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{{Short description|Visible mass of particles suspended in the atmosphere}}
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[[File:ISS-40 Thunderheads near Borneo.jpg|alt=|thumb|upright=1.5|Cloudscape over [[Borneo]], taken by the [[International Space Station]]]]
{{Weather}}
In [[meteorology]], a '''cloud''' is an [[aerosol]] consisting of a visible mass of miniature [[liquid]] [[droplets]], [[ice crystals|frozen crystals]], or other [[particulates|particles]], suspended in the [[atmosphere]] of a [[planet]]ary body or similar space.<ref>{{Cite web |title=Weather Terms |url=http://www.erh.noaa.gov/box/glossary.htm |access-date=21 June 2013 |publisher=[[National Weather Service]]}}</ref> [[Water]] or various other chemicals may compose the droplets and crystals. On [[Earth]], clouds are formed as a result of saturation of the [[air]] when it is cooled to its [[dew point]], or when it gains sufficient [[moisture]] (usually in the form of [[water vapor]]) from an adjacent source to raise the dew point to the ambient [[temperature]].

Clouds are seen in the Earth's [[homosphere]], which includes the [[troposphere]], [[stratosphere]], and [[mesosphere]].
'''Nephology''' is the science of clouds, which is undertaken in the [[cloud physics]] branch of [[meteorology]]. The World Meteorological Organization uses two methods of naming clouds in their respective layers of the homosphere, [[Latin]] and [[common name]].

[[Genus]] types in the troposphere, the atmospheric layer closest to Earth's surface, have [[Latin]] names because of the universal adoption of [[Luke Howard]]'s [[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 five main forms are [[stratus cloud|stratiform]] sheets or veils, [[cumuliform]] heaps, [[stratocumulus cloud|stratocumuliform]] bands, rolls, or ripples, [[cumulonimbus cloud|cumulonimbiform]] towers often with fibrous tops, and [[cirrus cloud|cirriform]] wisps or patches. ''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 the case of stratocumuliform clouds, the prefix ''strato-'' is applied to the low-level genus type but is dropped from the mid- and high-level variants to avoid double-prefixing with alto- and cirro-. 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 also have common names for their main types. They may have the appearance of veils or sheets, wisps, or bands or ripples, but not heaps or towers as in the troposphere. They are seen infrequently, mostly in the polar regions of Earth. Clouds have been observed in the atmospheres of other [[planet]]s and [[moons]] in the [[Solar System]] and beyond. However, due to their different temperature characteristics, they are often composed of other substances such as [[methane]], [[ammonia]], and [[sulfuric acid]], as well as water.

Tropospheric clouds can have a direct effect on [[climate change]] 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 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 the 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]].<ref>{{Cite web |last1=Ceppi |first1=Paulo |last2=Williams |first2=Ric |title=Why clouds are the missing piece in the climate change puzzle |url=http://theconversation.com/why-clouds-are-the-missing-piece-in-the-climate-change-puzzle-140812 |access-date=21 January 2021 |website=The Conversation |date=11 September 2020 |language=en}}</ref>

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== Etymology ==
The origin of the term "cloud" can be found in the [[Old English]] words {{Lang|ang|clud}} or ''{{Lang|ang|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 ''{{lang|ang|weolcan}}'', which had been the literal term for clouds in general.<ref name="Harper">{{Cite web |last=Harper, Douglas |year=2012 |title=Cloud |url=http://www.etymonline.com/index.php?term=cloud |access-date=13 November 2014 |website=Online Etymology Dictionary}}</ref><ref>{{Cite web |title=Cloud |url=http://www.thefreedictionary.com/cloud |access-date=13 November 2014 |website=The Free Dictionary |publisher=Farlex}}</ref>

== Homospheric nomenclatures and cross-classification ==
The table that follows is very broad in scope like the cloud genera template upon which it is partly based. There are some variations in styles of nomenclature between the classification scheme used for the troposphere (strict Latin except for surface-based aerosols) and the method used for the higher levels of the homosphere (common terms, some informally derived from Latin). However, these two schemes, both of which are authorized and used by the World Meteorological Organization, share a cross-classification of physical forms and altitude levels to derive the 10 tropospheric genera,<ref name="identification guide">{{Cite web |year=2017 |editor-last=World Meteorological Organization |title=Cloud Identification Guide, International Cloud Atlas |url=https://cloudatlas.wmo.int/cloud-identification-guide.html |access-date=4 April 2017}}</ref> 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.

{| class="wikitable"
!{{diagonal split header|<br />Level<ref name="Definitions">{{Cite web |year=2017 |editor-last=World Meteorological Organization |title=Definitions, International Cloud Atlas |url=https://cloudatlas.wmo.int/clouds-definitions.html |access-date=30 March 2017}}</ref>|Form<ref name="LANDSAT identification">{{Cite web |last=E.C. Barrett and C.K. Grant |year=1976 |title=The identification of cloud types in LANDSAT MSS images |url=https://ntrs.nasa.gov/citations/19760014556 |publisher=[[NASA]] |access-date=22 August 2012}}</ref>}}
!Stratiform <br> non-convective
!Cirriform <br> mostly non-convective
!Stratocumuliform <br> limited-convective
!Cumuliform <br> free-convective
!Cumulonimbiform <br> strong-convective
|-
!Extreme-level
|[[Noctilucent]] veils ||Noctilucent billows or whirls ||Noctilucent bands || ||
|-
!Very high-level<ref name="Polar-stratospheric">{{Cite web |year=2017 |editor-last=World Meteorological Organization |title=Upper atmospheric clouds, International Cloud Atlas |url=https://cloudatlas.wmo.int/upper-atmospheric-clouds.html |access-date=31 July 2017}}</ref>
|[[Nitric acid]] and [[water]] [[Polar stratospheric cloud|PSC]] veils||[[Cirrus cloud|Cirriform]] [[Nacreous cloud|nacreous PSC]]||[[Lenticular cloud|Lenticular]] [[Nacreous cloud|nacreous PSC]]|| ||
|-
!High-level
|[[Cirrostratus]]||[[Cirrus cloud|Cirrus]]||[[Cirrocumulus]]|| ||
|-
!Mid-level
|[[Altostratus]]|| ||[[Altocumulus]]|| ||
|-
!Towering vertical<ref name="automated Cb and Tcu detection">{{Cite web |last1=de Valk, Paul |last2=van Westhrenen, Rudolf |last3=Carbajal Henken, Cintia |year=2010 |title=Automated CB and TCU detection using radar and satellite data: from research to application |url=http://www.eumetsat.int/Home/Main/AboutEUMETSAT/Publications/ConferenceandWorkshopProceedings/2010/groups/cps/documents/document/pdf_conf_p57_s7_08_devalk_v.pdf |url-status=dead |archive-url=https://web.archive.org/web/20111116001457/http://www.eumetsat.int/Home/Main/AboutEUMETSAT/Publications/ConferenceandWorkshopProceedings/2010/groups/cps/documents/document/pdf_conf_p57_s7_08_devalk_v.pdf |archive-date=16 November 2011 |access-date=15 September 2011}}</ref>
| || || ||[[Cumulus congestus]]||[[Cumulonimbus]]
|-
!Multi-level or moderate vertical
|[[Nimbostratus]]|| || || [[Cumulus mediocris]]||
|-
!Low-level
|[[Stratus cloud|Stratus]]|| ||[[Stratocumulus]]||[[Cumulus humilis]] or [[fractus]]||
|-
!Surface-level
|[[Fog]] or [[mist]]|| || || ||
|}

== History of cloud science ==
{{Main|Timeline of meteorology}}Ancient cloud studies were not made in isolation, but were observed in combination with other [[weather]] elements and even other natural sciences. Around 340 BC, Greek philosopher [[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]], 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]].<ref name="Meteorologica">{{Cite journal |last=Frisinger |first=H. Howard |year=1972 |title=Aristotle and his ''Meteorologica'' |journal=Bulletin of the American Meteorological Society |volume=53 |pages=634 |doi=10.1175/1520-0477(1972)053<0634:AAH>2.0.CO;2 |issn=1520-0477 |doi-access=free}}</ref>[[File:Cloud_types_en.svg|thumb|upright=1.35|Tropospheric cloud classification by altitude of occurrence. Multi-level and vertical genus-types are not limited to a single altitude level; these include nimbostratus, cumulonimbus, and some of the larger cumulus species.|alt=g]]
After centuries of speculative theories about the formation and behavior of clouds, the first truly scientific studies were undertaken by [[Luke Howard]] in England and [[Jean-Baptiste Lamarck]] in France. Howard was a methodical observer with a strong grounding in the Latin language, and used his background to formally classify the various tropospheric cloud types during 1802. He believed that scientific observations of the changing cloud forms in the sky could unlock the key to weather forecasting.

Lamarck had worked independently on cloud classification the same year and had come up with a different naming scheme that failed to make an impression even in his home country of [[France]] because it used unusually descriptive and informal French names and phrases for cloud types. His system of nomenclature included 12 categories of clouds, with such names as (translated from French) hazy clouds, dappled clouds, and broom-like clouds. By contrast, Howard used universally accepted Latin, which caught on quickly after it was published in 1803.<ref name="Preface">{{Cite book |url=https://archive.org/details/manualonobservat00worl/page/ |title=International Cloud Atlas, preface to the 1939 edition |year=1975 |isbn=978-92-63-10407-6 |editor-last=World Meteorological Organization |volume=I |pages=[https://archive.org/details/manualonobservat00worl/page/ IX–XIII] |publisher=Secretariat of the World Meteorological Organization |access-date=6 December 2014}}</ref> As a sign of the popularity of the naming scheme, German dramatist and poet [[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.<ref name="Preface" /> This system covered only the tropospheric cloud types.  However, the discovery of clouds above the troposphere during the late 19th century eventually led to the creation of separate classification schemes that reverted to the use of descriptive common names and phrases that somewhat recalled Lamarck's methods of classification.  These very high clouds, although classified by these different methods, are nevertheless broadly similar to some cloud forms identified in the troposphere with Latin names.<ref name="Polar-stratospheric" />

== Formation ==
{{main|Cloud physics}}
Terrestrial clouds can be found throughout most of the homosphere, which includes the troposphere, stratosphere, and mesosphere. Within these layers of the [[atmosphere]], air can become saturated as a result of being cooled to its [[dew point]] or by having moisture added from an adjacent source.<ref name="auto">{{Cite web |last1=Bart van den Hurk |last2=Eleanor Blyth |year=2008 |title=Global maps of Local Land-Atmosphere coupling |url=http://www.knmi.nl/~hurkvd/Loco_workshop/Workshop_report.pdf |url-status=dead |archive-url=https://web.archive.org/web/20090225074154/http://www.knmi.nl/~hurkvd/Loco_workshop/Workshop_report.pdf |archive-date=25 February 2009 |access-date=2 January 2009 |publisher=KNMI}}</ref> In the latter case, saturation occurs when the dew point is raised to the ambient air temperature.

