The Aeronomy of Ice in the Mesosphere (AIM) mission is the first satellite dedicated to the study of noctilucent clouds. Noctilucent clouds, sometimes called Polar Mesospheric Clouds, were first reported in 1885. Forming at altitudes above 50 miles, they are so faint that they can only be seen from the ground in the reflected light of the Sun after it has set below the horizon. Since their discovery, their cause has been a subject of study as a possible indicator of climate change.

Polar mesospheric clouds

http://en.wikipedia.org/wiki/Polar_mesospheric_clouds

Noctilucent cloud

http://en.wikipedia.org/wiki/Noctilucent_clouds

Related

Reaction of Ozone and Climate to Increasing Stratospheric Water Vapor

http://www.giss.nasa.gov/research/briefs/shindell_05/