Editing Polar stratospheric cloud

{{short description|Clouds occurring in the stratosphere in high-latitude regions}}
{{Infobox Cloud
| name           = Polar stratospheric cloud
| image location = Nacreous clouds Antarctica.jpg
| image name     = Antarctic stratospheric cloud (nacreous clouds)
| abbreviation   = PSC
| symbol         =
| genus          =
| species        =
| variety        =
| altitude_m     = 15,000–25,000
| altitude_ft    = 49,000–82,000
| level          = other
| appearance     = Glowing brightly with vivid [[Cloud iridescence|iridescent]] colors
| precipitation  = No
| thickness      = 
| Ice content    = 
}}
[[File:Polar stratospheric cloud seen from Brissago, Ticino, Switzerland.jpg|alt=First documented appearance of a polar stratospheric cloud over Switzerland and Italy, seen from Brissago, Ticino, Switzerland|thumb|First documented appearance of a polar stratospheric cloud over Switzerland and Italy, seen from Brissago, Ticino, Switzerland on December 22, 2023]]

A '''polar stratospheric cloud''' ('''PSC''') is a [[cloud]] that forms in the winter polar [[stratosphere]] at altitudes from {{cvt|15000|to|25000|m|ft|sp=us}}. They are best observed during [[Twilight#Civil twilight|civil twilight]], when the [[Sun]] is between [[angular distance|1° and 6° below]] the [[horizon]], as well as in [[winter]] and in more northerly latitudes.<ref name=AAD /> One main type of PSC is composed of mostly [[supercooling|supercooled]] droplets of water and [[nitric acid]] and is implicated in the formation of [[ozone depletion|ozone holes]].<ref>{{cite web |publisher=[[United States Environmental Protection Agency|EPA]] |url=https://epa.gov/ozone/science/hole/whyant.html#psc |archive-url=https://web.archive.org/web/20060930070256/http://epa.gov/ozone/science/hole/whyant.html#psc |archive-date=2006-09-30 |title=Why is the ozone hole over Antarctica?}}</ref> The other main type consists only of [[ice crystal]]s, which are not harmful. This type of PSC is also called '''nacreous''' ({{IPAc-en|ˈ|n|eɪ|k|r|i|ə|s}}; from ''nacre'', or [[mother of pearl]]), due to its [[cloud iridescence|iridescence]].

==Formation==
[[File:Perlemorskyer over Tysnes 22. desember 2023.jpg|thumb|Polar stratospheric clouds over [[Western Norway]]]]
The stratosphere is very dry; unlike the [[troposphere]], it rarely allows clouds to form. In the extreme cold of the polar winter, however, stratospheric clouds of different types may form, which are classified according to their [[physical state]] (super-cooled liquid or ice) and [[chemical composition]].<ref>{{cite journal| doi = 10.1016/j.jastp.2007.09.011| url = https://core.ac.uk/download/pdf/71369.pdf| title = Polar stratospheric cloud microphysics and chemistry| journal = Journal of Atmospheric and Solar-Terrestrial Physics| volume = 70| pages = 13–40| year = 2008| last1 = Lowe| first1 = Douglas| last2 = MacKenzie| first2 = A. Robert| issue = 1| bibcode = 2008JASTP..70...13L| access-date = 2018-03-14| archive-url = https://web.archive.org/web/20180314180922/https://core.ac.uk/download/pdf/71369.pdf| archive-date = 2018-03-14| url-status = dead}}</ref>

Due to their high altitude and the curvature of the surface of the [[Earth]], these clouds will receive sunlight from below the horizon and reflect it to the ground, shining brightly well before [[dawn]] or after [[dusk]].

PSCs form at very low temperatures, below {{convert|-78|C|F}}. These temperatures can occur in the lower [[stratosphere]] in polar winter. In the [[Antarctic]], temperatures below {{convert|-88|C|F}} frequently cause type II PSCs. Such low temperatures are rarer in the [[Arctic]]. In the [[Northern hemisphere]], the generation of [[lee waves]] by mountains may locally cool the lower stratosphere and lead to the formation of lenticular (lens-shaped) PSCs.

