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.

UPDATE A 2018 study shows that methane emissions are responsible for the increase of noctilucent clouds, which glow eerily at night.

• Noctilucent cloud http://en.wikipedia.org/wiki/Noctilucent_clouds
• Reaction of Ozone and Climate to Increasing Stratospheric Water Vapor http://www.giss.nasa.gov/research/briefs/shindell_05/