Polar mesospheric clouds - also known as noctilucent or "night shining" clouds - form between 76 to 85 km (47 to 53 mi) above the Earth's surface, near the boundary of the mesosphere and thermosphere, a region known as the mesopause. At these altitudes, water vapor can freeze into clouds of ice crystals. When the sun is below the horizon and the ground is in darkness, these high clouds may still be illuminated, lending them their ethereal, "night shining" qualities.
Polar mesospheric clouds have been observed from all human vantage points in both the Northern and Southern Hemispheres - from the surface, in aircraft, and from spacecraft - and tend to be most visible during the late spring and early summer. Image courtesy of NASA.