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In
meteorology Meteorology is a branch of the (which include and ), with a major focus on . The study of meteorology dates back , though significant progress in meteorology did not begin until the 18th century. The 19th century saw modest progress in the f ...
, a cloud is an
aerosol An aerosol (abbreviation of "aero-solution") is a suspension Suspension or suspended may refer to: Science and engineering * Suspension (topology), in mathematics * Suspension (dynamical systems), in mathematics * Suspension of a ring, in math ...

aerosol
consisting of a visible mass of minute
liquid A liquid is a nearly incompressible In fluid mechanics Fluid mechanics is the branch of physics concerned with the mechanics Mechanics (Ancient Greek, Greek: ) is the area of physics concerned with the motions of physical objects, ...

liquid
droplets Raindrops in a plant. A drop or droplet is a small column of liquid A liquid is a nearly incompressible fluid In physics, a fluid is a substance that continually Deformation (mechanics), deforms (flows) under an applied shear stress, ...
,
frozen crystals
frozen crystals
, or other
particles In the Outline of physical science, physical sciences, a particle (or corpuscule in older texts) is a small wikt:local, localized physical body, object to which can be ascribed several physical property, physical or chemical , chemical properties ...
suspended in the
atmosphere An atmosphere (from the greek words ἀτμός ''(atmos)'', meaning 'vapour', and σφαῖρα ''(sphaira)'', meaning 'ball' or 'sphere') is a layer or a set of layers of gases surrounding a planet or other material body, that is held in ...

atmosphere
of a
planet A planet is an astronomical body orbiting a star or Stellar evolution#Stellar remnants, stellar remnant that is massive enough to be Hydrostatic equilibrium, rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and ...

planet
ary body or similar space.
Water Water (chemical formula H2O) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known li ...

Water
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 harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remaining 70.8% is Water distribution on Earth, covered wi ...

Earth
, clouds are formed as a result of saturation of the air when it is cooled to its
dew point Warmer air File:Atmosphere gas proportions.svg, Composition of Earth's atmosphere by volume, excluding water vapor. Lower pie represents trace gases that together compose about 0.043391% of the atmosphere (0.04402961% at April 2019 concentr ...
, 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 Humidity is the concentration of water vapor, water vapour present in the air. Water vap ...
(usually in the form of
water vapor (99.9839 °C) , - , Boiling point The boiling point of a substance is the temperature at which the vapor pressure 280px, The ''pistol test tube'' experiment. The tube contains alcohol and is closed with a piece of cork. By heating th ...
) from an adjacent source to raise the dew point to the
ambient Ambient or Ambiance or Ambience may refer to: Music and sound * Ambience (sound recording), also known as atmospheres or backgrounds * Ambient music, a genre of music that puts an emphasis on tone and atmosphere * Ambient (album), ''Ambient'' (albu ...
temperature. 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 Planetary means relating to a planet A planet is an astronomical body orbiting a star or Stellar evo ...
,
stratosphere The stratosphere () is the second layer of the atmosphere of the Earth, located above 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 atmosphe ...

stratosphere
, and
mesosphere The mesosphere (; from Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population i ...
. 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 The troposphere is the first and lowest layer of the atmosphere of th ...
branch of
meteorology Meteorology is a branch of the (which include and ), with a major focus on . The study of meteorology dates back , though significant progress in meteorology did not begin until the 18th century. The 19th century saw modest progress in the f ...
. There are two methods of naming clouds in their respective layers of the homosphere, Latin and common. Genus types in the troposphere, the atmospheric layer closest to Earth's surface, have
Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the Roman Republic, it became ...

Latin
names because of the universal adoption of
Luke Howard Luke Howard, (28 November 1772 – 21 March 1864) was a British manufacturing chemist A chemist (from Greek ''chēm(ía)'' alchemy; replacing ''chymist'' from Medieval Latin Medieval Latin was the form of Latin Latin (, or , ) ...

Luke Howard
's
nomenclature Nomenclature (, ) is a system A system is a group of interacting Interaction is a kind of action that occurs as two or more objects have an effect upon one another. The idea of a two-way effect is essential in the concept of interaction, a ...