[[File:Perlemorskyer.jpg|thumb|Polar stratospheric cloud in Elverum, Norway.]]
[[Forward scatter]]ing of sunlight within the clouds produces a pearly-white appearance. Particles within the [[Thin-film optics|optically thin]] clouds cause colored [[interference fringe]]s by [[diffraction]]. The visibility of the colors may be enhanced with a [[polarising filter]].<ref name=AAD /><ref name=wegener />

==Types==
[[File:Polar stratospheric cloud type 2.jpg|thumb|right|A lenticular type II (water) PSC showing iridescence]]

PSCs are classified into two main types, each of which consists of several subtypes.

*Type I clouds have a generally stratiform appearance resembling cirrostratus or haze.<ref name="PSC">{{cite web |editor=World Meteorological Organization |title=Nitric acid and water PSC, International Cloud Atlas |year=2017 |url=https://cloudatlas.wmo.int/nitric-acid-and-water-polar-stratospheric-clouds.html|access-date=3 April 2019}}</ref> They are sometimes sub-classified according to their [[chemical composition]] which can be measured using [[LIDAR]]. The technique also determines the height and ambient temperature of the cloud.<ref name=wegener /> They contain water, [[nitric acid]] and/or [[sulfuric acid]] and are a source of polar [[ozone depletion]].<ref>{{cite web |url=http://www.atoptics.co.uk/highsky/psc1.htm|title=Nacreous and Polar Stratospheric Clouds |website=atoptics.co.uk |date=16 September 2023 |access-date=24 December 2023}}</ref> The effects on [[ozone depletion]] arise because they support chemical reactions that produce active [[chlorine]] which catalyzes [[ozone]] destruction, and also because they remove gaseous [[nitric acid]], perturbing [[nitrogen cycle|nitrogen]] and chlorine cycles in a way which increases ozone depletion.<ref>{{cite web |publisher=[[World Meteorological Organization]] |url=http://www.wmo.int/disasters/DRR_related_Publications/Ozone/WMO_Scientific_Assessment_of_Ozone_Depletion_(2002)_e.pdf |title=Scientific Assessment of Ozone Depletion |year=2002 |access-date=2006-10-28 |archive-date=2020-03-13 |archive-url=https://web.archive.org/web/20200313150114/https://public.wmo.int/en/search?search_api_views_fulltext=disasters&sort_by=search_api_relevance%2FDRR_related_Publications%2FOzone%2FWMO_Scientific_Assessment_of_Ozone_Depletion_%282002%29_e.pdf |url-status=dead }}particularly section 3.2.2 (pages 3.21, i.e. 195 of the PDF file, and following).</ref>
**Type Ia clouds consist of large, aspherical particles, consisting of nitric acid trihydrate (NAT).<ref name=wegener />
**Type Ib clouds contain small, spherical particles (non-depolarising), of a liquid [[supercooled]] ternary solution (STS) of sulfuric acid, nitric acid, and water.<ref name=wegener />
**Type Ic clouds consist of [[Metastability|metastable]] water-rich nitric acid in a solid phase.<ref>{{cite web|publisher=[[Journal of Geophysical Research]]|url=http://www.agu.org/journals/jd/v101/iD04/96JD00062/96JD00062.pdf|title=The presence of metastable HNO<sub>3</sub>/H<sub>2</sub>O solid phases in the stratosphere inferred from ER 2 data|access-date=2012-08-17|archive-date=2020-01-26|archive-url=https://web.archive.org/web/20200126223114/http://www.agu.org/journals/jd/v101/iD04/96JD00062/96JD00062.pdf|url-status=dead}}</ref>

