Editing Cirrus cloud (section)

== Formation ==
Cirrus clouds are usually formed as warm, dry air rises,<ref name="MetOffice" /> causing water vapor to undergo [[Deposition (phase transition)|deposition]] onto particles, including mostly mineral dust and metallic particles<ref name =sciart/><ref name="cirrus-origins">{{cite web|url=
https://csl.noaa.gov/news/2013/139_0509.html|title=The origins of cirrus: Earth's highest clouds have dusty core|date=9 May 2013|access-date=17 March 2022|website=NOAA Research|publisher=National Oceanic and Aerospace Administration|archive-date=21 May 2022|archive-url=https://web.archive.org/web/20220521143550/https://research.noaa.gov/article/ArtMID/587/ArticleID/1503/The-origins-of-cirrus-Earth%E2%80%99s-highest-clouds-have-dusty-core|url-status=live}}</ref> at high altitudes. Particles gathered by research aircraft from cirrus clouds over several locations above North America and Central America included mineral dust (containing aluminum, potassium, calcium, iron, and silicon), metallic particles in elemental, sulfate and oxide forms (containing sodium, potassium, iron, nickel, copper, zinc, tin, silver, molybdenum and lead), possible biological particles (containing oxygen, carbon, nitrogen and phosphorus) and elemental carbon. The authors concluded that mineral dust contributed the largest number of ice nuclei to cirrus cloud formation.<ref name =sciart>{{cite journal | url=https://www.science.org/doi/10.1126/science.1234145 | doi=10.1126/science.1234145 | title=Clarifying the Dominant Sources and Mechanisms of Cirrus Cloud Formation | date=2013 | last1=Cziczo | first1=Daniel J. | last2=Froyd | first2=Karl D. | last3=Hoose | first3=Corinna | last4=Jensen | first4=Eric J. | last5=Diao | first5=Minghui | last6=Zondlo | first6=Mark A. | last7=Smith | first7=Jessica B. | last8=Twohy | first8=Cynthia H. | last9=Murphy | first9=Daniel M. | journal=Science | volume=340 | issue=6138 | pages=1320–1324 | pmid=23661645 | bibcode=2013Sci...340.1320C | url-access=subscription }}</ref>

The average cirrus cloud altitude increases as [[latitude]] decreases, but the altitude is always capped by the [[tropopause]].<ref name="D&R-973" /> These conditions commonly occur at the leading edge of a [[warm front]].<ref name="audubon-447"/> Because [[absolute humidity]] is low at such high altitudes, this genus tends to be fairly transparent.<ref name="usatoday">{{cite news |url=https://www.usatoday.com/weather/wcirrus.htm |title=Cirrus Clouds |last=Palmer |first=Chad |access-date=13 September 2008 |work=[[USA Today]] |date=16 October 2005|archive-url= https://web.archive.org/web/20081108034842/https://www.usatoday.com/weather/wcirrus.htm |archive-date= 8 November 2008}}</ref> Cirrus clouds can also form inside [[fallstreak hole]]s (also called "cavum").<ref>{{cite web|url=https://cloudatlas.wmo.int/en/clouds-supplementary-features-cavum.html|title=Cavum|access-date=26 September 2022|publisher=World Meteorological Organization|website=International Cloud Atlas}}</ref>