===Adiabatic cooling===
[[Adiabatic lapse rate#Dry adiabatic lapse rate|Adiabatic cooling]] 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.<ref name="adiabatic process">{{Cite web |last=Nave, R. |year=2013 |title=Adiabatic Process |url=http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/adiab.html |access-date=18 November 2013 |publisher=gsu.edu}}</ref> 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]] such as [[salt]] or dust particles that are small enough to be held aloft by normal [[Atmospheric circulation|circulation]] of the air.<ref name="humidity, saturation, and stability">{{Cite web |last=Elementary Meteorology Online |year=2013 |title=Humidity, Saturation, and Stability |url=http://apollo.lsc.vsc.edu/~wintelsw/MET1010LOL/chapter06/ |url-status=dead |archive-url=https://web.archive.org/web/20140502055741/http://apollo.lsc.vsc.edu/~wintelsw/MET1010LOL/chapter06/ |archive-date=2 May 2014 |access-date=18 November 2013 |publisher=vsc.edu}}</ref><ref name="cloud drops">{{Cite web |last=Horstmeyer, Steve |year=2008 |title=Cloud Drops, Rain Drops |url=http://www.shorstmeyer.com/wxfaqs/float/float.html |access-date=19 March 2012}}</ref>

[[File:Cloud evolution in under a minute.ogv|thumb|upright=1.35|Animation of cloud evolution from cumulus humilis to cumulonimbus capillatus incus]]
One agent is the convective upward motion of air caused by daytime solar heating at surface level.<ref name="humidity, saturation, and stability" /> Low level airmass instability allows for the formation of cumuliform clouds in the troposphere that can produce showers if the air is sufficiently moist.<ref>{{Cite journal |last1=Freud |first1=E. |last2=Rosenfeld |first2=D. |year=2012 |title=Linear relation between convective cloud drop number concentration and depth for rain initiation |journal=Journal of Geophysical Research |volume=117 |issue=D2 |pages=n/a |bibcode=2012JGRD..117.2207F |doi=10.1029/2011JD016457 |doi-access=free | issn=0148-0227}}</ref> On moderately rare occasions, convective lift can be powerful enough to penetrate the tropopause and push the cloud top into the stratosphere.<ref name="Tropopause penetrations">{{Cite web |last1=Long, Michael J. |last2=Hanks, Howard H. |last3=Beebe, Robert G. |date=June 1965 |title=TROPOPAUSE PENETRATIONS BY CUMULONIMBUS CLOUDS |url=http://oai.dtic.mil/oai/oai?verb=getRecord&metadataPrefix=html&identifier=AD0621573 |url-status=dead |archive-url=https://web.archive.org/web/20160303235551/http://oai.dtic.mil/oai/oai?verb=getRecord&metadataPrefix=html&identifier=AD0621573 |archive-date=3 March 2016 |access-date=9 November 2014}}</ref>

Frontal and [[cyclonic]] lift occur in the troposphere when [[Atmospheric instability|stable]] air is forced aloft at [[weather fronts]] and around centers of [[low pressure]] by a process called [[convergence zone|convergence]].<ref name="frontal clouds">{{Cite web |last=Elementary Meteorology Online |year=2013 |title=Lifting Along Frontal Boundaries |url=http://ww2010.atmos.uiuc.edu/%28Gh%29/guides/mtr/cld/dvlp/frnt.rxml/~wintelsw/MET1010LOL/chapter06/ |access-date=20 March 2015 |publisher=vsc.edu}}</ref> [[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.<ref name="Mackerel sky" /> [[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.<ref name="G&N:207-212">{{Cite book |last1=Lee M. Grenci |url=https://books.google.com/books?id=oh8lqM5obuYC&pg=PA212 |title=A World of Weather: Fundamentals of Meteorology: A Text / Laboratory Manual |last2=Jon M. Nese |publisher=Kendall/Hunt Publishing Company |year=2001 |isbn=978-0-7872-7716-1 |edition=3 |pages=207–212 |oclc=51160155}}</ref>

[[File:Dreamy Twilight.jpg|thumb|Windy evening [[twilight]] enhanced by the Sun's angle. Clouds can visually mimic a [[tornado]] resulting from orographic lift.]]
A third source of lift is wind circulation forcing air over a physical barrier such as a [[mountain]] ([[orographic lift]]).<ref name="humidity, saturation, and stability" /> 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]]s may appear.<ref name="MT">Pidwirny, M. (2006). [http://www.physicalgeography.net/fundamentals/8e.html "Cloud Formation Processes"] {{webarchive|url=https://web.archive.org/web/20081220230524/http://www.physicalgeography.net/fundamentals/8e.html |date=20 December 2008 }}, chapter 8 in ''Fundamentals of Physical Geography'', 2nd ed.</ref>

Clouds formed by any of these lifting agents are initially seen in the troposphere where these agents are most active.  However, water vapor that has been lifted to the top of troposphere can be carried even higher by gravity waves where further condensation can result in the formation of clouds in the stratosphere and mesosphere.<ref name=Atoptics>[http://www.atoptics.co.uk/highsky/nlc2.htm About NLCs, Polar Mesospheric Clouds], from Atmospheric optics</ref>

===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]].<ref name="fog formation">[[#Ackerman|Ackerman]], p. 109</ref><ref>{{Cite web |last=Glossary of Meteorology |year=2009 |title=Radiational cooling |url=http://amsglossary.allenpress.com/glossary/search?p=1&query=radiational+cooling&submit=Search |url-status=dead |archive-url=https://web.archive.org/web/20110512161339/http://amsglossary.allenpress.com/glossary/search?p=1&query=radiational+cooling&submit=Search |archive-date=12 May 2011 |access-date=27 December 2008 |publisher=[[American Meteorological Society]]}}</ref><ref>{{Cite web |last=Fovell, Robert |year=2004 |title=Approaches to saturation |url=http://www.atmos.ucla.edu/~fovell/AS3downloads/saturation.pdf |url-status=dead |archive-url=https://web.archive.org/web/20090225074155/http://www.atmos.ucla.edu/~fovell/AS3downloads/saturation.pdf |archive-date=25 February 2009 |access-date=7 February 2009 |publisher=[[University of California in Los Angeles]]}}</ref>

===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]] from surface water or moist ground,<ref name="convection">{{Cite book |last=Pearce, Robert Penrose |url=https://books.google.com/books?id=QECy_UBdyrcC&pg=PA66 |title=Meteorology at the Millennium |publisher=Academic Press |year=2002 |isbn=978-0-12-548035-2 |page=66}}</ref><ref name="auto"/><ref>{{Cite web |last=JetStream |year=2008 |title=Air Masses |url=http://www.srh.weather.gov/srh/jetstream/synoptic/airmass.htm |url-status=dead |archive-url=https://web.archive.org/web/20081224062959/http://www.srh.weather.gov/srh/jetstream/synoptic/airmass.htm |archive-date=24 December 2008 |access-date=2 January 2009 |publisher=[[National Weather Service]]}}</ref> precipitation or [[virga]],<ref>{{Cite web |last=[[National Weather Service]] Office |year=2009 |title=Virga and Dry Thunderstorms |url=http://www.wrh.noaa.gov/otx/outreach/ttalk/virga.php |access-date=2 January 2009 |location=Spokane, Washington |publisher=[[National Oceanic and Atmospheric Administration]]}}</ref> and [[transpiration]] from plants.<ref>{{Cite book |last1=Reiley, H. Edward |url=https://books.google.com/books?id=jZvsnsLIkNsC&pg=PA40 |title=Introductory horticulture |last2=Shry, Carroll L. |publisher=Cengage Learning |year=2002 |isbn=978-0-7668-1567-4 |page=40}}</ref>

== Tropospheric classification ==
{{Further|List of cloud types}}
Classification in the troposphere is based on a hierarchy of categories with physical forms and altitude levels at the top.<ref name="LANDSAT identification" /><ref name="Definitions"/> 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.<ref name="Principles">{{Cite web |year=2017 |editor-last=World Meteorological Organization |title=Principles, International Cloud Atlas |url=https://cloudatlas.wmo.int/principles-of-cloud-classification.html |access-date=9 May 2017}}</ref>

[[File:Cirrus-fibratus.jpg|thumb|[[Cirrus fibratus]] clouds in March]]
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.<ref>{{Cite web |last1=E.C. Barrett |last2=C.K. Grant |year=1976 |title=The identification of cloud types in LANDSAT MSS images |url=https://ntrs.nasa.gov/search.jsp?R=19760014556 |publisher=[[NASA]] |access-date=22 August 2012}}</ref> They are given below in approximate ascending order of instability or [[Atmospheric convection|convective]] activity.<ref name="meteorology">{{Cite web |year=2016 |editor-last=Pilotfriend |title=Meteorology |url=http://www.pilotfriend.com/av_weather/meteo/clouds.htm |access-date=19 March 2016 |publisher=Pilotfriend}}</ref>

* Nonconvective [[Stratus cloud|stratiform clouds]] appear in stable airmass conditions and, in general, have flat, sheet-like structures that can form at any altitude in the troposphere.<ref name="Stratiform">{{Cite web |year=2015 |editor-last=NASA |title=Stratiform or Stratus Clouds |url=http://www.grc.nasa.gov/WWW/k-12/Aero2000/studweb/glossary/stratcld.html |access-date=23 January 2015 |archive-date=23 January 2015 |archive-url=https://web.archive.org/web/20150123141538/http://www.grc.nasa.gov/WWW/k-12/Aero2000/studweb/glossary/stratcld.html |url-status=dead }}</ref> The stratiform group is divided by altitude range into the genera [[cirrostratus]] (high-level), [[altostratus]] (mid-level), [[stratus cloud|stratus]] (low-level), and [[nimbostratus]] (multi-level).<ref name="Definitions" /> Fog is commonly considered a surface-based cloud layer.<ref name="MT" /> 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.
* [[Cirrus cloud|Cirriform clouds]] in the troposphere are of the genus cirrus and have the appearance of detached or semi-merged 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]] where the air is partly unstable.<ref name="cirrus">{{Cite web |year=2017 |editor-last=World Meteorological Organization |title=Cirrus, International Cloud Atlas |url=https://cloudatlas.wmo.int/clouds-genera-cirrus.html |access-date=16 May 2017}}</ref> Clouds resembling cirrus, cirrostratus, and cirrocumulus can be found above the troposphere but are classified separately using common names.
* [[Stratocumulus cloud|Stratocumuliform clouds]] both cumuliform and stratiform characteristics in the form of rolls, ripples, or elements.<ref name="identification guide" /> They generally form as a result of limited convection in an otherwise mostly stable airmass topped by an inversion layer.<ref name="Multi-regime convection">{{Cite journal |last1=Laufersweiler |first1=M. J. |last2=Shirer |first2=H. N. |year=1995 |title=A theoretical model of multi-regime convection in a stratocumulus-topped boundary layer |journal=Boundary-Layer Meteorology |volume=73 |issue=4 |pages=373–409 |bibcode=1995BoLMe..73..373L |doi=10.1007/BF00712679 |s2cid=123031505}}</ref> 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.<ref name="castellanus">{{Cite web |year=2017 |editor-last=World Meteorological Organization |title=Altocumulus Castellanus, International Cloud Atlas |url=https://cloudatlas.wmo.int/species-altocumulus-castellanus-ac-cas.html |access-date=4 April 2017}}</ref> The stratocumuliform group is divided into [[cirrocumulus]] (high-level, strato- prefix dropped), [[altocumulus]] (mid-level, strato- prefix dropped), and [[stratocumulus]] (low-level).<ref name="identification guide" />
* [[Cumulus cloud|Cumuliform clouds]] generally appear in isolated heaps or tufts.<ref name="USA Today">{{Cite news |date=16 October 2005 |title=Cumulus clouds |work=Weather |publisher=[[USA Today]] |url=http://usatoday30.usatoday.com/weather/wcumulus.htm |access-date=16 October 2012}}</ref><ref name="Stommel91">{{Cite journal |last=Stommel |first=H. |year=1947 |title=Entrainment of Air into a Cumulus Cloud |journal=Journal of Meteorology |volume=4 |issue=3 |pages=91–94 |bibcode=1947JAtS....4...91S |doi=10.1175/1520-0469(1947)004<0091:EOAIAC>2.0.CO;2 |doi-access=free}}</ref> 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.<ref name="Mossop632-634">{{Cite journal |last1=Mossop |first1=S. C. |last2=Hallett |first2=J. |year=1974 |title=Ice Crystal Concentration in Cumulus Clouds: Influence of the Drop Spectrum |journal=Science |volume=186 |issue=4164 |pages=632–634 |bibcode=1974Sci...186..632M |doi=10.1126/science.186.4164.632 |pmid=17833720 |s2cid=19285155}}</ref> Depending on their vertical size, clouds of the [[cumulus]] genus type may be low-level or multi-level with moderate to towering vertical extent.<ref name="Definitions" />
* [[Cumulonimbus clouds]] are largest free-convective clouds, which has a towering vertical extent. They occur in highly unstable air<ref name="humidity, saturation, and stability" /> and often have fuzzy outlines at the upper parts of the clouds that sometimes include anvil tops.<ref name="identification guide" /> These clouds are the product of very strong convection that can penetrate the lower stratosphere.

