Saturday, April 11, 2009

Pretty Clouds

I am very familiar with the common 22° halo, as well as a few rarer arcs also caused by atmospheric ice crystals. However, when doing research for my post of the Perlan project, i came across some truly weird types of clouds. All pics are from wikpedia and either from NASA if not noted (no copyright casue they're gov't works) or under a Creative Commons 2.0 liscence, with author noted.
The following 3 pictures are polar stratospheric clouds, also known to astronomers as nacreous clouds, which form in the stratosphere during winter between 50,000 and 80,000 feet. They form in temperatures below 78° C (-108° F), which occur more commonly in the Antarctic than Arctic. Peculiarly, their formation (they are formed from water, nitric acid, and sometimes uslfuric acid) removes nitric acid from the stratosphere, disrupting nitrogen and chlorine cycles and causing ozone depletion. Every rose has its thorn.

(from Wikipedia contributor Mathiasm)

Noctilucent clouds are created by ice crystals in the mesosphere (third layer of the atmosphere) at altitudes between 47 and 53 miles at temperatures below -120° C (-184° F). Like polar stratospheric clouds, they're only visible at higher latitudes, although they're getting more frequent, possibly due to global warming. They are much more common and brighter in the Northern hemisphere for reasons unknown. They have also been formed by the Space Shuttle's water vapor exhaust at 64-71 miles altitude. They show high radar reflectivity between 50 MHz and 1.3 GHz, possibly due to the ice crystals becoming coated by sodium and iron from vaporized micrometeroites. Amazing stuff.
This first picture was taken from the ISS.

(From Mika Yrjölä

Finally, there's plain old cloud irridescence, where unifrom water droplets in low-to-medium altitude clouds coherently diffract light. (Coherence is not my strong suit, especially not at 2:23 am)
The first is an especially great shot by Mila Zinkova; the second, by an unnamed contributor, is closer to my personal experience.

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