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

cirrus
,
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 cloud, alt ...
,
cumulus Cumulus clouds are cloud In meteorology, a cloud is an aerosol consisting of a visible mass of minute liquid drop (liquid), droplets, ice crystals, frozen crystals, or other particulates, particles suspended in the atmosphere of a planeta ...

cumulus
, and
cumulonimbus Cumulonimbus (from Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the Roman Republ ...

cumulonimbus
. ''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 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
planet A planet is an astronomical body orbiting a star or Stellar evolution#Stellar remnants, stellar remnant that is massive enough to be Hydrostatic equilibrium, rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and ...

planet
s and
moons A natural satellite, or moon, is, in the most common usage, an astronomical body that orbit In physics, an orbit is the gravitationally curved trajectory of an physical body, object, such as the trajectory of a planet around a star or ...

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 "Sola ...

Solar System
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 A chemical formula is a way of presenting information about the chemical proportions of s that constitute a particular or molecule, using symbols, numbers, and sometimes a ...
,
ammonia Ammonia is a chemical compound, compound of nitrogen and hydrogen with the chemical formula, formula NH3. A Binary compounds of hydrogen, stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct ch ...

ammonia
, and
sulfuric acid Sulfuric acid (American spelling Despite the various English dialects spoken from country to country and within different regions of the same country, there are only slight regional variations in English orthography English orthogra ...

sulfuric acid
, as well as water. Tropospheric clouds can have a direct effect on
climate change Contemporary climate change includes both the global warming caused by humans, and its impacts on Earth's weather patterns. There have been previous periods of climate change, but the current changes are more rapid than any known even ...
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 the Earth's climate will cool or warm after a change in the climate system, for instance, how much it will warm for doubling in carbon dioxide () concentrations. In technical terms, climate sensitivity ...
.


Tabular overview

The table that follows is very broad in scope. It draws from several methods of cloud classification, both formal and informal, used in different levels of the Earth's homosphere by a number of cited authorities, especially with respect to forms, altitude levels, forms and levels, towering vertical clouds, and clouds above the troposphere. Despite some differences in nomenclature, the classification schemes seen in this article can be combined by using an informal 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. This table should not be seen as a strict or singular classification, but as an ''illustration'' of how various major cloud types are related to each other and defined through a full range of altitude levels from Earth's surface to the "edge of space."


Etymology and history of cloud science and nomenclature


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 English is a West Germanic language of the Indo-European language family The Indo-European languages are a language family A language ...
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.


Aristotle

Ancient cloud studies were not made in isolation, but were observed in combination with other
weather Weather is the state of the atmosphere An atmosphere (from the greek words ἀτμός ''(atmos)'', meaning 'vapour', and σφαῖρα ''(sphaira)'', meaning 'ball' or 'sphere') is a layer or a set of layers of gases surrounding a p ...

weather
elements and even other natural sciences. Around 340 BC, Greek philosopher
Aristotle Aristotle (; grc-gre, Ἀριστοτέλης ''Aristotélēs'', ; 384–322 BC) was a Greek philosopher A philosopher is someone who practices philosophy Philosophy (from , ) is the study of general and fundamental questio ...

Aristotle
wrote '' Meteorologica'', 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
meteorology Meteorology is a branch of the (which include and ), with a major focus on . The study of meteorology dates back , though significant progress in meteorology did not begin until the 18th century. The 19th century saw modest progress in the f ...
, 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, describes the continuous movement of water Water is an Inorganic compound, inorganic, Transparency and translucency, transparent, tasteless, odorless, ...
.


First comprehensive classification

After centuries of speculative theories about the formation and behavior of clouds, the first truly scientific studies were undertaken by
Luke Howard Luke Howard, (28 November 1772 – 21 March 1864) was a British manufacturing chemist A chemist (from Greek ''chēm(ía)'' alchemy; replacing ''chymist'' from Medieval Latin Medieval Latin was the form of Latin Latin (, or , ) ...

Luke Howard
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 Natural history is a domain of inquiry involving organisms, including animals, fu ...

Jean-Baptiste Lamarck
in France. Howard was a methodical observer with a strong grounding in the Latin language, and used his background to classify the various tropospheric cloud types during 1802. He believed that 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 (french: link=no, République française), is a transcontinental country This is a list of countries located on more than one continent A continent is one of several large landmasses ...

France
because it used unusual French names 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 German(s) may refer to: Common uses * of or related to Germany * Germans, Germanic ethnic group, citizens of Germany or people of German ancestry * For citizens of G ...

Johann Wolfgang von Goethe
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, but the discovery of clouds above the troposphere during the late 19th century eventually led to the creation of separate classification schemes using common names for these very high clouds, which were still broadly similar to some cloud forms identified in the troposphere.


