused by adiabatic cooling. Moisture is scarce in the stratosphere, so nacreous and non-nacreous cloud at this altitude range is restricted to polar regions in the winter where the air is coldest. PSC's show some variation in structure according to their chemical makeup and atmospheric conditions, but are limited to a single very high range of altitude of about , so they are not classified into altitude levels, genus types, species, or varieties. There is no Latin nomenclature in the manner of tropospheric clouds, but rather descriptive names using common English. Supercooled nitric acid and water PSC's, sometimes known as type 1, typically have a stratiform appearance resembling cirrostratus or haze, but because they are not frozen into crystals, do not show the pastel colours of the nacreous types. This type of PSC has been identified as a cause of ozone depletion in the stratosphere. The frozen nacreous types are typically very thin with mother-of-pearl colorations and an undulating cirriform or lenticular (stratocumuliform) appearance. These are sometimes known as type 2.

Polar mesospheric

Polar mesospheric clouds form at an extreme-level altitude range of about . They are given the Latin name noctilucent because of their illumination well after sunset and before sunrise. They typically have a bluish or silvery white coloration that can resemble brightly illuminated cirrus. Noctilucent clouds may occasionally take on more of a red or orange hue. They are not common or widespread enough to have a significant effect on climate. However, an increasing frequency of occurrence of noctilucent clouds since the 19th century may be the result of climate change. Noctilucent clouds are the highest in the atmosphere and form near the top of the mesosphere at about ten times the altitude of tropospheric high clouds. From ground level, they can occasionally be seen illuminated by the sun during deep

. Ongoing research indicates that convective lift in the mesosphere is strong enough during the polar

summer Summer is the hottest of the four temperate seasons, occurring after spring and before autumn. At or centred on the summer solstice, the earliest sunrise and latest sunset occurs, daylight hours are longest and dark hours are shortest, wit ...

to cause adiabatic cooling of small amount of water vapour to the point of saturation. This tends to produce the coldest temperatures in the entire atmosphere just below the mesopause. These conditions result in the best environment for the formation of polar mesospheric clouds. There is also evidence that smoke particles from burnt-up meteors provide much of the condensation nuclei required for the formation of noctilucent cloud. Noctilucent clouds have four major types based on physical structure and appearance. Type I veils are very tenuous and lack well-defined structure, somewhat like cirrostratus fibratus or poorly defined cirrus. Type II bands are long streaks that often occur in groups arranged roughly parallel to each other. They are usually more widely spaced than the bands or elements seen with cirrocumulus clouds. Type III billows are arrangements of closely spaced, roughly parallel short streaks that mostly resemble cirrus. Type IV whirls are partial or, more rarely, complete rings of cloud with dark centres. Distribution in the mesosphere is similar to the stratosphere except at much higher altitudes. Because of the need for maximum cooling of the water vapor to produce noctilucent clouds, their distribution tends to be restricted to polar regions of Earth. A major seasonal difference is that convective lift from below the mesosphere pushes very scarce water vapor to higher colder altitudes required for cloud formation during the respective summer seasons in the northern and southern hemispheres. Sightings are rare more than 45 degrees south of the north pole or north of the south pole.

Extraterrestrial

Cloud cover has been seen on most other planets in the

Venus Venus is the second planet from the Sun. It is sometimes called Earth's "sister" or "twin" planet as it is almost as large and has a similar composition. As an interior planet to Earth, Venus (like Mercury) appears in Earth's sky never fa ...

's thick clouds are composed of

sulfur dioxide Sulfur dioxide (IUPAC-recommended spelling) or sulphur dioxide (traditional Commonwealth English) is the chemical compound with the formula . It is a toxic gas responsible for the odor of burnt matches. It is released naturally by volcanic activ ...

(due to volcanic activity) and appear to be almost entirely stratiform. They are arranged in three main layers at altitudes of 45 to 65 km that obscure the

's surface and can produce

. No embedded cumuliform types have been identified, but broken stratocumuliform wave formations are sometimes seen in the top layer that reveal more continuous layer clouds underneath. On

Mars Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury (planet), Mercury. In the English language, Mars is named for the Mars (mythology), Roman god of war. Mars is a terr ...

, noctilucent, cirrus, cirrocumulus and stratocumulus composed of water-ice have been detected mostly near the poles. Water-ice fogs have also been detected on Mars. Both

Jupiter Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but ...

and

Saturn Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It is a gas giant with an average radius of about nine and a half times that of Earth. It has only one-eighth the average density of Earth; h ...

have an outer cirriform cloud deck composed of ammonia, an intermediate stratiform haze-cloud layer made of

ammonium hydrosulfide Ammonium hydrosulfide is the chemical compound with the formula . Composition It is the salt derived from the ammonium cation and the hydrosulfide anion. The salt exists as colourless, water-soluble, micaceous crystals. On Earth the compound ...

, and an inner deck of cumulus water clouds. Embedded cumulonimbus are known to exist near the Great Red Spot on

. The same category-types can be found covering

Uranus Uranus is the seventh planet from the Sun. Its name is a reference to the Greek god of the sky, Uranus (mythology), Uranus (Caelus), who, according to Greek mythology, was the great-grandfather of Ares (Mars (mythology), Mars), grandfather ...