At latitudes of [[65th parallel north|65°&nbsp;N]] or [[65th parallel south|S]], close to [[polar regions of Earth|polar regions]], cirrus clouds form, on average, only {{convert|7000|m|ft|abbr=on}} above sea level. In temperate regions, at roughly [[45th parallel north|45°&nbsp;N]] or [[45th parallel south|S]], their average altitude increases to {{convert|9500|m|ft|abbr=on}} above sea level. In [[tropics|tropical regions]], at roughly [[5th parallel north|5°&nbsp;N]] or [[5th parallel south|S]], cirrus clouds form {{convert|13500|m|ft|abbr=on}} above sea level on average. Across the globe, cirrus clouds can form anywhere from {{convert|4000|to|20000|m|ft|abbr=on}} above sea level.<ref name="D&R-973">{{harvnb|Dowling|Radke|1990|p=973}}</ref> Cirrus clouds form with a vast range of thicknesses. They can be as little as {{convert|100|m|ft|abbr=on|sigfig=2}} from top to bottom to as thick as {{convert|8000|m|ft|abbr=on|sigfig=2}}. Cirrus cloud thickness is usually somewhere between those two extremes, with an average thickness of {{convert|1500|m|ft|abbr=on}}.<ref name="D&R-977" />

The [[jet stream]], a high-level wind band, can stretch cirrus clouds long enough to cross continents.<ref name="D&R-974">{{harvnb|Dowling|Radke|1990|p=974}}</ref> [[Jet streak]]s, bands of faster-moving air in the jet stream, can create arcs of cirrus cloud hundreds of kilometers long.<ref name="cirrus-arc">{{cite web|url=https://earthobservatory.nasa.gov/images/145948/a-cirrus-arc|title=A Cirrus Arc|date=28 November 2019|publisher=National Aeronautics and Space Administration|website=NASA Earth Observatory|access-date=18 March 2022|archive-date=18 March 2022|archive-url=https://web.archive.org/web/20220318151229/https://earthobservatory.nasa.gov/images/145948/a-cirrus-arc|url-status=live}}</ref>

Cirrus cloud formation may be effected by organic [[aerosol]]s (particles produced by plants) acting as additional [[nucleation]] points for ice crystal formation.<ref>{{harvnb|Wolf|Zhang|Zawadowicz|Goodell|2020|p=1}}</ref><ref>{{Cite web|url=https://www.purdue.edu/newsroom/releases/2020/Q4/a-better-understanding-of-how-cirrus-clouds-form.html|title=A better understanding of how cirrus clouds form|date=1 October 2020|access-date=14 March 2022|last=Cziczo|first=Daniel|publisher=[[Purdue University]]|archive-date=3 May 2022|archive-url=https://web.archive.org/web/20220503184135/https://www.purdue.edu/newsroom/releases/2020/Q4/a-better-understanding-of-how-cirrus-clouds-form.html|url-status=live}}</ref> However, research suggests that cirrus clouds more commonly form on mineral dust or metallic particles rather than on organic ones.<ref name="cirrus-origins" />

=== Tropical cyclones ===
[[File:Isabel 2003-09-10 1640Z.jpg|thumb|alt=A picture showing the vast shield of cirrus clouds accompanying Hurricane Isabel in 2003|A vast shield of cirrus clouds accompanying the west side of [[Hurricane Isabel]]]]Sheets of cirrus clouds commonly fan out from the [[eye (cyclone)|eye walls]] of tropical cyclones.<ref name="cirrus-detection"/> (The eye wall is the ring of storm clouds surrounding the eye of a tropical cyclone.<ref>{{cite web|url=https://www.weather.gov/jetstream/tc_structure|title=Tropical Cyclone Structure|access-date=18 March 2022|website=NWS JetStream|publisher=National Oceanic and Atmospheric Administration|archive-date=16 November 2021|archive-url=https://web.archive.org/web/20211116033142/https://www.weather.gov/jetstream/tc_structure|url-status=live}}</ref>) A [[Central dense overcast|large shield of cirrus]] and [[cirrostratus cloud|cirrostratus]] typically accompanies the high altitude [[outflow (meteorology)|outflowing winds]] of tropical cyclones,<ref name="cirrus-detection"/> and these can make the underlying [[rain band|bands of rain]]—and sometimes even the eye—difficult to detect in satellite photographs.<ref>{{cite web|url=http://www.nrlmry.navy.mil/sat_training/tropical_cyclones/ssmi/composite/index.html|title=Tropical Cyclone SSMI – Composite Tutorial|publisher=[[United States Navy]]|access-date=18 February 2011|archive-date=4 December 2010|archive-url=https://web.archive.org/web/20101204103432/http://www.nrlmry.navy.mil/sat_training/tropical_cyclones/ssmi/composite/index.html|url-status=dead}}</ref>

