Polar mesospheric clouds (noctilucent or “night shining” clouds) form between 76 to 85 kilometers (47 to 53 miles) above the Earth’s surface, near the boundary of the mesosphere and thermosphere, a region known as the mesopause. Image © Astronaut William L. Stefanov, Jacobs/ESCG at NASA-JSC.
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 the International Space Station (ISS) —and tend to be most visible during the late spring and early summer. Some atmospheric scientists seek to understand their mechanisms of formation, while others have identified them as potential indicators of atmospheric changes resulting from increases in greenhouse gas concentrations.