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.