, and

Neptune Neptune is the eighth planet from the Sun and the farthest known planet in the Solar System. It is the fourth-largest planet in the Solar System by diameter, the third-most-massive planet, and the densest giant planet. It is 17 times ...

, but are all composed of

. Saturn's moon Titan has cirrus clouds believed to be composed largely of methane. The ''

Cassini–Huygens ''Cassini–Huygens'' ( ), commonly called ''Cassini'', was a space research, space-research mission by NASA, the European Space Agency (ESA), and the Italian Space Agency (ASI) to send a space probe to study the planet Saturn and its system, i ...

'' Saturn mission uncovered evidence of polar stratospheric clouds and a methane cycle on Titan, including lakes near the poles and fluvial channels on the surface of the moon. Some planets outside the Solar System are known to have atmospheric clouds. In October 2013, the detection of high altitude optically thick clouds in the

exoplanet An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917 but was not recognized as such. The first confirmation of detection occurred in 1992. A different planet, init ...

Kepler-7b Kepler-7b is one of the first five exoplanets to be confirmed by NASA's Kepler spacecraft, and was confirmed in the first 33.5 days of Kepler's science operations. It orbits a star slightly hotter and significantly larger than the Sun that is exp ...

was announced, and, in December 2013, in the atmospheres of

GJ 436 b Gliese 436 b (sometimes called GJ 436 b) is a Neptune-sized exoplanet orbiting the red dwarf Gliese 436. It was the first hot Neptune discovered with certainty (in 2007) and was among the smallest-known transiting planets in mass and radius, ...

and

GJ 1214 b Gliese 1214 b (often shortened to GJ 1214 b) is an exoplanet that orbits the star Gliese 1214, and was discovered in December 2009. Its parent star is 48 light-years from the Sun, in the constellation Ophiuchus. As of 2017, GJ 1214 b is the most ...

In culture and religion

Benjamin West - Joshua passing the River Jordan with the Ark of the Covenant - Google Art Project

Clouds play an important mythical or non-scientific role in various cultures and religious traditions. The ancient

Akkadians The Akkadian Empire () was the first ancient empire of Mesopotamia after the long-lived civilization of Sumer. It was centered in the city of Akkad () and its surrounding region. The empire united Akkadian and Sumerian speakers under one rul ...

believed that the clouds (in meteorology, probably the supplementary feature ''mamma'') were the breasts of the sky goddess

Antu Antu may refer to: * Antu (goddess), a goddess, in Akkadian mythology * Antu (Mapuche mythology), the Pillan spirit in the Mapuche mythology * Antu, India, a town in Pratapgarh District, Uttar Pradesh, India * Antu County, in Jilin, China * Alpha ...

and that rain was milk from her breasts. In ,

Yahweh Yahweh *''Yahwe'', was the national god of ancient Israel and Judah. The origins of his worship reach at least to the early Iron Age, and likely to the Late Bronze Age if not somewhat earlier, and in the oldest biblical literature he posse ...

is described as guiding the

Israelites The Israelites (; , , ) were a group of Semitic-speaking tribes in the ancient Near East who, during the Iron Age, inhabited a part of Canaan. The earliest recorded evidence of a people by the name of Israel appears in the Merneptah Stele o ...

through the desert in the form of a "

pillar of cloud The pillars of fire and cloud are a dual theophany (manifestation of God) described in various places in the first five books of the Hebrew Bible. The pillars are said to have guided the Israelites through the desert during the Exodus from Egypt ...

" by day and a " pillar of fire" by night. In

Mandaeism Mandaeism (Classical Mandaic: ࡌࡀࡍࡃࡀࡉࡉࡀ ; Arabic: المندائيّة ), sometimes also known as Nasoraeanism or Sabianism, is a Gnostic, monotheistic and ethnic religion. Its adherents, the Mandaeans, revere Adam, Abel, S ...

uthra An uthra or ʿutra ( myz, ࡏࡅࡕࡓࡀ; plural: ʿutri) is a "divine messenger of the light" in Mandaeism. Charles G. Häberl and James F. McGrath translate it as "excellency". Jorunn J. Buckley defines them as "Lightworld beings, called 'utr ...

s (celestial beings) are also occasionally mentioned as being in ''anana'' ("clouds"; e.g., in ''

Right Ginza The Right Ginza is one of the two parts of the Ginza Rabba, the longest and the most important holy scripture of Mandaeism. The other part of the Ginza Rabba is the Left Ginza. Summaries of each book (or tractate), based mostly on Häberl (2007 ...

'' Book 17, Chapter 1), which can also be interpreted as female consorts. In the

ancient Greek comedy Ancient Greek comedy was one of the final three principal dramatic forms in the theatre of classical Greece (the others being tragedy and the satyr play). Athenian comedy is conventionally divided into three periods: Old Comedy, Middle Comedy, an ...

The Clouds ''The Clouds'' ( grc, Νεφέλαι ''Nephelai'') is a Greek comedy play written by the playwright Aristophanes. A lampooning of intellectual fashions in classical Athens, it was originally produced at the City Dionysia in 423BC and was not as ...