Formation in the homosphere: How air becomes saturated

Terrestrial clouds can be found throughout most of the homosphere, which includes the troposphere, stratosphere, and mesosphere. Within these layers of the
atmosphere An atmosphere (from the greek words ἀτμός ''(atmos)'', meaning 'vapour', and σφαῖρα ''(sphaira)'', meaning 'ball' or 'sphere') is a layer or a set of layers of gases surrounding a planet or other material body, that is held in ...

atmosphere
, air can become saturated as a result of being cooled to its
dew point Warmer air File:Atmosphere gas proportions.svg, Composition of Earth's atmosphere by volume, excluding water vapor. Lower pie represents trace gases that together compose about 0.043391% of the atmosphere (0.04402961% at April 2019 concentr ...
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 Thermodynamics is a branch of physics that deals with heat, Work (thermodynamics), work, and temperature, and their relation to energy, radiation, and physical properties of matter. The behavior of these quantities is govern ...
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 or CCNs (also known as cloud seeds) are small particles typically 0.2 µm, or 1/100 the size of a cloud In meteorology Meteorology is a branch of the atmospheric sciences which includes atmospheric chemistry a ...
such as
salt Salt is a mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure fo ...

salt
or dust particles that are small enough to be held aloft by normal
circulation Circulation may refer to: Science and technology * Atmospheric circulation, the large-scale movement of air * Circulation (physics), the path integral of the fluid velocity around a closed curve in a fluid flow field * Circulatory system, a biolo ...

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 lift occur when
stable A stable is a building in which livestock Livestock are the domesticated Domestication is a sustained multi-generational relationship in which one group of organisms assumes a significant degree of influence over the reproduction and c ...
air is forced aloft at
weather fronts A weather front is a boundary separating air mass upright=1.25, Different air masses which affect North America as well as other continents, tend to be separated by frontal boundaries In meteorology Meteorology is a branch of the atmosphe ...

weather fronts
and around centers of
low pressure File:Low pressure system over Iceland.jpg, 250 px, This low-pressure system over Iceland spins counter-clockwise due to balance between the Coriolis and pressure gradient force. In meteorology, a low-pressure area, low area or low is a region whe ...
by a process called
convergence Convergence may refer to: Arts and media Literature *Convergence (book series), ''Convergence'' (book series), edited by Ruth Nanda Anshen *Convergence (comics), "Convergence" (comics), two separate story lines published by DC Comics: **A four-par ...
.
Warm front A warm front is a density discontinuity located at the leading edge of a homogeneous warm air mass upright=1.25, Different air masses which affect North America as well as other continents, tend to be separated by frontal boundaries In meteo ...

Warm front
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 Trough may refer to: In science * Trough (geology), a long depression less steep than a tren ...

Cold front
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. 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. A mountain differs from a plateau in having a limited summit area, and is larger than a hill, typically rising at least ...

mountain
(
orographic lift cloud pattern—analogous to a ship wake Image:Kelvin Wake Fr=2.png, 280px, Kelvin wake simulation plot. In fluid dynamics, a wake may either be: * the region of recirculating flow immediately behind a moving or stationary blunt body, caused by ...
). If the air is generally stable, nothing more than lenticular cap 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 Lightning is a naturally occurring electrostatic discharge Electrostatic discharge (ESD) is the sudden fl ...

thunderstorm
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 An aerosol (abbreviation of "aero-solution") is a suspension Suspension or suspended may refer to: Science and engineering * Suspension (topology), in mathematics * Suspension (dynamical systems), in mathematic ...

fog
. Ackerman, 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 Vaporization (or vaporisation) of an element or compound is a phase transition from the liquid phase to vapor. There are two types of vaporization: evaporation and boiling. Evaporation is a surface phe ...

evaporation
from surface water or moist ground, precipitation or
virga In meteorology Meteorology is a branch of the (which include and ), with a major focus on . The study of meteorology dates back , though significant progress in meteorology did not begin until the 18th century. The 19th century saw m ...

virga
, and
transpiration Transpiration is the process of water Water (chemical formula H2O) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Eart ...

transpiration
from plants.


Classification: How clouds are identified in the troposphere

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 Homogeneity and heterogeneity are concepts often used in the sciences and statistics Statistics ...
activity.


Stratiform

''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 In meteorology, a cloud is an aerosol consisting of a visible mass of minute liquid drop (liquid), droplets, ice crystals, frozen crystals, or othe ...
(high-level),
altostratus Altostratus is a middle altitude cloud In meteorology, a cloud is an aerosol consisting of a visible mass of minute liquid drop (liquid), droplets, ice crystals, frozen crystals, or other particulates, particles suspended in the atmosphere o ...
(mid-level), (low-level), and
nimbostratus A nimbostratus cloud is a multi-level, amorphous, nearly uniform and often dark grey cloud In meteorology, a cloud is an aerosol consisting of a visible mass of minute liquid drop (liquid), droplets, ice crystals, frozen crystals, or other p ...
(multi-level). 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 is lifted above surface level during breezy conditions.


