Polar stratospheric cloud

Polar Stratospheric Cloud
Antarctic stratospheric cloud (nacreous clouds)
Abbreviation	PSC
Altitude	15,000–25,000 m (50,000–80,000 ft)
Classification	Other
Appearance	Glowing brightly with vivid iridescent colours
Precipitation cloud?	no
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Polar stratospheric clouds or PSCs, also known as nacreous clouds ( /ˈneɪkriːəs/, from nacre, or mother of pearl, due to its iridescence), are clouds in the winter polar stratosphere at altitudes of 15,000–25,000 meters (50,000–80,000 ft). They are implicated in the formation of ozone holes;^[1] their 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 destruction.^[2]

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 and chemical composition.

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. Because of this, an observer, unfamiliar with this type of cloud — also a noctilucent cloud — may think one to be a UFO.

PSCs form at very low temperatures, below −78 °C. These temperatures can occur in the lower stratosphere in polar winter. In the Antarctic, temperatures below −88 °C 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 PSCs.

Types of PSC

A type 2 (water) PSC

PSCs are classified into three types Ia, Ib and II according to their chemical composition.

• Type I clouds contain nitric acid and water.
• Type Ia clouds consist of crystals formed from nitric acid and water.
• Type Ib cloud droplets additionally contain sulfuric acid and are present in the form of supercooled ternary solution.
• Type II clouds consist of water ice only.

See also

• Cloud iridescence
• Noctilucent clouds
• Ozone depletion
• Aurora (astronomy)
• Circumhorizontal arc

References

1. ^ United States Environmental Protection Agency — see [1]
2. ^ World Meteorological Organization Scientific Assessment of Ozone Depletion (2002), particularly section 3.2.2 (pages 3.21, i.e. 195 of the PDF file, and following).

• 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

• "Rare cloud spotted". AAP. Sydney Morning Herald. August 1, 2006. http://www.smh.com.au/news/science/extreme-weather-makes-rare-cloud/2006/08/01/1154198117951.html. 
• http://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
• Macrae, Fiona (01 August 2006). "Rare clouds 'could indicate global warming'". Daily Mail. http://www.dailymail.co.uk/pages/live/articles/news/worldnews.html?in_article_id=398592&in_page_id=1811.