Clouds of unknown composition which occur at great heights. These clouds are extremely rare, seen only at twilight and in the summer months in both hemispheres.

Weakly-luminous clouds, seen at night at heights of about 50 miles (80 km) above the Earth.

Wavy, thin, bluish-white clouds that are best seen at twilight in polar latitudes. They form at altitudes about 80 to 90 km above the Earth's surface.

Clouds formed at extremely high-altitude that shine at night. A bit of a mystery, scientists aren't sure why or how they are formed.

Wavy, thin clouds that are seen at twilight in polar regions. They form at altitudes of 80-90 km and are made visible by sunlight reflecting off the underside of the cloud.

Rarely seen clouds of tiny ice particles that form approximately 75 to 90 kilometers above the earth's surface. They have been seen only during twilight ( dusk and dawn) during the summer months in the higher latitudes. They may appear bright against a dark night sky, with a blue-silver color or orange-red.

Clouds of unknown composition which occur at great heights, 75 to 90 kilometers. They resemble thin cirrus, but usually with a bluish or silverish color, although sometimes orange to red, standing out against a dark night sky. Sometimes called luminous clouds. These clouds have been seen rarely, and then only during twilight, especially with the sun between 5 and 13 degrees below the horizon; they have been observed only during summer months in both hemispheres (between latitudes 50 to 75 degrees N and 40 to 60 degrees S), and only in some parts of these latitude belts.