Cirriform

Cirriform clouds in the troposphere are of the 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 ...

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 Spontaneous process, spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When t ...

convection
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 of this structure have 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 cloud In meteorology, a cloud is an aerosol consisting of a visible mass of minute liquid drop (liquid), droplets, ice crystals, frozen crystals, or other partic ...
(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 elem ...

altocumulus
(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 cloud, alt ...
(low-level).


Cumuliform

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 cloud In meteorology, a cloud is an aerosol consisting of a visible mass of minute liquid drop (liquid), droplets, ice crystals, frozen crystals, or other particulates, particles suspended in the atmosphere of a planeta ...

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


Cumulonimbiform

The largest free-convective clouds comprise the genus
cumulonimbus Cumulonimbus (from Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the Roman Republ ...

cumulonimbus
, which have 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 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 ...
es,
surface weather observation, Victoria Victoria most commonly refers to: * Victoria (Australia), a state of the Commonwealth of Australia * Victoria, British Columbia, provincial capital of British Columbia, Canada * Victoria (mythology), Roman goddess of Victory * Victoria, ...
s, and
weather map 250px, A surface weather analysis for the United States on October 21, 2006. A weather map, also known as synoptic weather chart, displays various meteorological Meteorology is a branch of the atmospheric sciences which includes atmospheric ...
s.JetStream (2008)
How to read weather maps.
National Weather Service The National Weather Service (NWS) is an agency Agency may refer to: * a governmental or other institution Institutions, according to Samuel P. Huntington, are "stable, valued, recurring patterns of behavior". Institutions can refer to m ...
. Retrieved on 16 May 2007.
The base-height range for each level varies depending on the latitudinal
geographical zone The five main latitude In geography Geography (from Greek: , ''geographia'', literally "earth description") is a field of science devoted to the study of the lands, features, inhabitants, and phenomena of the Earth and planets. T ...
. 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 zones Zone or The Zone may refer to: Places Climate and altitude zones * Death zone (originally the lethal zone), altitudes above a certain point where the amount of oxygen is insufficient to sustain ...
s, in the
temperate regions In geography Geography (from Greek: , ''geographia'', literally "earth description") is a field of science devoted to the study of the lands, features, inhabitants, and phenomena of the Earth and planets. The first person to use ...
, and in the
tropics The tropics are the region of Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remaining 70.8% ...

tropics
. 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 (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 thermal wind, air currents in the Atmosphere of Earth, atmospheres of some planets, including Earth. On Earth, the main jet streams are located near the altitude of the tropopause and are wester ...
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 cirrocumulus (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 cirrostratus (Cs): :Cirrostratus is a thin nonconvective stratiform ice crystal veil that typically gives rise to halos caused by refraction of the . 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 altocumulus (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 altostratus (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 stratocumulus (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. * Genus cumulus (Cu); species humilis – ''little vertical extent'': :These are small detached fair-weather cumuliform clouds that have nearly horizontal bases and flattened tops, and do not produce rain showers. * Genus stratus (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 Water (chemical formula H2O) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main const ...

mist
.


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 (Ns); ''multi-level'': This is a diffuse, dark grey, multi-level stratiform layer with great horizontal extent and usually moderate to deep vertical development that looks feebly illuminated from the inside. 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. Ackerman, 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 United Nations Specialized Agencies are autonomous organizations working with the United Nations The United Nations (UN) is an intergovernmental organization that aims ...
(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. * Genus cumulus (Cu); species mediocris – ''moderate vertical extent'': :These 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. * Genus cumulus (Cu); species congestus – ''great vertical extent'': :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). * Genus cumulonimbus (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
thunderstorm A thunderstorm, also known as an electrical storm or a lightning storm, is a storm characterized by the presence of lightning Lightning is a naturally occurring electrostatic discharge Electrostatic discharge (ESD) is the sudden fl ...

thunderstorm
s, local very heavy downpours of
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rain
that may cause
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s, and a variety of types of
lightning Lightning is a naturally occurring electrostatic discharge Electrostatic discharge (ESD) is the sudden flow of electricity Electricity is the set of physical phenomena associated with the presence and motion Image:Leaving Yongsan ...

lightning
including cloud-to-ground that can cause
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wildfire
s. Other convective severe weather may or may not be associated with thunderstorms and include heavy
snow Snow comprises individual ice Ice is water Water (chemical formula H2O) is an , transparent, tasteless, odorless, and , which is the main constituent of 's and the s of all known living organisms (in which it acts as a ). ...

snow
showers,
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hail
, strong
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,
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downburst
s, and
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tornado
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 Trough may refer to: In science * Trough (geology), a long depression less steep than a tren ...

cold front
s.