===Levels and genera===
{{See also|Weather map|Station model}}

Tropospheric clouds form in any of three levels (formerly called [[wikt:étage|''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]]es, [[surface weather observation]]s,<ref name="Definitions" /> and [[weather map]]s.<ref name="HOWTOREAD">JetStream (2008). [http://www.srh.weather.gov/srh/jetstream/synoptic/wxmaps.htm How to read weather maps.] {{Webarchive|url=https://web.archive.org/web/20150101062001/http://www.srh.weather.gov/srh/jetstream/synoptic/wxmaps.htm |date=1 January 2015 }} [[National Weather Service]]. Retrieved on 16 May 2007.</ref> The base-height range for each level varies depending on the latitudinal [[geographical zone]].<ref name="Definitions" /> Each altitude level comprises two or three genus-types differentiated mainly by physical form.<ref name="appearance">{{Cite web |year=2017 |editor-last=World Meteorological Organization |title=Appearance of Clouds, International Cloud Atlas |url=https://cloudatlas.wmo.int/appearance-of-clouds.html |access-date=26 April 2017}}</ref><ref name="identification guide" />

The standard levels and genus-types are summarised below in approximate descending order of the altitude at which each is normally based.<ref name="wmo-cloud classifications" /> Multi-level clouds with significant vertical extent are separately listed and summarized in approximate ascending order of instability or convective activity.<ref name="meteorology" />

====High-level====
[[File:Cirrus sky panorama.jpg|thumb|High [[cirrus cloud|cirrus]] upper-left merging into [[cirrostratus]] right and some [[cirrocumulus]] far right]]

High clouds form at altitudes of {{convert|10000|to|25000|ft|abbr=on|order=flip|-2}} in the [[polar region]]s, {{convert|16500|to|40000|ft|abbr=on|order=flip|-2}} in the [[temperate regions]], and {{convert|20000|to|60000|ft|abbr=on|order=flip|-2}} in the [[tropics]].<ref name="Definitions" /> 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 analyzed 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).<ref name="Cloud type identification">{{Cite web |year=2015 |editor-last=Colorado State University Dept. of Atmospheric Science |title=Cloud type identification by satellites |url=http://rammb.cira.colostate.edu/wmovl/VRL/Texts/SATELLITE_METEOROLOGY/CHAPTER-2.PDF |archive-url=https://web.archive.org/web/20060411213048/http://rammb.cira.colostate.edu/wmovl/VRL/Texts/SATELLITE_METEOROLOGY/CHAPTER-2.PDF |archive-date=11 April 2006 |url-status=live |access-date=30 December 2015 |publisher=[[Colorado State University]]}}</ref>

* Genus [[Cirrus cloud|cirrus]] (Ci) – these are mostly fibrous wisps of delicate, white, cirriform, ice crystal clouds that show up clearly against the blue sky.<ref name="cirrus" /> Cirrus are generally non-convective except castellanus and floccus subtypes which show limited convection. They often form along a high altitude [[jetstream]]<ref name="Jetstream clouds">{{Cite journal |last=Vincent J. Schaefer |date=October 1952 |title=Cloud Forms of the Jet Stream |journal=Tellus |volume=5 |issue=1 |pages=27–31 |bibcode=1953Tell....5...27S |doi=10.1111/j.2153-3490.1953.tb01032.x}}</ref> 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.<ref name="wmo-cloud classifications">{{Cite web |year=1995 |editor-last=World Meteorological Organization |title=WMO cloud classifications |url=http://www.weatheranswer.com/public/Clouds_WMO.pdf |archive-url=https://web.archive.org/web/20050226160200/http://www.weatheranswer.com/public/Clouds_WMO.pdf |archive-date=26 February 2005 |url-status=live |access-date=1 February 2012}}</ref>
* 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.<ref name="cirrocumulus">{{Cite web |year=2017 |editor-last=World Meteorological Organization |title=Cirrocumulus, International Cloud Atlas |url=https://cloudatlas.wmo.int/clouds-genera-cirrocumulus.html |access-date=16 May 2017}}</ref><ref name="YDN-364">{{Cite book |last1=Miyazaki |first1=R. |title=Proceedings Ninth Pacific Conference on Computer Graphics and Applications. Pacific Graphics 2001 |last2=Yoshida |first2=S. |last3=Dobashi |first3=Y. |last4=Nishita |first4=T. |year=2001 |isbn=978-0-7695-1227-3 |pages=363 |chapter=A method for modeling clouds based on atmospheric fluid dynamics |citeseerx=10.1.1.76.7428 |doi=10.1109/PCCGA.2001.962893 |s2cid=6656499}}</ref> 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.<ref name="Mackerel sky">{{Cite web |title=Mackerel sky |url=http://www.weatheronline.co.uk/reports/wxfacts/Sometimes-a-bit-fishy.htm |access-date=21 November 2013 |publisher=Weather Online}}</ref>
* Genus [[cirrostratus]] (Cs) – cirrostratus is a thin nonconvective stratiform ice crystal veil that typically gives rise to halos caused by refraction of the [[sunlight|Sun's rays]]. The Sun and Moon are visible in clear outline.<ref name="cirrostratus">{{Cite web |year=2017 |editor-last=World Meteorological Organization |title=Cirrostratus, International Cloud Atlas |url=https://cloudatlas.wmo.int/clouds-genera-cirrostratus.html |access-date=16 May 2017}}</ref> Cirrostratus does not produce precipitation, but often thickens into altostratus ahead of a warm front or low-pressure area, which sometimes does.<ref name="Altostratus">{{Cite book |url=https://archive.org/details/manualonobservat00worl/page/35 |title=Altostratus, International Cloud Atlas |year=1975 |isbn=978-92-63-10407-6 |editor-last=World Meteorological Organization |volume=I |pages=[https://archive.org/details/manualonobservat00worl/page/35 35–37] |publisher=Secretariat of the World Meteorological Organization |access-date=26 August 2014}}</ref>

====Mid-level====
[[File:Sunrise (Abbottabad).jpg|thumb|Sunrise scene giving a shine to an altocumulus stratiformis perlucidus cloud]]
[[File:As 1.jpg|thumb|Altostratus translucidus near top of photo thickening into altostratus opacus near bottom (see also 'species and varieties')]]
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 {{convert|6500|ft|abbr=on|order=flip|-2}} above surface at any latitude, but may be based as high as {{convert|13000|ft|abbr=on|order=flip|-2}} near the poles, {{convert|23000|ft|abbr=on|order=flip|-2}} at midlatitudes, and {{convert|25000|ft|abbr=on|order=flip|-2}} in the tropics.<ref name="Definitions" /> 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.<ref name="Cloud type identification" />

* 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.<ref name="altocumulus">{{Cite web |year=2017 |editor-last=World Meteorological Organization |title=Altocumulus, International Cloud Atlas |url=https://cloudatlas.wmo.int/clouds-genera-altocumulus.html |access-date=16 May 2017}}</ref> 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-gray shading.<ref name="ac-compared">{{Cite web |year=2017 |editor-last=World Meteorological Organization |title=Ac Compared With Cc, International Cloud Atlas |url=https://cloudatlas.wmo.int/ac-compared-with-cc.html |access-date=6 April 2018}}</ref> Altocumulus can produce virga, very light precipitation that evaporates before reaching the ground.<ref name="mid-level-clouds">{{Cite web |year=2017 |editor-last=Met Office |title=Mid Level Clouds – Altocumulus |url=https://www.metoffice.gov.uk/learning/clouds/mid-level-clouds/altocumulus |access-date=6 April 2018}}</ref>
* Genus [[altostratus]] (As) – Altostratus is a midlevel opaque or translucent nonconvective veil of gray/blue-gray 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.<ref name="altostratus">{{Cite web |year=2017 |editor-last=World Meteorological Organization |title=Altostratus, International Cloud Atlas |url=https://cloudatlas.wmo.int/clouds-genera-altostratus.html |access-date=16 May 2017}}</ref>

====Low-level====
[[File:Clouds over Africa.jpg|thumb|[[Cumulus humilis]] with stratocumulus stratiformis in the foreground (see also 'species and varieties')]]
[[File:Cumulus humilis clouds in Ukraine.jpg|thumb|[[Cumulus humilis cloud]]s in May]]
Low clouds are found from near the surface up to {{convert|6500|ft|abbr=on|order=flip|-2}}.<ref name="Definitions" /> 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.<ref name="Cloud type identification" />
* 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.<ref name="stratocumulus">{{Cite web |year=2017 |editor-last=World Meteorological Organization |title=Stratocumulus, International Cloud Atlas |url=https://cloudatlas.wmo.int/clouds-genera-stratocumulus.html |url-status=dead |archive-url=https://web.archive.org/web/20170510035919/https://cloudatlas.wmo.int/clouds-genera-stratocumulus.html |archive-date=10 May 2017 |access-date=16 May 2017}}</ref> Stratocumulus is often present during wet weather originating from other rain clouds, but can only produce very light precipitation on its own.<ref name="stratocumulus-precipitation">{{Cite web |year=2016 |editor-last=Met Office |title=Stratocumulus |url=https://www.metoffice.gov.uk/learning/clouds/low-level-clouds/stratocumulus |access-date=10 April 2018}}</ref>
* Species [[cumulus humilis]] – These are small detached fair-weather cumuliform clouds that have nearly horizontal bases and flattened tops, and do not produce rain showers.<ref name="cumulus">{{Cite web |year=2017 |editor-last=World Meteorological Organization |title=Cumulus, International Cloud Atlas |url=https://cloudatlas.wmo.int/clouds-genera-cumulus.html |access-date=16 May 2017}}</ref>
* Genus [[Stratus cloud|stratus]] (St) – This is a flat or sometimes ragged nonconvective stratiform type that sometimes resembles elevated fog.<ref name="stratus">{{Cite web |year=2017 |editor-last=World Meteorological Organization |title=Stratus, International Cloud Atlas |url=https://cloudatlas.wmo.int/clouds-genera-stratus.html |access-date=16 May 2017}}</ref> Only very weak precipitation can fall from this cloud, usually drizzle or snow grains.<ref name="drizzle">{{Cite web |year=2017 |editor-last=World Meteorological Organization |title=Drizzle, International Cloud Atlas |url=https://cloudatlas.wmo.int/drizzle.html |access-date=9 April 2018}}</ref><ref name="snow-grains">{{Cite web |year=2017 |editor-last=World Meteorological Organization |title=Snow Grains, International Cloud Atlas |url=https://cloudatlas.wmo.int/snow-grains.html |access-date=9 April 2018}}</ref> 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 {{Cvt|1|km}}.<ref name="stratus-fog">{{Cite web |year=2000 |editor-last=Colorado State University |title=Stratus and Fog |url=http://rammb.cira.colostate.edu/wmovl/vrl/tutorials/satmanu-eumetsat/SatManu/CMs/FgStr/backgr.htm |access-date=9 April 2018}}</ref> If the visibility is 1&nbsp;km or higher, the visible condensation is termed [[mist]].<ref name="mist-fog">{{Cite web |year=2017 |editor-last=Met Office |title=Difference Between Mist and Fog |url=https://www.metoffice.gov.uk/learning/fog/difference-mist-and-fog |access-date=9 April 2018}}</ref>

====Multi-level or moderate vertical====
[[File:Nimbostratus virga.JPG|thumb|Nimbostratus with virga]]
These clouds have low- to mid-level bases that form anywhere from near the surface to about {{convert|8000|ft|abbr=on|order=flip|-2}} and tops that can extend into the mid-altitude range and sometimes higher in the case of nimbostratus.
* Genus [[nimbostratus]] (Ns) – This is a diffuse, dark gray, multi-level stratiform layer with great horizontal extent and usually moderate to deep vertical development that looks feebly illuminated from the inside.<ref name="nimbostratus">{{Cite web |year=2017 |editor-last=World Meteorological Organization |title=Nimbostratus, International Cloud Atlas |url=https://cloudatlas.wmo.int/clouds-genera-nimbostratus.html |access-date=16 May 2017}}</ref> Nimbostratus normally forms from mid-level altostratus, and develops at least moderate vertical extent<ref name="cloud atlas" /><ref name="Plymouth State Meteorology" /> 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.<ref name="glossary">{{Cite web |last=American Meteorological Society |year=2012 |title=Glossary of Meteorology |url=http://glossary.ametsoc.org/wiki/Nimbostratus |access-date=9 January 2014}}</ref> The ''nimbo-'' prefix refers to its ability to produce continuous rain or snow over a wide area, especially ahead of a warm front.<ref name="precipitating clouds">[[#Ackerman|Ackerman]], p. 118</ref> This thick cloud layer lacks any towering structure of its own, but may be accompanied by embedded towering cumuliform or cumulonimbiform types.<ref name="Plymouth State Meteorology" /><ref name="nimbostratus associated with deep convection">{{Cite book |last=Houze, Robert A. |url=https://books.google.com/books?id=5DKWGZwBBEYC&pg=PA211 |title=Cloud Dynamics |publisher=Academic Press |year=1994 |isbn=978-0-08-050210-6 |page=211}}</ref> Meteorologists affiliated with the [[World Meteorological Organization]] (WMO) officially classify nimbostratus as mid-level for synoptic purposes while informally characterizing it as multi-level.<ref name="Definitions" /> Independent meteorologists and educators appear split between those who largely follow the WMO model<ref name="cloud atlas">{{Cite web |last=Clouds Online |year=2012 |title=Cloud Atlas |url=http://www.clouds-online.com |access-date=1 February 2012}}</ref><ref name="Plymouth State Meteorology">{{Cite web |last=Koermer, Jim |year=2011 |title=Plymouth State Meteorology Program Cloud Boutique |url=http://vortex.plymouth.edu/cloudboutique/ |url-status=dead |archive-url=https://web.archive.org/web/20140701173917/http://vortex.plymouth.edu/cloudboutique/ |archive-date=1 July 2014 |access-date=1 September 2015 |publisher=[[Plymouth State University]]}}</ref> and those who classify nimbostratus as low-level, despite its considerable vertical extent and its usual initial formation in the middle altitude range.<ref>{{Cite web |last=Hatheway, Becca |year=2009 |title=Cloud Types |url=http://www.windows2universe.org/earth/Atmosphere/clouds/cloud_types.html |access-date=15 September 2011 |website=Windows to the Universe, US National Earth Science Teachers Association (NESTA)}}</ref><ref name="classification of clouds">{{Cite web |title=cloud: Classification of Clouds |url=http://www.infoplease.com/ce6/weather/A0857400.html |website=Infoplease.com}}</ref>
* Species [[cumulus mediocris]] – These cumuliform clouds of free convection have clear-cut, medium-gray, flat bases and white, domed tops in the form of small sproutings and generally do not produce precipitation.<ref name="cumulus" /> They usually form in the low level of the troposphere except during conditions of very low relative humidity, when the cloud 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.<ref name="Definitions" />

