Noctilucent

Bright (or noctilucent) night clouds resemble faint Cirrus, but are usually bluish or silver in color, sometimes orange or red, and stand out against the dark background of the night sky. They are the highest clouds on Earth, as shown by measurements, which show that they are at altitudes between 75 and 90 kilometers.

The physical constitution of noctilucents is still unknown, but there are certain reasons to think that they consist of very fine cosmic dust. Amazingly, recent evidence indicates that at least some of these noctilucent clouds result from the frozen water expelled by NASA space shuttles.

Noctilucents are only rarely observed in the summer months at latitudes between 50º and 70º north and south of Ecuador and with the sun between 5º and 13º below the horizon. The first time noctilucent clouds were observed was in 1885, two years after the eruption of the Krakatoa volcano on the island of Java.