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 w ...
(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, in the lower left corner at the tip of the triangular formation, in the far southern Indian Ocean The Indian Ocean is the third-largest of the world's five oceanic divisions, covering or 19.8% of the water on Earth's surface. It is bounded b ...

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

convection
. 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
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, 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 ge ...
in
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Northern Australia
. Associated with a powerful "ripple" in the atmosphere, the cloud may be "surfed" in 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'. 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. 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.


Accessory clouds, supplementary features, and other derivative 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 An anvil is a metalworking Metalworking is the process of shaping and reshaping metal A metal (from Ancient Greek, Greek μέταλλον ''métallon'', "mine, quarry, metal") is a mat ...

cumulonimbus incus
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 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 w ...
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, 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
World Meteorological Organization The World Meteorological Organization (WMO) is a specialized agency United Nations Specialized Agencies are autonomous organizations working with the United Nations The United Nations (UN) is an intergovernmental organization that aims ...
(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 cloud, arcus and roll clouds, shelf cloud, wall ...
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 the World meteorological Organization is the ''flumen'', also known more informally as the ''beaver's tail''. It is formed by the warm, humid inflow 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 missile A tactical ballistic missile (TBM), or battlefield range ballistic missile (BRBM), is a ballistic missile A ballistic missile follows a ballistic trajectory to deliver on ...
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.


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').
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Contrail
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: * , 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
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honeycomb
. * 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 which is named after the engineer and fluid dynamicist
Theodore von Kármán Theodore von Kármán ( hu, (szőlőskislaki) Kármán Tódor ; 11 May 18816 May 1963) was a Hungarian-American mathematician, aerospace engineer, and physicist who was active primarily in the fields of aeronautics and astronautics. He was respon ...
,. 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: Where tropospheric clouds are most and least prevalent


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 Geography (from Greek: , ''geographia'', literally "earth description") is a field of science devoted to the study of the lands, features, inhabitants, and phenomena of the Earth and planets. The first person to use the ...

latitude
. 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 Hemisphere, Northern and Southern Hemisphere, Southern hemispheres. It is an imaginary line located at 0 degrees latitude, halfway between the N ...

equator
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 (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
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s where
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es of polar origin meet and clash with those of tropical or subtropical origin. This leads to the formation of weather-making
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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 latitude In geography Geography (from Greek: , ''geographia'', literally "earth description") is a field of science devoted to the study of the lands, features, inhabitants, and phenomena of the Earth ...
. The presence of a large-scale high-pressure
subtropical ridge The horse latitudes are the latitude In geography Geography (from Greek: , ''geographia'', literally "earth description") is a field of science devoted to the study of the lands, features, inhabitants, and phenomena of the Eart ...
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 The electromagnetic spectrum is the range of frequency, frequencies (the spectrum) of electromagnetic radiation and their respective wavelengths and photon energy, photon en ...
. 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 light efficiently, so that the intensity of the
solar radiation Solar irradiance is the power Power typically refers to: * Power (physics) In physics, power is the amount of energy transferred or converted per unit time. In the International System of Units, the unit of power is the watt, equal to one j ...
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 Altitude or height (also sometimes known as depth) is a distance measurement, usually in the vertical or "up" direction, between a reference and a point or object. The exact d ...

cloud base
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 within the portion of the electromagnetic spectrum that is visual perception, perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nan ...

light
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
rain Rain is liquid water in the form of droplet Rain water flux from a canopy. Among the forces that govern drop formation: cohesion, Van der Waals force">Cohesion_(chemistry).html" ;"title="surface tension, Cohesion (chemistry)">cohesion, ...

rain
. 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
reflectionReflection or reflexion may refer to: Philosophy * Self-reflection Science * Reflection (physics), a common wave phenomenon ** Specular reflection, reflection from a smooth surface *** Mirror image, a reflection in a mirror or in water ** Signal r ...
/
absorption Absorption may refer to: Chemistry and biology *Absorption (chemistry), diffusion of particles of gas or liquid into liquid or solid materials *Absorption (skin), a route by which substances enter the body through the skin *Absorption (pharmacolo ...
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 ...