====Towering vertical====
[[File:Towering Verticle Thunderhead.jpg|thumb|Isolated cumulonimbus calvus cloud over the [[Mojave Desert]], releasing a heavy shower]]
[[File:Cumulonimbus-incus mykonos.jpg|thumb|Single-cell Cumulonimbus capillatus incus]]
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.<ref name="automated Cb and Tcu detection"/>

* Species [[cumulus congestus]] – 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 gray coloration and the top commonly resembles a cauliflower. This cloud type can produce moderate to heavy showers<ref name="cumulus" /> 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-gray to nearly black base and a very high top in the form of a mountain or huge tower.<ref name="cumulonimbus">{{Cite web |year=2017 |editor-last=World Meteorological Organization |title=Cumulonimbus, International Cloud Atlas |url=https://cloudatlas.wmo.int/clouds-genera-cumulonimbus.html |access-date=16 May 2017}}</ref> Cumulonimbus can produce [[thunderstorm]]s, local very heavy downpours of [[rain]] that may cause [[flash flood]]s, and a variety of types of [[lightning]] including cloud-to-ground that can cause [[wildfire]]s.<ref name="Scott2000">{{Cite journal |last=Scott A |year=2000 |title=The Pre-Quaternary history of fire |journal=Palaeogeogr Palaeoclimatol Palaeoecol |volume=164 |issue=1–4 |pages=281–329 |bibcode=2000PPP...164..281S |doi=10.1016/S0031-0182(00)00192-9}}</ref> Other convective severe weather may or may not be associated with thunderstorms and include heavy [[snow]] showers, [[hail]],<ref name="ncar">{{Cite web |last=National Center for Atmospheric Research |year=2008 |title=Hail |url=http://www.ncar.ucar.edu/research/meteorology/storms/hail.php |url-status=dead |archive-url=https://web.archive.org/web/20100527170048/http://www.ncar.ucar.edu/research/meteorology/storms/hail.php |archive-date=27 May 2010 |access-date=18 July 2009 |publisher=University Corporation for Atmospheric Research}}</ref> strong [[wind shear]], [[downburst]]s,<ref>[[Ted Fujita|Fujita, Ted]] (1985). "The Downburst, microburst and macroburst". SMRP Research Paper 210.</ref> and [[tornado]]es.<ref name="Renno">{{Cite journal |last=Renno |first=N. O. |year=2008 |title=A thermodynamically general theory for convective vortices |url=https://deepblue.lib.umich.edu/bitstream/2027.42/73164/1/j.1600-0870.2008.00331.x.pdf |archive-url=https://web.archive.org/web/20190502122954/https://deepblue.lib.umich.edu/bitstream/2027.42/73164/1/j.1600-0870.2008.00331.x.pdf |archive-date=2 May 2019 |url-status=live |journal=Tellus A |volume=60 |issue=4 |pages=688–699 |bibcode=2008TellA..60..688R |doi=10.1111/j.1600-0870.2008.00331.x |hdl-access=free |hdl=2027.42/73164}}</ref> 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]]s.<ref name="G&N:207-212" />

===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.<ref name="species">{{Cite web |year=2017 |editor-last=World Meteorological Organization |title=Species, International Cloud Atlas |url=https://cloudatlas.wmo.int/principles-of-cloud-classification-species.html |access-date=2 June 2017}}</ref> 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.<ref name="meteorology" />

{| class="wikitable"
!Forms and levels !! Stratiform <br> non-convective !! Cirriform <br> mostly nonconvective !! Stratocumuliform <br> limited-convective !! Cumuliform <br> free-convective !! Cumulonimbiform <br> strong convective
|-
!High-level
| [[Cirrostratus]]{{blist| nebulosus| fibratus }}||[[Cirrus cloud|Cirrus]]<br> '''non-convective'''{{blist| uncinus| fibratus| spissatus}} '''limited convective'''{{blist| castellanus| floccus}} || [[Cirrocumulus]]{{blist| stratiformis| lenticularis| castellanus| floccus}}|| ||
|-
!Mid-level
| [[Altostratus]]{{blist| no differentiated species<br>(always nebulous)}} || || [[Altocumulus]]{{blist| stratiformis| lenticularis|  castellanus| floccus| volutus}}||
|-
!Low-level
| [[Stratus cloud|Stratus]]{{blist| nebulosus| [[fractus]]}}|| || [[Stratocumulus]]{{blist| stratiformis| lenticularis| castellanus| floccus| volutus}}|| [[Cumulus]]{{blist| [[Cumulus humilis|humilis]]| [[fractus]]}}  ||
|-
!Multi-level or moderate vertical
| [[Nimbostratus]]{{blist| no differentiated species<br>(always nebulous)}} || || || Cumulus{{blist| [[Cumulus mediocris|mediocris]]}} ||
|-
!Towering vertical
| || || || Cumulus{{blist| [[Cumulus congestus|congestus]] || [[Cumulonimbus]]{{blist| calvus| capillatus}}
|-
|}

====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.<ref name="nebulosus">{{Cite web |year=2017 |editor-last=World Meteorological Organization |title=Nebulosus, International Cloud Atlas |url=https://cloudatlas.wmo.int/clouds-species-nebulosus.html |access-date=2 June 2017}}</ref> Cirrostratus ''fibratus'' is a species made of semi-merged filaments that are transitional to or from cirrus.<ref name="fibratus" /> 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<ref name="nebulosus" /> except when broken up into ragged sheets of stratus [[fractus]] (see below).<ref name="cloud atlas" /><ref name="species" /><ref name="clouds - species and varieties" />

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.<ref name="fibratus">{{Cite web |year=2017 |editor-last=World Meteorological Organization |title=Fibratus, International Cloud Atlas |url=https://cloudatlas.wmo.int/clouds-species-fibratus.html |access-date=2 June 2017}}</ref> The species ''uncinus'' is similar but has upturned hooks at the ends. Cirrus ''spissatus'' appear as opaque patches that can show light gray shading.<ref name="species" />

[[File:Lenticular Cloud in Wyoming 0034b.jpg|thumb|[[Lenticular cloud|Altocumulus lenticularis]] forming over mountains in Wyoming with lower layer of cumulus mediocris and higher layer of cirrus spissatus]]

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.<ref name="stratiformis">{{Cite web |year=2017 |editor-last=World Meteorological Organization |title=Stratiformis, International Cloud Atlas |url=https://cloudatlas.wmo.int/clouds-species-stratiformis.html |access-date=2 June 2017}}</ref> 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]]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.<ref name="cloud atlas" /><ref name="species" /><ref name="clouds - species and varieties" />

====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).<ref name="species" /><ref name="clouds - species and varieties" /><ref name="fractus">{{Cite web |year=2017 |editor-last=World Meteorological Organization |title=Species Fractus, International Cloud Atlas |url=https://cloudatlas.wmo.int/clouds-species-fractus.html |access-date=5 April 2018}}</ref> 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).<ref name="pannus">{{Cite web |year=2017 |editor-last=World Meteorological Organization |title=Accessory Cloud Pannus, International Cloud Atlas |url=https://cloudatlas.wmo.int/clouds-accessory-pannus.html |access-date=5 April 2018}}</ref>

====Partly unstable====
[[File:Stratocumulus castellanus 3oct.jpg|thumb|Example of a castellanus cloud formation]]
These species are subdivisions of genus types that can occur in partly unstable air with limited [[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.<ref name="Elevated convection">{{Cite journal |last1=Stephen F. Corfidi |last2=Sarah J. Corfidi |last3=David M Schultz |year=2008 |title=Elevated Convection and Castellanus: Ambiguities, Significance, and Questions |url=https://zenodo.org/record/1234533 |journal=Weather and Forecasting |volume=23 |issue=6 |page=1282 |bibcode=2008WtFor..23.1280C |doi=10.1175/2008WAF2222118.1|doi-access=free }}</ref> 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.<ref name="species-castellanus">{{Cite web |year=2017 |editor-last=World Meteorological Organization |title=Species Castellanus, International Cloud Atlas |url=https://cloudatlas.wmo.int/clouds-species-castellanus.html |access-date=5 April 2018}}</ref> 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.<ref name="floccus">{{Cite web |year=2017 |editor-last=World Meteorological Organization |title=Species Floccus, International Cloud Atlas |url=https://cloudatlas.wmo.int/clouds-species-floccus.html |access-date=5 April 2018}}</ref>

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.<ref name="ICA2017">{{Cite news |last=Sutherland |first=Scott |date=23 March 2017 |title=Cloud Atlas leaps into 21st century with 12 new cloud types |work=The Weather Network |agency=Pelmorex Media |url=https://www.theweathernetwork.com/news/articles/cloud-atlas-leaps-into-21st-century-with-12-new-cloud-types/80685/ |access-date=24 March 2017 |archive-date=31 May 2022 |archive-url=https://web.archive.org/web/20220531022305/https://www.theweathernetwork.com/news/articles/cloud-atlas-leaps-into-21st-century-with-12-new-cloud-types/80685/ |url-status=dead }}</ref> There are some volutus clouds that form as a consequence of interactions with specific geographical features rather than with a parent cloud. Perhaps the strangest geographically specific cloud of this type is the [[Morning Glory cloud|Morning Glory]], a rolling cylindrical cloud that appears unpredictably over the [[Gulf of Carpentaria]] in [[Northern Australia]]. Associated with a powerful "ripple" in the atmosphere, the cloud may be "surfed" in [[Glider (sailplane)|glider]] aircraft.<ref name="stg">{{Cite web |last=Abbie Thomas |date=7 August 2003 |title=Soaring the glory |url=http://www.abc.net.au/science/articles/2003/08/07/2045486.htm |access-date=30 August 2014 |website=ABC Science |publisher=Australian Broadcasting Corporation}}</ref>

====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<ref name="species" /> which is the same type that the International Civil Aviation Organization refers to as 'towering cumulus'.<ref name="automated Cb and Tcu detection" />

[[File:Cumulus mediocris atmospheric instability 01.jpg|thumb|Cumulus mediocris cloud, about to turn into a cumulus congestus]]
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.<ref name="species" /><ref name="clouds - species and varieties">{{Cite web |last=Boyd, Sylke |year=2008 |title=Clouds – Species and Varieties |url=http://www.morris.umn.edu/~sboyd/weather/cloudsystems/Clouds_files/speciesandvarieties.htm |url-status=dead |archive-url=https://web.archive.org/web/20101230041442/http://www.morris.umn.edu/~sboyd/weather/cloudsystems/Clouds_files/speciesandvarieties.htm |archive-date=30 December 2010 |access-date=4 February 2012 |website=University of Minnesota}}</ref>

===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.<ref name="varieties">{{Cite web |year=2017 |editor-last=World Meteorological Organization |title=Varieties, International Cloud Atlas |url=https://cloudatlas.wmo.int/principles-of-cloud-classification-varieties.html |access-date=1 February 2018}}</ref>

====Opacity-based====
[[File:Sleepy Twilight.jpg|thumb|left|A layer of stratocumulus stratiformis perlucidus hiding the setting sun with a background layer of stratocumulus cumulogenitus resembling distant mountains]]
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.<ref name="varieties" /><ref name="Aerographer2012">{{Cite web |last=Aerographer/Meteorology |year=2012 |title=Cloud Variety |url=http://meteorologytraining.tpub.com/14269/css/14269_20.htm |url-status=dead |archive-url=https://web.archive.org/web/20121221005913/http://meteorologytraining.tpub.com/14269/css/14269_20.htm |archive-date=21 December 2012 |access-date=2 July 2012 |website=meteorologytraining.tpub.com}}</ref>

====Pattern-based====
[[File:Sculpting La Silla’s Skies.jpg|thumb|left|Cirrus fibratus radiatus over ESO's [[La Silla Observatory]]<ref>{{Cite web |title=Sculpting La Silla's Skies |url=http://www.eso.org/public/images/potw1433a/ |access-date=23 August 2014 |website=www.eso.org |publisher=ESO}}</ref>]]

