Polar stratospheric cloud explained

Polar stratospheric cloud
Image Location:	Nacreous clouds Antarctica.jpg
Image Name:	Antarctic stratospheric cloud (nacreous clouds)
Abbreviation:	PSC
Altitude M:	15,000–25,000
Altitude Ft:	49,000–82,000
Level:	other
Appearance:	Glowing brightly with vivid iridescent colors
Precipitation:	No

Polar stratospheric clouds (PSCs) are clouds in the winter polar stratosphere at altitudes of . They are best observed during civil twilight, when the Sun is between 1 and 6 degrees below the horizon, as well as in winter and in more northerly latitudes. One main type of PSC is made up mostly of supercooled droplets of water and nitric acid and is implicated in the formation of ozone holes.^[1] The other main type consists only of ice crystals which are not harmful. This type of PSC is also referred to as nacreous (from nacre, or mother of pearl, due to its iridescence).

Formation

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.^[2]

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 -78C. These temperatures can occur in the lower stratosphere in polar winter. In the Antarctic, temperatures below -88C 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.

Forward scattering of sunlight within the clouds produces a pearly-white appearance. Particles within the optically thin clouds cause colored interference fringes by diffraction. The visibility of the colors may be enhanced with a polarising filter.

Types

PSCs are classified into two main types each of which consists of several sub-types

• Type I clouds have a generally stratiform appearance resembling cirrostratus or haze.^[3] 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. They contain water, nitric acid and/or sulfuric acid and are a source of polar ozone depletion.^[4] 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 and chlorine cycles in a way which increases ozone depletion.^[5]
  • Type Ia clouds consist of large, aspherical particles, consisting of nitric acid trihydrate (NAT).
  • Type Ib clouds contain small, spherical particles (non-depolarising), of a liquid supercooled ternary solution (STS) of sulfuric acid, nitric acid, and water.
  • Type Ic clouds consist of metastable water-rich nitric acid in a solid phase.^[6]
• Type II clouds, which are very rarely observed in the Arctic, have cirriform and lenticular sub-types ^[7] and consist of water ice only.^[8]

Only Type II clouds are necessarily nacreous^[9] whereas Type I clouds can be iridescent under certain conditions, just as 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.^[10]

See also

• Aurora
• Circumhorizontal arc
• Cloud iridescence
• Noctilucent clouds

References

• ETH PSC page
• meteoros.de

External links

Research

• Nacreous Clouds at atoptics.co.uk
• Polar Stratospheric Clouds Above Spitsbergen at Alfred Wegener Institute

News reports

• News: Rare cloud spotted . AAP . Sydney Morning Herald . August 1, 2006.
• https://news.yahoo.com/s/ap/20060801/ap_on_sc/antarctica_clouds_4
• http://www.cnn.com/2006/TECH/science/08/01/antarctica.clouds.ap/index.html

Notes and References

1. Web site: . Why is the ozone hole over Antarctica?. https://web.archive.org/web/20060930070256/http://epa.gov/ozone/science/hole/whyant.html#psc . 2006-09-30 .
2. 10.1016/j.jastp.2007.09.011. Polar stratospheric cloud microphysics and chemistry. Journal of Atmospheric and Solar-Terrestrial Physics. 70. 13–40. 2008. Lowe. Douglas. MacKenzie. A. Robert. 1. 2008JASTP..70...13L. 2018-03-14. https://web.archive.org/web/20180314180922/https://core.ac.uk/download/pdf/71369.pdf. 2018-03-14. dead.
3. Web site: World Meteorological Organization . Nitric acid and water PSC, International Cloud Atlas . 2017 . 3 April 2019.
4. Web site: Nacreous and Polar Stratospheric Clouds . atoptics.co.uk . 16 September 2023 . 24 December 2023.
5. Web site: . Scientific Assessment of Ozone Depletion . 2002 . 2006-10-28 . 2020-03-13 . 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 . dead . particularly section 3.2.2 (pages 3.21, i.e. 195 of the PDF file, and following).
6. Web site: Journal of Geophysical Research. The presence of metastable HNO₃/H₂O solid phases in the stratosphere inferred from ER 2 data. 2012-08-17. 2020-01-26. https://web.archive.org/web/20200126223114/http://www.agu.org/journals/jd/v101/iD04/96JD00062/96JD00062.pdf. dead.
7. Web site: World Meteorological Organization . Nacreous PSC, International Cloud Atlas . 2017 . 3 April 2019.
8. Web site: Maturilli. Maturilli. Polar Stratospheric Clouds Above Spitsbergen. Alfred Wegener Institute for Polar and Marine Research. dead. https://web.archive.org/web/20070824200937/http://www.awi.de/en/research/research_divisions/climate_science/atmospheric_circulations/research_themes/aerosol/polar_stratospheric_clouds/. 2007-08-24.
9. Web site: Australian Antarctic Division. Polar stratospheric clouds / Observations. https://web.archive.org/web/20110602123302/http://www.antarctica.gov.au/about-antarctica/fact-files/atmosphere/polar-stratospheric-clouds/observations. dead. June 2, 2011.
10. Web site: World Meteorological Organization . Upper atmospheric clouds, International Cloud Atlas . 2017 . 31 July 2017.