Editing Cloud (section)

===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" />