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,<ref name="Cumulus Humilis-Skynews">{{Cite web |last=Cumulus-skynews |year=2013 |title=Clouds: Their curious natures |url=http://cumulus-skynews.tumblr.com/post/49212742113/5a-cumulus-humilis-radiatus-the-latin-term |access-date=26 August 2014}}</ref>{{Unreliable source?|date=February 2022|reason=is there a better source than tumblr? If reliable maybe add a hidden comment to say why}}<ref name="Pretor-Pinney20">{{Cite book |last=Pretor-Pinney |first=Gavin |url=https://books.google.com/books?id=ix4qy7FihDcC |title=The Cloudspotter's Guide: The Science, History, and Culture of Clouds |date=2007 |publisher=Penguin Group |isbn=978-1-101-20331-6 |page=20 |author-link=Gavin Pretor-Pinney}}</ref> and with the genus altostratus.<ref name="radiatus">{{Cite web |year=2017 |editor-last=World Meteorological Organization |title=Variety Radiatus, International Cloud Atlas |url=https://cloudatlas.wmo.int/clouds-varieties-radiatus.html |access-date=5 April 2018}}</ref>
[[File:Morning Sky 7.jpg|thumbnail|upright|Altocumulus stratiformis duplicatus at sunrise in the California Mojave Desert, US (higher layer orange to white; lower layer gray)]]

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.<ref name="varieties" /><ref name="Aerographer2012" />

====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.<ref name="clouds - species and varieties" /><ref name="varieties" /><ref name="Aerographer2012" />

===Other types===
{{see also|List of cloud types}}
[[File:Clouds in the drakensberge 03.jpg|alt=Clouds on mountains|thumb|Clouds on mountains]]
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.<ref name="clouds - species and varieties" /><ref name="Aerographer2012" /> 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.<ref name="features">{{Cite web |year=2017 |editor-last=World Meteorological Organization |title=Features, International Cloud Atlas |url=https://cloudatlas.wmo.int/principles-of-cloud-classification-supplementary-features.html |access-date=1 February 2018}}</ref>

====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.<ref name="features" />

When the precipitation reaches the ground without completely evaporating, it is designated as the feature ''praecipitatio''.<ref name="Dunlop77-78">{{harvnb|Dunlop|2003|pp=77–78}}</ref> 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 because this cloud genus lies too close to the ground to allow the formation of virga.<ref name="clouds - species and varieties" /><ref name="Aerographer2012" /><ref name="features" />

====Cloud-based supplementary features====
''Incus'' is the most type-specific supplementary feature, seen only with cumulonimbus of the species capillatus. A [[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]] where the air no longer continues to get colder with increasing altitude.<ref name="cumulonimbus-incus">{{Cite web |date=5 August 2009 |title=Cumulonimbus Incus |url=http://epod.usra.edu/blog/2009/08/cumulonimbus-incus.html |access-date=23 October 2012 |publisher=Universities Space Research Association}}</ref>

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.<ref name="features" />

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.<ref name="features" /><ref name="Aerographer">{{Cite web |last=Aerographer/Meteorology |year=2012 |title=Roll cloud formation on cumulonimbus |url=http://meteorologytraining.tpub.com/14269/css/14269_27.htm |url-status=dead |archive-url=https://web.archive.org/web/20130518082044/http://meteorologytraining.tpub.com/14269/css/14269_27.htm |archive-date=18 May 2013 |access-date=5 July 2012}}</ref><ref name="Dunlop79">{{harvnb|Dunlop|2003|p=79}}</ref>

An ''[[Arcus cloud|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.<ref name="Ludlum473">{{Cite book |last=Ludlum |first=David McWilliams |url=https://archive.org/details/audubonsocietyfi00ludl/page/473 |title=National Audubon Society Field Guide to Weather |publisher=Alfred A. Knopf |year=2000 |isbn=978-0-679-40851-2 |page=[https://archive.org/details/audubonsocietyfi00ludl/page/473 473] |oclc=56559729}}</ref> A large arcus formation can have the appearance of a dark menacing arch.<ref name="features" />

Several new supplementary features have been formally recognized by the [[World Meteorological Organization]] (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 cloud|Kelvin–Helmholtz (wave) cloud]]. This phenomenon has also been observed in cloud formations over other planets and even in the Sun's atmosphere.<ref>{{Cite web |last=Fox |first=Karen C. |date=30 December 2014 |title=NASA's Solar Dynamics Observatory Catches "Surfer" Waves on the Sun |url=http://www.nasa.gov/mission_pages/sunearth/news/sun-surfing.html |website=NASA-The Sun-Earth Connection: Heliophysics |publisher=NASA |access-date=20 November 2014 |archive-date=20 November 2021 |archive-url=https://web.archive.org/web/20211120110428/https://www.nasa.gov/mission_pages/sunearth/news/sun-surfing.html |url-status=dead }}</ref> 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 that 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.<ref name=ICA2017/>

====Accessory clouds====
Supplementary cloud formations detached from the main cloud are known as [[accessory cloud]]s.<ref name="clouds - species and varieties" /><ref name="Aerographer2012" /><ref name="features" /> 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.<ref name="pannus" />

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,<ref name="GarretI">{{Cite journal |last1=Garrett |first1=T. J. |last2=Dean-Day |first2=J. |last3=Liu |first3=C. |last4=Barnett |first4=B. |last5=Mace |first5=G. |last6=Baumgardner |first6=D. |last7=Webster |first7=C. |last8=Bui |first8=T. |last9=Read |first9=W. |last10=Minnis |first10=P. |year=2006 |title=Convective formation of pileus cloud near the tropopause |journal=Atmospheric Chemistry and Physics |volume=6 |issue=5 |pages=1185–1200 |bibcode=2006ACP.....6.1185G |doi=10.5194/acp-6-1185-2006 |hdl-access=free |hdl=2060/20080015842 |s2cid=14440075 |doi-access=free }}</ref> 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.<ref name="features" /> An accessory cloud recently officially recognized by the World Meteorological Organization is the ''flumen'', also known more informally as the ''beaver's tail''. It is formed by the warm, humid [[inflow (meteorology)|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 (cloud)|scud]] clouds and does not rotate.<ref name=ICA2017/>

==== Mother clouds ====
[[File:Port of Piraeus Panoramic View.JPG|thumb|Cumulus partly spreading into stratocumulus cumulogenitus over the port of [[Piraeus]] in Greece]]

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.<ref name="mother-clouds">{{Cite web |year=2017 |editor-last=World Meteorological Organization |title=Mother clouds, International Cloud Atlas |url=https://cloudatlas.wmo.int/principles-of-cloud-classification-mother-clouds.html |access-date=2 June 2017}}</ref>
[[File:Sunset and Clouds.jpg|thumb|Cumulonimbus mother cloud dissipating into stratocumulus cumulonimbogenitus at dusk]]

====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, the latter having reached altitudes of {{cvt|57|km|mi}},<ref name="z578">{{cite web | title=Tonga volcano had highest plume ever recorded, new study confirms | website=University of Oxford | date=2022-01-15 | url=https://www.ox.ac.uk/news/2022-11-04-tonga-volcano-had-highest-plume-ever-recorded-new-study-confirms | access-date=2025-03-18}}</ref> and nuclear [[mushroom cloud]]s having an upward extend of up to {{cvt|67|km}}.<ref>{{cite magazine |last=Veselov |first=A.V. |title=Tsar-bomba |magazine=Atompress |year=2006 |volume=43 |issue=726 |page=7}}</ref> Smaller low-level "pyrocumulus" or "fumulus" clouds formed by contained industrial activity are now classified as cumulus ''homogenitus'' (Latin for 'man-made'). [[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.<ref name="mother-clouds" />

=== Large scale patterns ===

Sometimes, certain atmospheric processes cause clouds to become organized into patterns that can cover large areas.  These patterns are usually difficult to identify from surface level and are best seen from an aircraft or spacecraft.

==== Stratocumulus fields ====

Stratocumulus clouds can be organized into "fields" that take on certain specially classified shapes and characteristics. In general, these fields are more discernible from high altitudes than from ground level. They can often be found in the following forms:
* [[Actinoform cloud|Actinoform]], which resembles a leaf or a spoked wheel.
* Closed cell, which is cloudy in the center and clear on the edges, similar to a filled [[honeycomb]].<ref name="marine clouds">{{Cite journal |last1=Kore.n |first1=I. |last2=Feingold |first2=G. |year=2013 |title=Adaptive behavior of marine cellular clouds |journal=Scientific Reports |volume=3 |pages=2507 |bibcode=2013NatSR...3.2507K |doi=10.1038/srep02507 |pmc=3753593 |pmid=23978979}}</ref>
* Open cell, which resembles an empty honeycomb, with clouds around the edges and clear, open space in the middle.<ref>{{Cite web |date=5 October 2005 |title=Cloud Formations off the West Coast of South America |url=http://earthobservatory.nasa.gov/IOTD/view.php?id=5904 |access-date=29 March 2013 |publisher=[[NASA Earth Observatory]]}}</ref>

====Vortex streets====
{{Main|Karman vortex street}}

[[File:Whirpool Clouds.jpg|thumb|Cirrus fibratus intortus formed into a Kármán vortex street at evening twilight]]
These patterns are formed from a phenomenon known as a [[Kármán vortex]] which is named after the engineer and fluid dynamicist [[Theodore von Kármán]].<ref>Theodore von Kármán, ''Aerodynamics''. McGraw-Hill (1963): {{ISBN|978-0-07-067602-2}}. Dover (1994): {{ISBN|978-0-486-43485-8}}.</ref> Wind driven clouds, usually mid level altocumulus or high level cirrus, can form into parallel rows that follow the wind direction. When the wind and clouds encounter high elevation land features such as vertically prominent islands, they can form eddies around the high land masses that give the clouds a twisted appearance.<ref name="vortex-street">{{Cite web |year=2001 |editor-last=National Aeronautics and Space Administration |title=Vortex Streets |url=https://earthobservatory.nasa.gov/IOTD/view.php?id=2270 |access-date=5 April 2018}}</ref>

== Distribution ==

===Convergence along low-pressure zones===
{{Main|Intertropical Convergence Zone|Extratropical cyclone|Cold front|Warm front}}
[[File:Worldclouds 2009.jpg|thumb|Global cloud cover, averaged over the month of October 2009. [[NASA]] composite satellite image.<ref>For a larger image see [http://earthobservatory.nasa.gov/images/imagerecords/41000/41292/cldfrc_TMO_200910_lrg.png this image] {{Webarchive|url=https://web.archive.org/web/20100529185735/http://earthobservatory.nasa.gov/images/imagerecords/41000/41292/cldfrc_TMO_200910_lrg.png |date=29 May 2010 }} at earthobservatory.nasa.gov</ref>]]

[[File:MODAL2 M CLD FR.ogv|thumb|These maps display the fraction of Earth's area that was cloudy on average during each month from January 2005 to August 2013. The measurements were collected by the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA's Terra satellite. Colors range from blue (no clouds) to white (totally cloudy). Like a digital camera, MODIS collects information in gridded boxes, or pixels. Cloud fraction is the portion of each pixel that is covered by clouds. Colors range from blue (no clouds) to white (totally cloudy).<ref>{{Cite web |title=Cloud Fraction : Global Maps |url=http://earthobservatory.nasa.gov/GlobalMaps/view.php?d1=MODAL2_M_CLD_FR |access-date=26 October 2014 |website=nasa.gov}}</ref> (''click for more detail'')]]

