Dec 022009
 

Polar Mesospheric Clouds PMCs (also known as noctilucent clouds) exist at a height of about 76 to 85 kilometers. They are transient, upper atmospheric phenomena observed usually in the summer months at high latitudes (greater than 50 degrees)
 of both the Northern and Southern Hemispheres. PMCs are made of crystals of water ice formed at about −120 °C, although their formation mechanism are still not really understood nowadays. For more information, I let you refer to the wikipedia website : http://en.wikipedia.org/wiki/Noctilucent_cloud.

This image was acquired at an altitude of just over 200 miles in the pre-dawn hours of July 22, 2008 as the International Space Station was passing over western Mongolia in central Asia (credit NASA).
PMCs have been increasing over the past 30 years… sign of climate changes?
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