environment
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, bushfire, wildland fire or rural fire is an unplanned, unwanted, uncontrolled fire BBQ. Fire is the rapid oxidation of a material in the exothermic chemical process of combustion, releasing heat, light, and various reaction ...

forest fire
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 In meteorology Meteorology is a branch of the atmospheric sciences which includes atmospheric chemistry and atmospheric physics, with a major f ...

cloud iridescence
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 , List of renamed Indian cities and states, officially known as Bengaluru (), is the Capital city, capital and the largest city of the Indian state of Karnataka. It has a population of more than and a metropolitan area, metropolit ...

Bangalore
, India. File:Regnbyge.jpg, Late-summer
rainstorm Rain is liquid water in the form of Drop (liquid), droplets that have condensation, condensed from atmosphere, atmospheric water vapor and then precipitation (meteorology), become heavy enough to fall under gravity. Rain is a major component ...

rainstorm
in
Denmark Denmark ( da, Danmark, ) is a Nordic country The Nordic countries, or the Nordics, are a geographical and cultural region In geography, regions are areas that are broadly divided by physical characteristics ( physical geography), hu ...

Denmark
. Nearly black color of base indicates main cloud in foreground probably cumulonimbus. File:Burning Yellow Sunset.jpg, Particles in the
atmosphere An atmosphere (from the greek words ἀτμός ''(atmos)'', meaning 'vapour', and σφαῖρα ''(sphaira)'', meaning 'ball' or 'sphere') is a layer or a set of layers of gases surrounding a planet or other material body, that is held in ...

atmosphere
and the
sun The Sun is the star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many othe ...

sun
's angle enhance colors of stratocumulus cumulogenitus at evening
twilight Twilight is the illumination of the lower atmosphere An atmosphere (from the greek words ἀτμός ''(atmos)'', meaning 'vapour', and σφαῖρα ''(sphaira)'', meaning 'ball' or 'sphere') is a layer or a set of layers of gases s ...

twilight


Effects on the troposphere, climate, and climate change

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 (prounounced ; la, albedo, meaning 'whiteness') is the measure of the diffuse reflection Diffuse reflection is the reflectionReflection or reflexion may refer to: Philosophy * Self-reflection Science * Reflection (physics), a comm ...

albedo
. 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 the process by which radiation from a planet's atmosphere warms the planet's surface to a temperature above what it would be without this atmosphere. Radiatively active gases (i.e., greenhouse gas A greenhou ...

greenhouse effect
of
greenhouse gases A greenhouse gas (GHG or GhG) is a gas Gas is one of the four fundamental states of matter In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics ...

greenhouse gases
and
water vapor (99.9839 °C) , - , Boiling point The boiling point of a substance is the temperature at which the vapor pressure 280px, The ''pistol test tube'' experiment. The tube contains alcohol and is closed with a piece of cork. By heating th ...
. High-level genus-types particularly show this duality with both short-wave
albedo Albedo (prounounced ; la, albedo, meaning 'whiteness') is the measure of the diffuse reflection Diffuse reflection is the reflectionReflection or reflexion may refer to: Philosophy * Self-reflection Science * Reflection (physics), a comm ...

albedo
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. Ackerman, 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
weather Weather is the state of the atmosphere An atmosphere (from the greek words ἀτμός ''(atmos)'', meaning 'vapour', and σφαῖρα ''(sphaira)'', meaning 'ball' or 'sphere') is a layer or a set of layers of gases surrounding a p ...

weather
and
climate Climate is the long-term pattern of weather Weather is the state of the atmosphere An atmosphere (from the greek words ἀτμός ''(atmos)'', meaning 'vapour', and σφαῖρα ''(sphaira)'', meaning 'ball' or 'sphere') is a la ...

climate
remains a leading source of uncertainty in
global warming Contemporary climate change includes both the global warming caused by humans, and its impacts on Earth's weather patterns. There have been previous periods of climate change, but the current changes are more rapid than any known even ...

global warming
projections.