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]]. It is most prevalent in and along low pressure zones of surface tropospheric convergence which encircle the Earth close to the [[equator]] and near the 50th parallels of latitude in the northern and southern [[Hemispheres of the Earth|hemispheres]].<ref>{{Cite book |last=Kondratʹev, Kirill Iakovlevich |url=https://books.google.com/books?id=tcCTsENwgGYC&pg=PA411 |title=Atmospheric aerosol properties: formation, processes and impacts |publisher=Springer |year=2006 |isbn=978-3-540-26263-3 |page=403}}</ref> 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.<ref name="Convergence/divergence">{{Cite web |last=Wei-hung, Leung |year=2010 |title=Meteorology Basics: Convergence and Divergence |url=http://www.hko.gov.hk/education/edu01met/wxphe/ele-condiv-e.htm |url-status=dead |archive-url=https://web.archive.org/web/20191026053144/http://www.hko.gov.hk/education/edu01met/wxphe/ele-condiv-e.htm |archive-date=26 October 2019 |access-date=8 December 2014 |publisher=Hong Kong Observatory}}</ref> 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.<ref name="noaa">{{Cite web |date=24 October 2007 |title=Inter-Tropical Convergence Zone |url=http://www.srh.noaa.gov/jetstream//tropics/itcz.htm |access-date=4 June 2009 |website=JetStream – Online School for Weather |publisher=[[NOAA]]}}</ref> Clouds of virtually any type can form along the mid-latitude convergence zones depending on the stability and moisture content of the air. These extratropical convergence zones are occupied by the [[polar front]]s where [[air mass]]es of polar origin meet and clash with those of tropical or subtropical origin.<ref name="The Climate System: General Circulation and Climate Zones">{{Cite web |last=Kushnir, Yochanan |year=2000 |title=The Climate System: General Circulation and Climate Zones |url=http://eesc.columbia.edu/courses/ees/climate/lectures/gen_circ/index.html |url-status=dead |archive-url=https://web.archive.org/web/20040822022845/http://eesc.columbia.edu/courses/ees/climate/lectures/gen_circ/index.html |archive-date=22 August 2004 |access-date=13 March 2012}}</ref> This leads to the formation of weather-making [[extratropical cyclone]]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.<ref>{{Cite news |last=Williams, Jack |date=27 June 1997 |title=Extratropical storms are major weather makers |work=USA Today |url=https://www.usatoday.com/weather/wstorm.htm |access-date=13 March 2012}}</ref>

===Divergence along high pressure zones===
{{Main|Subtropical ridge|Polar high}}

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.<ref name="Convergence/divergence"/> 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 presence of a large-scale high-pressure [[subtropical ridge]] on each side of the equator reduces cloudiness at these low latitudes.<ref name="subtropical-ridge">{{Cite journal |last1=Cai |first1=Wenju |last2=Van Rensch |first2=Peter |last3=Cowan |first3=Tim |year=2011 |title=Subtropical Ridge |journal=Journal of Climate |volume=24 |issue=23 |pages=6035 |bibcode=2011JCli...24.6035C |doi=10.1175/2011JCLI4149.1 |s2cid=59145525|doi-access=free }}</ref> Similar patterns also occur at higher latitudes in both hemispheres.<ref name="pressure-belts">{{Cite web |year=2015 |editor-last=PMF IAS |title=Atmospheric Pressure Belts and Wind Systems PMF IAS Pressure Belts |url=https://www.pmfias.com/pressure-belts-pressure-systems-equatorial-low-sub-tropical-high-sub-polar-low-polar-high/ |access-date=5 April 2018}}</ref>

==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.<ref name="Luminance">{{Cite web |year=2017 |editor-last=World Meteorological Organization |title=Luminance, International Cloud Atlas |url=https://cloudatlas.wmo.int/appearance-of-clouds-luminance.html |access-date=10 May 2017}}</ref> In the troposphere, dense, deep clouds exhibit a high reflectance (70–95%) throughout the [[visible spectrum]]. 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.<ref name="Steven Salter and John Latham">[http://www.21stcenturychallenges.org/60-seconds/increasing-cloud-reflectivity/ Increasing Cloud Reflectivity] {{webarchive|url=https://web.archive.org/web/20130511051600/http://www.21stcenturychallenges.org/60-seconds/increasing-cloud-reflectivity/ |date=11 May 2013 }}, Royal Geographical Society, 2010.</ref> Cloud droplets tend to [[scattering|scatter]] light efficiently, so that the intensity of the [[solar radiation]] decreases with depth into the gases. As a result, the [[cloud base]] can vary from a very light to very-dark-gray depending on the cloud's thickness and how much [[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 gray shading because of its greater reflectivity.<ref name="Luminance" />

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]]. By this process of accumulation, the space between droplets becomes increasingly larger, permitting light to penetrate farther into the cloud. If the cloud is sufficiently large and the droplets within are spaced far enough apart, a percentage of the light that enters the cloud is not reflected back out but is absorbed, giving the cloud a darker look. A simple example of this is one's being able to see farther in heavy rain than in heavy fog. This process of [[Reflection (physics)|reflection]]/[[Absorption (electromagnetic radiation)|absorption]] is what causes the range of cloud color from white to black.<ref name="Bette Hileman">{{Cite journal |last=Hileman |first=B. |year=1995 |title=Clouds absorb more solar radiation than researchers previously thought |journal=Chemical & Engineering News |volume=73 |issue=7 |pages=33 |doi=10.1021/cen-v073n007.p033}}</ref>

Striking cloud colorations can be seen at any altitude, with the color of a cloud usually being the same as the incident light.<ref name="Coloration">{{Cite web |year=2017 |editor-last=World Meteorological Organization |title=Coloration, International Cloud Atlas |url=https://cloudatlas.wmo.int/appearance-of-clouds-colour.html |access-date=13 May 2017}}</ref> During daytime when the sun is relatively high in the sky, tropospheric clouds generally appear bright white on top with varying shades of gray underneath. Thin clouds may look white or appear to have acquired the color of their [[Natural environment|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 gray 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.<ref name="Coloration" />

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.<ref name="Curiosities">{{Cite web |year=2007 |editor-last=University of Wisconsin-Madison-News |title=Curiosities-Green sky before tornado |url=http://www.news.wisc.edu/14039 |access-date=17 January 2015}}</ref>

Yellowish clouds may be seen in the troposphere in the late spring through early fall months during [[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.<ref name="Garrett Nagle">{{Cite book |last=Nagle, Garrett |title=Hazards |publisher=Nelson Thornes |year=1998 |isbn=978-0-17-490022-1 |page=101 |chapter=10. Cities and Air Pollution |chapter-url=https://books.google.com/books?id=qC1l1RdWpIwC&pg=PA101}}</ref>

<gallery mode="packed" heights="160">
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]] with altocumulus volutus and cirrocumulus stratiformis
File:Red Color in Gray Clouds.JPG|Sunset reflecting shades of pink onto gray stratocumulus stratiformis translucidus (becoming perlucidus in the background)
File:Sharp View.JPG|Stratocumulus stratiformis perlucidus before sunset in [[Bangalore]], India
File:Regnbyge.jpg|Late-summer [[rainstorm]] in [[Denmark]]. Nearly black color of base indicates main cloud in foreground probably cumulonimbus.
File:Burning Yellow Sunset.jpg|Particles in the [[atmosphere]] and the [[sun]]'s angle enhance colors of stratocumulus cumulogenitus at evening [[twilight]].
</gallery>

==Effects==
{{see also|Cloud cover|Cloud feedback|Global dimming|Climate change}}
[[File:Cumulus_clouds_panorama.jpg|alt=|thumb|400x400px|Cumuliform cloudscape over [[Swifts Creek]], Australia]]
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.<ref name="cloud-heating" />

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]]. 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]] of [[greenhouse gases]] and [[water vapor]].<ref name="cloud-heating">{{Cite web |title=Cloud Climatology |url=http://isccp.giss.nasa.gov/role.html |access-date=12 July 2011 |website=International Satellite Cloud Climatology Program |publisher=National Aeronautics and Space Administration}}</ref>

High-level genus-types particularly show this duality with both short-wave albedo cooling and long-wave greenhouse warming effects. On the whole, ''ice-crystal'' clouds in the upper troposphere (cirrus) tend to favor net warming.<ref name="Clouds and the greenhouse effect" /><ref name="Nucleation">{{Cite journal |last=Franks |first=F. |year=2003 |title=Nucleation of ice and its management in ecosystems |journal=Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences |volume=361 |issue=1804 |pages=557–74 |bibcode=2003RSPTA.361..557F |doi=10.1098/rsta.2002.1141 |pmid=12662454 |s2cid=25606767}}</ref> However, the cooling effect is dominant with mid-level and low clouds, especially when they form in extensive sheets.<ref name="Clouds and the greenhouse effect" /> 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.<ref name="Clouds and the greenhouse effect">[[#Ackerman|Ackerman]], p. 124</ref>

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.<ref>{{Cite web |last=Wolchover |first=Natalie |date=25 February 2019 |title=A World Without Clouds |url=https://www.quantamagazine.org/cloud-loss-could-add-8-degrees-to-global-warming-20190225/ |website=Quanta Magazine}}</ref> 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 sensitivity|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.<ref>{{Cite journal |last=Bony |first=S. |year=2005 |title=Marine boundary layer clouds at the heart of tropical cloud feedback uncertainties in climate models |journal=Geophysical Research Letters |volume=32 |issue=20 |pages=L20806 |bibcode=2005GeoRL..3220806B |doi=10.1029/2005GL023851 |doi-access=free}}</ref><ref>{{Cite journal |last1=Medeiros |first1=B. |last2=Stevens |first2=B. |last3=Held |first3=I. M. |last4=Zhao |first4=M. |last5=Williamson |first5=D. L. |last6=Olson |first6=J. G. |last7=Bretherton |first7=C. S. |year=2008 |title=Aquaplanets, Climate Sensitivity, and Low Clouds |journal=Journal of Climate |volume=21 |issue=19 |pages=4974–4991 |bibcode=2008JCli...21.4974M |citeseerx=10.1.1.620.6314 |doi=10.1175/2008JCLI1995.1}}</ref> For these reasons the role of tropospheric clouds in regulating [[weather]] and [[climate]] remains a leading source of uncertainty in [[global warming]] projections.<ref>{{Cite book |last1=Forster |first1=P. |title={{Harvnb|IPCC AR6 WG1|2021}} |last2=Storelvmo |first2=T. |last3=Armour |first3=K. |last4=Collins |first4=W. |year=2021 |chapter=Chapter 7: The Earth's energy budget, climate feedbacks, and climate sensitivity |ref={{harvid|IPCC AR6 WG1 Ch7|2021}} |chapter-url=https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_Chapter07.pdf}} From pp 1022: "The cloud feedback could either amplify or offset some of the future warming and has long been the biggest source of uncertainty in climate projections."</ref><ref>{{Cite web |title=Will Clouds Speed or Slow Global Warming? |url=https://www.nsf.gov/news/special_reports/clouds/question.jsp |url-status=dead |archive-url=https://archive.today/20111114194612/https://www.nsf.gov/news/special_reports/clouds/question.jsp |archive-date=14 November 2011 |access-date=23 October 2012 |publisher=National Science Foundation}}</ref>

== Stratospheric classification and distribution ==
[[File:Nacreous clouds Antarctica.jpg|thumb|right|Lenticular nacreous clouds over Antarctica]]

[[Polar stratospheric cloud]]s (PSC's) are found in the lowest part of the stratosphere.  Moisture is scarce above the troposphere, so nacreous and non-nacreous clouds at this altitude range are restricted to polar regions in the winter where and when the air is coldest.<ref name="Polar-stratospheric" />

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 {{convert|15000|-|25000|m|abbr=on|-2}}  Accordingly, they are classified as a singular type with no differentiated altitude levels, genus types, species, or varieties. There is no Latin nomenclature in the manner of tropospheric clouds, but rather descriptive names of several general forms using common English.<ref name="Polar-stratospheric"/>

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 colors of the nacreous types.  This type of PSC has been identified as a cause of ozone depletion in the stratosphere.<ref name="PSC">{{Cite web |year=2017 |editor-last=World Meteorological Organization |title=Nitric acid and water PSC, International Cloud Atlas |url=https://cloudatlas.wmo.int/nitric-acid-and-water-polar-stratospheric-clouds.html |access-date=3 April 2019}}</ref> 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.<ref name="nacreous">{{Cite web |year=2017 |editor-last=World Meteorological Organization |title=Nacreous PSC, International Cloud Atlas |url=https://cloudatlas.wmo.int/nacreous-clouds.html |access-date=3 April 2019}}</ref><ref name="autogenerated2">{{Cite web |last=Les Cowley |year=2011 |title=Nacreous clouds |url=http://www.atoptics.co.uk/highsky/nacr1.htm |access-date=31 January 2012 |website=atoptics.co.uk}}</ref>

== Mesospheric classification and distribution ==
[[File:Helkivad ööpilved Kuresoo kohal.jpg|thumb|right|[[Noctilucent cloud]] over Estonia]]

[[Noctilucent cloud]]s are the highest in the atmosphere and are found near the top of the mesosphere at about {{convert|80|to|85|km|abbr=on}} or roughly ten times the altitude of tropospheric high clouds.<ref name="autogenerated1">{{Cite book |last1=Michael Gadsden |url=http://www.iugg.org/IAGA/iaga_pages/pdf/ONC_Sep06.pdf |title=Observing Noctilucent Clouds |last2=Pekka Parviainen |date=September 2006 |publisher=International Association of Geomagnetism & Aeronomy |page=9 |access-date=31 January 2011 |archive-date=31 October 2008 |archive-url=https://web.archive.org/web/20081031003710/http://www.iugg.org/IAGA/iaga_pages/pdf/ONC_Sep06.pdf |url-status=dead }}</ref>  They are given this Latin derived 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.<ref name="Polar-stratospheric" /> They are not common or widespread enough to have a significant effect on climate.<ref name="simulation studies" /> However, an increasing frequency of occurrence of noctilucent clouds since the 19th century may be the result of climate change.<ref name="noctilucent-cloud">{{Cite web |year=2017 |editor-last=Project Possum |title=About Noctiluent Clouds |url=http://projectpossum.org/research/noctilucent-cloud/about-noctilucent-clouds/ |access-date=6 April 2018}}</ref>