'', written by

Aristophanes Aristophanes (; grc, Ἀριστοφάνης, ; c. 446 – c. 386 BC), son of Philippus, of the deme In Ancient Greece, a deme or ( grc, δῆμος, plural: demoi, δημοι) was a suburb or a subdivision of Athens and other city-states ...

and first performed at the

City Dionysia The Dionysia (, , ; Greek: Διονύσια) was a large festival in ancient Athens in honor of the god Dionysus, the central events of which were the theatrical performances of dramatic tragedies and, from 487 BC, comedies. It was the sec ...

in 423 BC, the philosopher

Socrates Socrates (; ; –399 BC) was a Greek philosopher from Athens who is credited as the founder of Western philosophy and among the first moral philosophers of the ethical tradition of thought. An enigmatic figure, Socrates authored no te ...

declares that the Clouds are the only true deities and tells the main character Strepsiades not to worship any deities other than the Clouds, but to pay homage to them alone. In the play, the Clouds change shape to reveal the true nature of whoever is looking at them, turning into

centaur A centaur ( ; grc, κένταυρος, kéntauros; ), or occasionally hippocentaur, is a creature from Greek mythology with the upper body of a human and the lower body and legs of a horse. Centaurs are thought of in many Greek myths as being ...

s at the sight of a long-haired politician, wolves at the sight of the

embezzler Embezzlement is a crime that consists of withholding assets for the purpose of conversion of such assets, by one or more persons to whom the assets were entrusted, either to be held or to be used for specific purposes. Embezzlement is a type ...

Simon, deer at the sight of the coward Cleonymus, and mortal women at the sight of the effeminate

informer An informant (also called an informer or, as a slang term, a “snitch”) is a person who provides privileged information about a person or organization to an agency. The term is usually used within the law-enforcement world, where informan ...

Cleisthenes Cleisthenes ( ; grc-gre, Κλεισθένης), or Clisthenes (c. 570c. 508 BC), was an ancient Athenian lawgiver credited with reforming the constitution of ancient Athens and setting it on a democratic footing in 508 BC. For these accomplishm ...

. They are hailed the source of inspiration to comic poets and philosophers; they are masters of

rhetoric Rhetoric () is the art of persuasion, which along with grammar and logic (or dialectic), is one of the three ancient arts of discourse. Rhetoric aims to study the techniques writers or speakers utilize to inform, persuade, or motivate parti ...

, regarding

eloquence Eloquence (from French ''eloquence'' from Latin ''eloquentia'') is fluent, elegant, persuasive, and forceful speech, persuading an audience. Eloquence is both a natural talent and improved by knowledge of language, study of a specific subject ...

and

sophistry A sophist ( el, σοφιστής, sophistes) was a teacher in ancient Greece in the fifth and fourth centuries BC. Sophists specialized in one or more subject areas, such as philosophy, rhetoric, music, athletics, and mathematics. They taught ' ...

alike as their "friends". In China, clouds are symbols of luck and happiness. Overlapping clouds (in meteorology, probably ''duplicatus clouds'') are thought to imply eternal happiness and clouds of different colors are said to indicate "multiplied blessings". Cloud watching or cloud gazing is a popular children's activity involving watching the clouds and looking for shapes in them, a form of

pareidolia Pareidolia (; ) is the tendency for perception to impose a meaningful interpretation on a nebulous stimulus, usually visual, so that one sees an object, pattern, or meaning where there is none. Common examples are perceived images of animals, ...

References

Bibliography

* * *

External links

Current global map of total cloud water

Monthly maps of global cloud cover
from NASA's

Earth Observatory NASA Earth Observatory is an online publishing outlet for NASA which was created in 1999. It is the principal source of satellite imagery and other scientific information pertaining to the climate and the environment which are being provided by N ...

World Meteorological Organization’s (WMO) International Cloud Atlas

International Cloud Atlas The ''International Cloud Atlas'' or simply the ''Cloud Atlas'', is a cloud atlas that was first published in 1896 and has remained in print since. Its initial purposes included aiding the training of meteorologists and promoting more consisten ...

{{Authority control Articles containing video clips Clouds, fog and precipitation

Etymology

Cross classification

History of cloud science

Formation

Adiabatic cooling

Non-adiabatic cooling

Adding moisture to the air

Classification

Physical forms

Levels and genera

High-level

Mid-level

Low-level

Multi-level or moderate vertical

Towering vertical

Species

Stable or mostly stable

Ragged

Partly unstable

Unstable or mostly unstable

Varieties

Opacity-based

Pattern-based

Combinations

Other types

Precipitation-based supplementary features

Cloud-based supplementary features

Accessory clouds

Mother clouds

Other genitus and mutatus clouds

Stratocumulus fields

Vortex streets

Distribution

Convergence along low-pressure zones

Divergence along high pressure zones

Luminance, reflectivity, and coloration

Effects

Polar stratospheric

Polar mesospheric

Extraterrestrial

In culture and religion

See also

References

Bibliography

External links