=== Thunderstorms ===
[[File:Cirren von Cumulonimbus-Amboss und Cu&Sc.JPG|thumb|alt=A picture showing the cirrus clouds lancing out from the anvil of the thunderstorm, taken just before the lower mass of the cumulonimbus cloud went over the photographer|White cirrus in an anvil cloud]]
[[Thunderstorm]]s can form dense cirrus at their tops. As the cumulonimbus cloud in a thunderstorm grows vertically, the liquid water droplets freeze when the air temperature reaches the [[freezing point]].<ref name="lydolph-122">{{harvnb|Lydolph|1985|p=122}}</ref> The [[anvil cloud]] takes its shape because the [[temperature inversion]] at the tropopause prevents the warm, moist air forming the thunderstorm from rising any higher, thus creating the flat top.<ref name="G&N-212">{{harvnb|Grenci|Nese|2001|p=212}}</ref> In the tropics, these thunderstorms occasionally produce copious amounts of cirrus from their anvils.<ref>{{cite web|title=Computer-simulated Thunderstorms with Ice Clouds Reveal Insights for Next-generation Computer Models |url=http://www.pnl.gov/science/highlights/highlight.asp?id=709 |work=Atmospheric Sciences & Global Change Division Research Highlights |publisher=Pacific Northwest National Laboratory |access-date=30 January 2011 |page=42 |date=December 2009 |archive-url=https://web.archive.org/web/20110514115603/http://www.pnl.gov/science/highlights/highlight.asp?id=709 |archive-date=14 May 2011 }}</ref> High-altitude winds commonly push this dense mat out into an anvil shape that stretches [[downwind]] as much as several kilometers.<ref name="G&N-212"/>

Individual cirrus cloud formations can be the remnants of anvil clouds formed by thunderstorms. In the dissipating stage of a cumulonimbus cloud, when the normal column rising up to the anvil has evaporated or dissipated, the mat of cirrus in the anvil is all that is left.<ref name="G&N-213">{{harvnb|Grenci|Nese|2001|p=213}}</ref>

=== Contrails ===
[[Contrail]]s are an [[anthropogenic cloud|artificial type]] of cirrus cloud formed when water vapor from the exhaust of a [[jet engine]] condenses on particles, which come from either the surrounding air or the exhaust itself, and freezes, leaving behind a visible trail. The exhaust can trigger the formation of cirrus by providing [[ice nuclei]] when there is an insufficient naturally-occurring supply in the atmosphere.<ref name="McGraw-2"/> One of the [[Environmental impact of aviation|environmental impacts of aviation]] is that persistent contrails can form into large mats of cirrus,<ref name="NASA-cirrus">{{cite web|url=http://www.nasa.gov/home/hqnews/2004/apr/HQ_04140_clouds_climate.html|publisher=[[National Aeronautics and Space Administration]]|title=Clouds Caused By Aircraft Exhaust May Warm The U.S. Climate|date=27 April 2004|first1=Gretchen|last1=Cook-Anderson|first2=Chris|last2=Rink|first3=Julia|last3=Cole|access-date=24 June 2011|archive-date=18 May 2011|archive-url=https://web.archive.org/web/20110518194600/http://www.nasa.gov/home/hqnews/2004/apr/HQ_04140_clouds_climate.html|url-status=live}}</ref> and increased air traffic has been implicated as one possible cause of the increasing frequency and amount of cirrus in Earth's atmosphere.<ref name="NASA-cirrus"/><ref name="Minnis-1671">{{harvnb|Minnis|Ayers|Palikonda|Phan|2004|p=1671}}</ref>
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