Ongoing research indicates that convective lift in the mesosphere is strong enough during the polar [[summer]] to cause adiabatic cooling of small amount of water vapor to the point of saturation.  This tends to produce the coldest temperatures in the entire atmosphere just below the mesopause.<ref name="simulation studies">{{Cite journal |last1=Turco |first1=R. P. |last2=Toon |first2=O. B. |last3=Whitten |first3=R. C. |last4=Keesee |first4=R. G. |last5=Hollenbach |first5=D. |year=1982 |title=Noctilucent clouds: Simulation studies of their genesis, properties and global influences |journal=Planetary and Space Science |volume=30 |issue=11 |pages=1147–1181 |bibcode=1982P&SS...30.1147T |doi=10.1016/0032-0633(82)90126-X}}</ref> There is evidence that smoke particles from burnt-up meteors provide much of the condensation nuclei required for the formation of noctilucent cloud.<ref name="sounding rocket">{{Cite web |last=Fox, Karen C. |year=2013 |title=NASA Sounding Rocket Observes the Seeds of Noctilucent Clouds |url=http://www.nasa.gov/mission_pages/sunearth/news/CHAMPS-NLCs.html#.UkqY0BAxIgq |access-date=1 October 2013 |archive-date=24 September 2013 |archive-url=https://web.archive.org/web/20130924233857/http://www.nasa.gov/mission_pages/sunearth/news/CHAMPS-NLCs.html#.UkqY0BAxIgq |url-status=dead }}</ref>

Noctilucent clouds have four major types based on physical structure and appearance. Type I veils are very tenuous and lack well-defined structure, somewhat like cirrostratus fibratus or poorly defined cirrus.<ref name="veils">{{Cite web |year=2017 |editor-last=World Meteorological Organization |title=Type I Veils, International Cloud Atlas |url=https://cloudatlas.wmo.int/noctilucent-clouds-type-I-veils.html |access-date=18 July 2019}}</ref> 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.<ref name="bands">{{Cite web |year=2017 |editor-last=World Meteorological Organization |title=Type II Bands, International Cloud Atlas |url=https://cloudatlas.wmo.int/noctilucent-clouds-type-II-bands.html |access-date=18 July 2019}}</ref> Type III billows are arrangements of closely spaced, roughly parallel short streaks that mostly resemble cirrus.<ref name="billows">{{Cite web |year=2017 |editor-last=World Meteorological Organization |title=Type III Billows, International Cloud Atlas |url=https://cloudatlas.wmo.int/noctilucent-clouds-type-III-billows.html |access-date=18 July 2019}}</ref> Type IV whirls are partial or, more rarely, complete rings of cloud with dark centers.<ref name="whirls">{{Cite web |year=2017 |editor-last=World Meteorological Organization |title=Type IV Whirls, International Cloud Atlas |url=https://cloudatlas.wmo.int/noctilucent-clouds-type-IV-whirls.html |access-date=18 July 2019}}</ref>

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.  Sightings are rare more than 45 degrees south of the North Pole or north of the South Pole.<ref name="Polar-stratospheric" />

{{clear}}

== Extraterrestrial ==
{{See also|Extraterrestrial atmospheres}}
[[File:Neptune_clouds.jpg|alt=A composite black-and-white photograph showing cirrus clouds over the surface of Mars|thumb|Cirrus clouds on Neptune, captured during ''[[Voyager 2]]''<nowiki/>'s flyby]]
Cloud cover has been seen on most other planets in the [[Solar System]]. [[Venus]]'s thick clouds are composed of [[sulfur dioxide]] (due to volcanic activity) and appear to be almost entirely stratiform.<ref name="Bougher-127-129">{{Cite book |last1=Bougher |first1=Stephen Wesley |url=https://books.google.com/books?id=b93lEgkPquoC&pg=PA128 |title=Venus II: Geology, Geophysics, Atmosphere, and Solar Wind Environment |last2=Phillips |first2=Roger |date=1997 |publisher=University of Arizona Press |isbn=978-0-8165-1830-2 |pages=127–129}}</ref> They are arranged in three main layers at altitudes of 45 to 65&nbsp;km that obscure the [[planet]]'s surface and can produce [[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.<ref name="Mysterious waves">{{Cite magazine |last=Shiga, David |year=2006 |title=Mysterious waves seen in Venus's clouds |url=https://www.newscientist.com/article/dn10497-mysterious-waves-seen-in-venuss-clouds.html#.UnjSlRAxKQx |magazine=New Scientist |access-date=5 November 2013}}</ref> On [[Mars]], noctilucent, cirrus, cirrocumulus and stratocumulus composed of water-ice have been detected mostly near the poles.<ref>{{Cite web |last=SPACE.com staff |date=28 August 2006 |title=Mars Clouds Higher Than Any on Earth |url=http://www.space.com/scienceastronomy/060828_mars_clouds.html |publisher=SPACE.com}}</ref><ref name="NASA-photo">{{Cite web |date=19 September 2008 |title=Clouds Move Across Mars Horizon |url=http://www.nasa.gov/mission_pages/phoenix/images/press/16145-animated.html |access-date=15 April 2011 |website=Phoenix Photographs |publisher=[[National Aeronautics and Space Administration]] |archive-date=2 June 2016 |archive-url=https://web.archive.org/web/20160602213811/http://www.nasa.gov/mission_pages/phoenix/images/press/16145-animated.html |url-status=dead }}</ref> Water-ice fogs have also been detected on Mars.<ref name="Mars-clouds">{{Cite web |title=NASA SP-441: Viking Orbiter Views of Mars |date=January 1980 |url=https://history.nasa.gov/SP-441/ch12.htm |access-date=26 January 2013 |publisher=National Aeronautics and Space Administration |last1=Carr |first1=M. H. |last2=Baum |first2=W. A. |last3=Blasius |first3=K. R. |last4=Briggs |first4=G. A. |last5=Cutts |first5=J. A. |last6=Duxbury |first6=T. C. |last7=Greeley |first7=R. |last8=Guest |first8=J. |last9=Masursky |first9=H. |last10=Smith |first10=B. A. }}</ref>

Both [[Jupiter]] and [[Saturn]] have an outer cirriform cloud deck composed of ammonia,<ref>{{Cite web |last=Phillips |first=Tony |date=20 May 2010 |title=Big Mystery: Jupiter Loses a Stripe |url=https://science.nasa.gov/science-news/science-at-nasa/2010/20may_loststripe/ |access-date=15 April 2011 |website=Nasa Headline News – 2010 |publisher=[[National Aeronautics and Space Administration]] |archive-date=20 April 2011 |archive-url=https://web.archive.org/web/20110420012725/http://science.nasa.gov/science-news/science-at-nasa/2010/20may_loststripe/ |url-status=dead }}</ref><ref>{{Cite book |last1=Dougherty |first1=Michele |url=https://books.google.com/books?id=M56CHHxVMP4C&pg=PA118 |title=Saturn from Cassini-Huygens |last2=Esposito |first2=Larry |date=November 2009 |publisher=Springer |isbn=978-1-4020-9216-9 |edition=1 |page=118 |oclc=527635272}}</ref> an intermediate stratiform haze-cloud layer made of [[ammonium hydrosulfide]], and an inner deck of cumulus water clouds.<ref>{{Cite web |last1=Ingersoll, A.P. |last2=Dowling, T.E. |last3=Gierasch, P.J. |last4=Orton, G.S. |last5=Read, P.L. |last6=Sanchez-Lavega, A. |last7=Showman, A.P. |last8=Simon-Miller, A.A. |last9=Vasavada, A.R. |title=Dynamics of Jupiter's Atmosphere |url=http://www.lpl.arizona.edu/~showman/publications/ingersolletal-2004.pdf |archive-url=https://web.archive.org/web/20070418104027/http://www.lpl.arizona.edu/~showman/publications/ingersolletal-2004.pdf |archive-date=18 April 2007 |url-status=live |access-date=1 February 2007 |publisher=Lunar & Planetary Institute}}</ref><ref>{{Cite web |last=Monterrey Institute for Research in Astronomy |date=11 August 2006 |title=Saturn |url=http://www.mira.org/fts0/planets/100/text/txt002x.htm |access-date=31 January 2011}}</ref> Embedded cumulonimbus are known to exist near the [[Great Red Spot]] on [[Jupiter]].<ref name="Jupiter-clouds">{{Cite web |title=Thunderheads on Jupiter |url=http://photojournal.jpl.nasa.gov/catalog/pia00506 |access-date=26 January 2013 |website=Jet Propulsion Laboratory |publisher=National Aeronautics and Space Administration}}</ref><ref name="Saturn-clouds">{{Cite news |last=Minard |first=Anne |date=14 October 2008 |title=Mysterious Cyclones Seen at Both of Saturn's Poles |work=National Geographic News |publisher=National Geographic |url=http://news.nationalgeographic.com/news/2008/10/081014-saturn-cyclones.html |archive-url=https://web.archive.org/web/20081016145724/http://news.nationalgeographic.com/news/2008/10/081014-saturn-cyclones.html |url-status=dead |archive-date=16 October 2008 |access-date=26 January 2013}}</ref> The same category-types can be found covering [[Uranus]] and [[Neptune]], but are all composed of [[methane]].<ref name="Neptune's Atmosphere">{{Cite news |last=Taylor Redd, Nola |year=2012 |title=Neptune's Atmosphere: Composition, Climate, & Weather |work=Space.com |url=http://www.space.com/18922-neptune-atmosphere.html |access-date=5 November 2013}}</ref><ref name="Uranus-clouds">{{Cite magazine |last=Boyle |first=Rebecca |date=18 October 2012 |title=Check Out The Most Richly Detailed Image Ever Taken of Uranus |url=http://www.popsci.com/science/article/2012-10/most-richly-detailed-image-ever-taken-icy-distant-uranus |magazine=Popular Science}}</ref><ref name="Irwin-115">{{Cite book |last=Irwin |first=Patrick |url=https://books.google.com/books?id=p8wCsJweUb0C&pg=PA115 |title=Giant Planets of Our Solar System: Atmospheres, Composition, and Structure |date=July 2003 |publisher=Springer |isbn=978-3-540-00681-7 |edition=1 |page=115}}</ref><ref name="elkins-tanton">{{Cite book |last=Elkins-Tanton, Linda T. |url=https://archive.org/details/uranusneptuneplu00elki/page/79 |title=Uranus, Neptune, Pluto, and the Outer Solar System |publisher=Chelsea House |year=2006 |isbn=978-0-8160-5197-7 |location=New York |pages=[https://archive.org/details/uranusneptuneplu00elki/page/79 79–83]}}</ref> Saturn's moon [[Titan (moon)|Titan]] has cirrus clouds believed to be composed largely of methane.<ref name="Coustenis155">{{Cite book |last1=Athéna Coustenis |url=https://books.google.com/books?id=j3O47dxrDAQC |title=Titan: Exploring an Earthlike World |last2=F.W. Taylor |publisher=World Scientific |year=2008 |isbn=978-981-270-501-3 |pages=154–155}}</ref><ref>{{Cite web |date=3 February 2011 |title=Surprise Hidden in Titan's Smog: Cirrus-Like Clouds |url=http://www.nasa.gov/mission_pages/cassini/whycassini/titan-clouds_prt.htm |access-date=16 April 2011 |website=Mission News |publisher=[[National Aeronautics and Space Administration]] |archive-date=16 April 2011 |archive-url=https://web.archive.org/web/20110416182620/http://www.nasa.gov/mission_pages/cassini/whycassini/titan-clouds_prt.htm |url-status=dead }}</ref> The ''[[Cassini–Huygens]]'' Saturn mission uncovered evidence of polar stratospheric clouds<ref name="Clouds on titan">{{Cite web |last=Elizabeth Zubritsky |year=2016 |title=NASA Scientists find impossible cloud on titan |url=http://www.nasa.gov/feature/goddard/2016/nasa-scientists-find-impossible-cloud-on-titan-again |access-date=1 November 2016}}</ref> and a methane cycle on Titan, including lakes near the poles and fluvial channels on the surface of the moon.<ref name="methane-cycle">{{Cite web |year=2008 |editor-last=National Aeronautics and Space Administration |title=NASA Confirms Liquid Lake on Saturn Moon, Cassini Mission News |url=https://www.nasa.gov/mission_pages/cassini/media/cassini-20080730.html |access-date=5 April 2018 |archive-date=9 January 2019 |archive-url=https://web.archive.org/web/20190109125729/https://www.nasa.gov/mission_pages/cassini/media/cassini-20080730.html |url-status=dead }}</ref>