Polar stratospheric

Polar stratospheric cloud Polar stratospheric clouds (PSCs) are cloud In meteorology Meteorology is a branch of the atmospheric sciences Atmospheric science is the study of the Earth's atmosphere File:Atmosphere gas proportions.svg, Composition of Earth's ...
s (PSC's) form in the lowest part of the stratosphere during the
winter Winter is the coldest season of the year in Polar regions of Earth, polar and temperate climate, temperate zones. It occurs between autumn and spring (season), spring. The tilt of Earth's axis causes seasons; winter occurs when a Hemisphere ...

winter
, 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 cloud . Image:A Day of Night-Shining Clouds.png, These images show measurements of polar mesospheric clouds over the course of one day. Polar mesospheric clouds (PMCs) are a diffuse scattering layer of water ice crystals near the summer polar mesopause ...
s form at an extreme-level altitude range of about . They are given the Latin name 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
twilight Twilight is the illumination of the lower atmosphere An atmosphere (from the greek words ἀτμός ''(atmos)'', meaning 'vapour', and σφαῖρα ''(sphaira)'', meaning 'ball' or 'sphere') is a layer or a set of layers of gases s ...

twilight
. Ongoing research indicates that convective lift in the mesosphere is strong enough during the polar
summer Summer is the hottest of the four temperate In geography Geography (from Greek: , ''geographia'', literally "earth description") is a field of science devoted to the study of the lands, features, inhabitants, and phenom ...

summer
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 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
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 "Sola ...

Solar System
.
Venus Venus is the second planet from the Sun. It is named after the Venus (mythology), Roman goddess of love and beauty. As List of brightest natural objects in the sky, the brightest natural object in Earth's night sky after the Moon, Venus can ...

Venus
's thick clouds are composed of
sulfur dioxide Sulfur dioxide (IUPAC The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering OrganizationsNational Adhering Organizations in chemistry are the organizations that work as the autho ...
(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
planet A planet is an astronomical body orbiting a star or Stellar evolution#Stellar remnants, stellar remnant that is massive enough to be Hydrostatic equilibrium, rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and ...

planet
's surface and can produce
virga In meteorology Meteorology is a branch of the (which include and ), with a major focus on . The study of meteorology dates back , though significant progress in meteorology did not begin until the 18th century. The 19th century saw m ...

virga
. 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, being larger than only Mercury (planet), Mercury. In English, Mars carries the name of the Mars (mythology), Roman god of war and is often referred to ...

Mars
, 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 ...

Jupiter
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 only has one-eighth the average density of Earth; how ...

Saturn
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 A chemical compound is a chemical substance composed of many identical molecules (or molecular entity, molecular entities) composed of atoms from more than one chemical element, element held togeth ...
, and an inner deck of cumulus water clouds. Embedded cumulonimbus are known to exist near the
Great Red Spot The Great Red Spot is a persistent high-pressure region in the atmosphere of Jupiter The atmosphere of Jupiter is the largest planetary atmosphere in the Solar System. It is mostly made of molecular hydrogen and helium in roughly Sun#Compositi ...
on
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 ...

Jupiter
. 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, who, according to Greek mythology Greek mythology is the body of myths originally told by the Ancient Greece, ancient Greeks, and ...

Uranus
, and
Neptune Neptune is the eighth and farthest-known Solar planet from the Sun. In the Solar System, it is the fourth-largest planet by diameter, the third-most-massive planet, and the densest giant planet. It is 17 times the mass of Earth, slightly mo ...

Neptune
, but are all composed of
methane Methane (, ) is a chemical compound with the chemical formula A chemical formula is a way of presenting information about the chemical proportions of s that constitute a particular or molecule, using symbols, numbers, and sometimes a ...
. Saturn's moon
Titan Titan most often refers to: * Titan (moon), the largest moon of Saturn * Titans, a race of deities in Greek mythology Titan or Titans may also refer to: Arts and entertainment Fictional entities Fictional locations * Titan in fiction, fictional ...
has cirrus clouds believed to be composed largely of methane. The ''
Cassini–Huygens The ''Cassini–Huygens'' space-research mission ( ), commonly called ''Cassini'', involved a collaboration among NASA The National Aeronautics and Space Administration (NASA; ) is an independent agencies of the United States government, ...
'' 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
atmosphere An atmosphere (from the greek words ἀτμός ''(atmos)'', meaning 'vapour', and σφαῖρα ''(sphaira)'', meaning 'ball' or 'sphere') is a layer or a set of layers of gases surrounding a planet or other material body, that is held in ...

atmosphere
of
exoplanet An exoplanet or extrasolar planet is a outside the . 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. This was followed by the confirmation of a ...

exoplanet
Kepler-7b was announced, and, in December 2013, in the atmospheres of GJ 436 b and .


