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nacreous cloud

 
Dictionary: nacreous cloud

n.
A cloud resembling a cirrus, showing iridescent coloration when the sun is several degrees below the horizon.

[From NACRE (from the iridescent coloring of mother-of-pearl).]


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Geography Dictionary: nacreous cloud
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A rare, iridescent cloud formation, occurring in the stratosphere at a mean height of 24 km. It develops above high latitudes, shortly after sunset or before dawn. Its stationary nature suggests that this is a mountain wave cloud. See noctilucent cloud.

Wikipedia: Polar stratospheric cloud
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Polar Stratospheric Cloud
Arctic stratospheric cloud (nacreous clouds)
Arctic 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

Polar stratospheric clouds (PSCs), also known as nacreous clouds, 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]

Contents

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.

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.

See also

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).

External links

Research

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Copyrights:

Dictionary. The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.  Read more
Geography Dictionary. A Dictionary of Geography. Copyright © Susan Mayhew 1992, 1997, 2004. All rights reserved.  Read more
Wikipedia. This article is licensed under the Creative Commons Attribution/Share-Alike License. It uses material from the Wikipedia article "Polar stratospheric cloud" Read more