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]] of [[exoplanet]] [[Kepler-7b]] was announced,<ref name="MIT-20131002">{{Cite web |last=Chu |first=Jennifer |date=2 October 2013 |title=Scientists generate first map of clouds on an exoplanet |url=http://web.mit.edu/newsoffice/2013/scientists-generate-first-map-of-clouds-on-kepler-7b-1003.html |access-date=2 January 2014 |website=[[MIT]]}}</ref><ref>{{Cite journal |last1=Demory |first1=B. O. |last2=De Wit |first2=J. |last3=Lewis |first3=N. |last4=Fortney |first4=J. |last5=Zsom |first5=A. |last6=Seager |first6=S. |last7=Knutson |first7=H. |last8=Heng |first8=K. |last9=Madhusudhan |first9=N. |last10=Gillon |first10=M. |last11=Barclay |first11=T. |year=2013 |title=Inference of Inhomogeneous Clouds in an Exoplanet Atmosphere |journal=The Astrophysical Journal |volume=776 |issue=2 |pages=L25 |arxiv=1309.7894 |bibcode=2013ApJ...776L..25D |doi=10.1088/2041-8205/776/2/L25 |last12=Desert |first12=J. M. |last13=Parmentier |first13=V. |last14=Cowan |first14=N. B. |s2cid=701011}}</ref> and, in December 2013, in the atmospheres of [[GJ 436 b]] and [[GJ 1214 b]].<ref name="NASA-20131231">{{Cite web |last1=Harrington |first1=J.D. |last2=Weaver |first2=Donna |last3=Villard |first3=Ray |date=31 December 2013 |title=Release 13-383 – NASA's Hubble Sees Cloudy Super-Worlds With Chance for More Clouds |url=http://www.nasa.gov/press/2013/december/nasas-hubble-sees-cloudy-super-worlds-with-chance-for-more-clouds/ |access-date=1 January 2014 |website=[[NASA]]}}</ref><ref name="NAT-20140101a">{{Cite journal |last=Moses |first=J. |year=2014 |title=Extrasolar planets: Cloudy with a chance of dustballs |journal=Nature |volume=505 |issue=7481 |pages=31–32 |bibcode=2014Natur.505...31M |doi=10.1038/505031a |pmid=24380949 |s2cid=4408861}}</ref><ref name="NAT-20140101b">{{Cite journal |last1=Knutson |first1=H. A. |last2=Benneke |first2=B. R. |last3=Deming |first3=D. |last4=Homeier |first4=D. |year=2014 |title=A featureless transmission spectrum for the Neptune-mass exoplanet GJ 436b |journal=Nature |volume=505 |issue=7481 |pages=66–68 |arxiv=1401.3350 |bibcode=2014Natur.505...66K |doi=10.1038/nature12887 |pmid=24380953 |s2cid=4454617}}</ref><ref name="NAT-20140101c">{{Cite journal |last1=Kreidberg |first1=L. |last2=Bean |first2=J. L. |last3=Désert |first3=J. M. |last4=Benneke |first4=B. R. |last5=Deming |first5=D. |last6=Stevenson |first6=K. B. |last7=Seager |first7=S. |last8=Berta-Thompson |first8=Z. |last9=Seifahrt |first9=A. |last10=Homeier |first10=D. |year=2014 |title=Clouds in the atmosphere of the super-Earth exoplanet GJ 1214b |journal=Nature |volume=505 |issue=7481 |pages=69–72 |arxiv=1401.0022 |bibcode=2014Natur.505...69K |doi=10.1038/nature12888 |pmid=24380954 |s2cid=4447642}}</ref>

==In culture and religion==
[[File:Benjamin West - Joshua passing the River Jordan with the Ark of the Covenant - Google Art Project.jpg|thumb|upright=1.3|''Joshua Passing the River Jordan with the Ark of the Covenant'' (1800) by [[Benjamin West]], showing [[Yahweh]] leading the Israelites through the desert in the form of a [[pillar of cloud]], as described in {{Bibleverse||Exodus|13:21–22|9}}<ref name="Gertz" />]]
Clouds play an important mythical or non-scientific role in various cultures and religious traditions. The ancient [[Akkadians]] believed that the clouds (in meteorology, probably the supplementary feature ''mamma'') were the breasts of the sky goddess [[Antu (goddess)|Antu]],<ref name="Nemet-Nejat">{{Cite book |last=Nemet-Nejat |first=Karen Rhea |url=https://archive.org/details/dailylifeinancie00neme/page/182 |title=Daily Life in Ancient Mesopotamia |date=1998 |publisher=Greenwood |isbn=978-0313294976 |page=[https://archive.org/details/dailylifeinancie00neme/page/182 182] |author-link=Karen Rhea Nemet-Nejat}}</ref> and that rain was milk from her breasts.<ref name="Nemet-Nejat" /> In {{Bibleverse||Exodus|13:21–22|9}}, [[Yahweh]] is described as guiding the [[Israelites]] through the desert in the form of a "[[pillar of cloud]]" by day and a "[[Pillar of Fire (theophany)|pillar of fire]]" by night,<ref name="Gertz">{{Cite book |last=Gertz |first=Jan Christian |title=The Book of Exodus: Composition, Reception, and Interpretation |date=2014 |publisher=Brill |isbn=978-90-04-28266-7 |location=Leiden, The Netherlands |page=111 |chapter=The Miracle at the Sea: Remarks on the Recent Discussion about Origin and Composition of the Exodus Narrative |chapter-url=https://books.google.com/books?id=TmGeBQAAQBAJ&pg=PA111}}</ref> and in the [[Acts of the Apostles]], a cloud takes the risen [[Jesus]] away from the view of his disciples at his [[Ascension of Jesus|ascension]].<ref>{{bibleverse|Acts|1:9}}</ref>

In [[Mandaeism]], [[uthra]]s (celestial beings) are also occasionally mentioned as being in ''anana'' ("clouds"; e.g., in ''[[Right Ginza]]'' Book 17, Chapter 1), which can also be interpreted as female consorts.<ref name="GR Gelbert">{{Cite book |last=Gelbert |first=Carlos |url=https://livingwaterbooks.com.au/product/ginza-rba/ |title=Ginza Rba |publisher=Living Water Books |year=2011 |isbn=9780958034630 |location=Sydney}}</ref>

''[[The Cloud of Unknowing]]'' is a 14th-century work of [[Christian mysticism]] that advises a contemplative practice focused on experiencing God through love and "unknowing".{{citation needed|date=August 2024}}

In the [[ancient Greek comedy]] ''[[The Clouds]]'', written by [[Aristophanes]] and first performed at the [[City Dionysia]] in 423 BC, the philosopher [[Socrates]] declares that the Clouds are the only true deities<ref name="Strauss1966">{{Cite book |last=Strauss |first=Leo |url=https://books.google.com/books?id=Vce_OEKtTf4C |title=Socrates and Aristophanes |date=1966 |publisher=The University of Chicago Press |isbn=978-0-226-77719-1 |location=Chicago, Illinois |pages=17–21, 29}}</ref> and tells the main character Strepsiades not to worship any deities other than the Clouds, but to pay homage to them alone.<ref name="Strauss1966" /> In the play, the Clouds change shape  to reveal the true nature of whoever is looking at them,<ref name="Roche">{{Cite book |last=Roche |first=Paul |title=Aristophanes: The Complete Plays: A New Translation by Paul Roche |date=2005 |publisher=New American Library |isbn=978-0-451-21409-6 |location=New York City, New York |pages=149–150}}</ref><ref name="Strauss1966" /><ref name="Robson">{{Cite book |last=Robson |first=James |url=https://books.google.com/books?id=isKSDQAAQBAJ&pg=PA81 |title=Popular Culture in the Ancient World |date=2017 |publisher=Cambridge University Press |isbn=978-1-107-07489-7 |editor-last=Grig |editor-first=Lucy |location=Cambridge, England |page=81}}</ref> turning into [[centaur]]s at the sight of a [[long hair|long-haired]] politician, wolves at the sight of the [[embezzler]] Simon, deer at the sight of the coward [[Cleonymus of Athens|Cleonymus]], and mortal women at the sight of the effeminate [[Theoroi|informer]] [[Cleisthenes (son of Sibyrtius)|Cleisthenes]].<ref name="Roche" /><ref name="Robson" /><ref name="Strauss1966" /> They are hailed the source of inspiration to comic poets and philosophers;<ref name="Strauss1966" /> they are masters of [[rhetoric]], regarding [[eloquence]] and [[sophistry]] alike as their "friends".<ref name="Strauss1966" />

In China, clouds are symbols of luck and happiness.<ref name="Ding">{{Cite book |last=Ding |first=Ersu |url=https://books.google.com/books?id=hLeQ8eDWNGUC&pg=PT118 |title=Parallels, Interactions, and Illuminations: Traversing Chinese and Western Theories of the Sign |date=2010 |publisher=University of Toronto Press |isbn=978-1-4426-4048-1 |location=Toronto, Canada |page=118}}</ref> Overlapping clouds (in meteorology, probably ''duplicatus clouds'') are thought to imply eternal happiness<ref name="Ding" /> and clouds of different colors are said to indicate "multiplied blessings".<ref name="Ding" />

Informal cloud watching or cloud gazing is a popular activity involving watching the clouds and looking for shapes in them, a form of [[pareidolia]].<ref>{{Cite web|url=https://discovertheforest.org/activities/cloudgazing|title=Cloudgazing|website=Discover the Forest|access-date=23 November 2020|archive-date=4 October 2023|archive-url=https://web.archive.org/web/20231004143134/https://discovertheforest.org/activities/cloudgazing|url-status=dead}}</ref><ref>{{Cite web |date=20 July 2015 |title=Do You See Faces In The Clouds? The Science of Pareidolia |url=https://www.farmersalmanac.com/is-that-a-face-in-the-clouds-pareidolia-21911}}</ref>

== See also ==
{{portal|Science||Water|Weather}}
{{div col|colwidth=22em}}
* [[Atmospheric Radiation Measurement]] (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]]
{{div col end}}

== References ==
{{Reflist|30em}}

== Bibliography ==
* {{Cite book |last=Ackerman, Steven A. |url=https://books.google.com/books?id=N8Gwc-IXxbMC&pg=PA124 |title=Meteorology: Clouds and the Greenhouse Effect |publisher=Jones & Bartlett |year=2011 |isbn=978-0-7637-8927-5 |ref=Ackerman}}
* {{cite book |last=Dunlop |first=Storm |url=https://books.google.com/books?id=BR2ft4G5TgQC |title=The Weather Identification Handbook |date=June 2003 |publisher=Lyons Press |isbn=978-1-58574-857-0}}
* {{Cite book |ref={{harvid|IPCC AR6 WG1|2021}} |author=IPCC |author-link=IPCC |year=2021 |title=Climate Change 2021: The Physical Science Basis |series=Contribution of Working Group I to the [[Sixth Assessment Report]] of the Intergovernmental Panel on Climate Change |display-editors=4 |editor1-first=V. |editor1-last=Masson-Delmotte |editor2-first=P. |editor2-last= Zhai |editor3-first=A. |editor3-last=Pirani |editor4-first=S. L. |editor4-last=Connors |editor5-first=C. |editor5-last=Péan |editor6-first=S. |editor6-last=Berger |editor7-first=N. |editor7-last=Caud |editor8-first=Y. |editor8-last=Chen |editor9-first=L. |editor9-last=Goldfarb |editor10-first=M. I. |editor10-last=Gomis |publisher=Cambridge University Press |url=https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_Full_Report.pdf}}

== External links ==
{{Sister project links|Cloud}}
* [https://earth.nullschool.net/#current/wind/surface/level/overlay=total_cloud_water/winkel3/ Current global map of total cloud water]
* [https://earthobservatory.nasa.gov/global-maps/MODAL2_M_CLD_FR Monthly maps of global cloud cover], from NASA's [[Earth Observatory]]
* [https://cloudatlas.wmo.int/home.html World Meteorological Organization's (WMO) International Cloud Atlas]  [[International Cloud Atlas]]

{{Meteorological variables}}
{{Cloud types}}
{{Nature}}

{{Authority control}}

[[Category:Clouds| ]]
[[Category:Articles containing video clips]]
[[Category:Clouds, fog and precipitation]]