Editing Cloud (section)

== 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>