*Type II clouds, which are very rarely observed in the Arctic, have cirriform and lenticular sub-types <ref name="nacreous">{{cite web |editor=World Meteorological Organization |title=Nacreous PSC, International Cloud Atlas |year=2017 |url=https://cloudatlas.wmo.int/nacreous-clouds.html|access-date=3 April 2019}}</ref> and consist of [[ice|water ice]] only.<ref name=wegener>{{cite web|last=Maturilli|first=Maturilli|title=Polar Stratospheric Clouds Above Spitsbergen|url=http://www.awi.de/en/research/research_divisions/climate_science/atmospheric_circulations/research_themes/aerosol/polar_stratospheric_clouds/|publisher=[[Alfred Wegener Institute for Polar and Marine Research]]|url-status=dead|archive-url=https://web.archive.org/web/20070824200937/http://www.awi.de/en/research/research_divisions/climate_science/atmospheric_circulations/research_themes/aerosol/polar_stratospheric_clouds/|archive-date=2007-08-24}}</ref>

[[File:Polar Stratospheric Cloud type I above Cirrus.jpg|thumb|right|upright=2|A stratiform type I PSC (white cloud above the orange tropospheric clouds), showing fine horizontal structures in the veil]]

Only Type II clouds are necessarily nacreous<ref name=AAD>{{cite web|publisher=[[Australian Antarctic Division]]|url=http://www.antarctica.gov.au/about-antarctica/fact-files/atmosphere/polar-stratospheric-clouds/observations|archive-url=https://web.archive.org/web/20110602123302/http://www.antarctica.gov.au/about-antarctica/fact-files/atmosphere/polar-stratospheric-clouds/observations|url-status=dead|archive-date=June 2, 2011|title=Polar stratospheric clouds / Observations}}</ref> whereas Type I clouds can be iridescent under certain conditions, just as [[Cloud iridescence|any other cloud]].  The [[World Meteorological Organization]] no longer uses the alpha-numeric nomenclature seen in this article, and distinguishes only between super-cooled stratiform acid-water PSCs and cirriform-lenticular water ice nacreous PSCs.<ref name="Polar-stratospheric">{{cite web |editor=World Meteorological Organization |title=Upper atmospheric clouds, International Cloud Atlas |year=2017 |url=https://cloudatlas.wmo.int/upper-atmospheric-clouds.html|access-date=31 July 2017}}</ref>

==See also==
* [[Aurora]]
* [[Circumhorizontal arc]]
* [[Cloud iridescence]]
* [[Noctilucent cloud]]s

==References==
{{reflist}}
{{refbegin}}
*[https://web.archive.org/web/20060523205844/http://www.iac.ethz.ch/research/chemistry/polar_stratospheric_clouds/ ETH PSC page]
*[http://www.meteoros.de/psc/psce.htm meteoros.de]
{{refend}}

==External links==
{{commons and category|Polar stratospheric cloud}}

===Research===
*[http://www.atoptics.co.uk/highsky/nacr1.htm Nacreous Clouds] at atoptics.co.uk
*[https://web.archive.org/web/20070824200937/http://www.awi.de/en/research/research_divisions/climate_science/atmospheric_circulations/research_themes/aerosol/polar_stratospheric_clouds/ Polar Stratospheric Clouds Above Spitsbergen] at [[Alfred Wegener Institute for Polar and Marine Research|Alfred Wegener Institute]]

===News reports===
*{{cite news |title=Rare cloud spotted |url=http://www.smh.com.au/news/science/extreme-weather-makes-rare-cloud/2006/08/01/1154198117951.html |agency=AAP |publisher=Sydney Morning Herald |date=August 1, 2006}}
*https://news.yahoo.com/s/ap/20060801/ap_on_sc/antarctica_clouds_4{{dead link|date=September 2018}}
*http://www.cnn.com/2006/TECH/science/08/01/antarctica.clouds.ap/index.html{{dead link|date=September 2018}}

{{Cloud types}}

[[Category:Cloud types]]
[[Category:Ozone depletion]]