In culture and religion

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 Mesopotamia ( grc, Μεσοποταμία ''Mesopotamíā''; ar, بِلَاد ٱلرَّافِدَيْن ; syc, ܐܪܡ ܢܗܪ̈ܝܢ, or , ) is a historical region of ...
believed that the clouds (in meteorology,probably the supplementary feature ''mamma'') were the breasts of the sky goddess Antu and that rain was milk from her breasts. In ,
Yahweh Yahweh was the national god of ancient Kingdom of Israel (Samaria), Israel and Kingdom of Judah, Judah. His origins reach at least to the early Iron Age, and likely to the Late Bronze Age. In the oldest biblical literature, he is a Weather ...
is described as guiding the
Israelites The Israelites (; ) were a confederation of Iron Age ancient Semitic-speaking peoples, Semitic-speaking tribes of the ancient Near East, who inhabited a part of Canaan during the history of ancient Israel and Judah, tribal and monarchic peri ...

Israelites
through the desert in the form of a "" by day and a " pillar of fire" by night. In the
ancient Greek comedy Ancient Greek comedy was one of the final three principal drama Drama is the specific Mode (literature), mode of fiction Mimesis, represented in performance: a Play (theatre), play, opera, mime, ballet, etc., performed in a theatre, or on R ...
''
The Clouds ''The Clouds'' ( grc, Νεφέλαι ''Nephelai'') is a Greek comedy Ancient Greek comedy was one of the final three principal drama Drama is the specific Mode (literature), mode of fiction Mimesis, represented in performance: a Play (the ...

The Clouds
'', written by
Aristophanes Aristophanes (; grc, Ἀριστοφάνης, ; c. 446 – c. 386 BC), son of Philippus, of the deme 250px, Pinakia, identification tablets (name, father's name, deme) used for tasks like jury selection, Museum at the Ancient Agora of Athe ...

Aristophanes
and first performed at the
City Dionysia The Dionysia () (Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approxim ...
in 423 BC, the philosopher
Socrates Socrates (; ; –399 BC) was a Greek philosopher from Athens Athens ( ; el, Αθήνα, Athína ; grc, Ἀθῆναι, Athênai (pl.) ) is the capital city, capital and List of cities in Greece, largest city of Greece. Athens domi ...

Socrates
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 Greek mythology is the body of myth Myth is a folklore genre Folklore is the expressive body of culture share ...

centaur
s at the sight of a politician, wolves at the sight of the embezzler Simon, deer at the sight of the coward Cleonymus, and mortal women at the sight of the effeminate informer
Cleisthenes Cleisthenes ( ; grc-gre, Κλεισθένης, Kleisthénēs, ) or Clisthenes ( la, Clīsthenēs ) was an ancient Athenian lawgiver credited with reforming the constitution of ancient Athens , image_skyline = File:Ath ...
. They are hailed the source of inspiration to comic poets and philosophers; they are masters of
rhetoric Rhetoric () is the Art (skill), art of persuasion, which along with grammar and logic (or dialectic – see Martianus Capella), is one of the Trivium, three ancient arts of discourse. Rhetoric aims to study the techniques writers or sp ...
, regarding
eloquence ''Eloquence'' (from French ''eloquence'' from Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Lati ...
and
sophistry A sophist ( el, σοφιστής, ''sophistes'') was a teacher in ancient Greece Ancient Greece ( el, Ἑλλάς, Hellás) was a civilization belonging to a period of History of Greece, Greek history from the Greek Dark Ages of the 12th–9 ...
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 (physiology), stimulus, usually visual, so that one sees an object, pattern, or meaning where there is none. Common examples are Cloud#In c ...

pareidolia
.


See also

*
Atmospheric Radiation Measurement The Atmospheric Radiation Measurement Climate Research Facility (ARM Climate Research Facility) is a United States Department of Energy scientific user facility for the study of global climate change by the national and international research comm ...
(ARM) (US) * Bioprecipitation * Ceiling (cloud), Ceiling * Cloud albedo * Cloud Appreciation Society * Cloud cover * Cloud forcing * Cloud seeding * Clouds (sculpture) * Cloudscape (art) * Cloudscape photography * Coalescence (meteorology), Coalescence * Extraterrestrial sky * Fog * Mist * Mushroom cloud * Pileus (meteorology) * Precipitation * Sunshine duration * Undulatus asperatus * Weather lore


References


Bibliography

*


External links


Current global map of total cloud water

Monthly maps of global cloud cover
from NASA's Earth Observatory
World Meteorological Organization’s (WMO) International Cloud Atlas
International Cloud Atlas {{Authority control Clouds, Articles containing video clips Clouds, fog and precipitation