noctilucent clouds


noctilucent clouds

Clouds of unknown composition that occur at great heights, 45 to 55 miles (70–90 km). They resemble thin cirrus clouds but usually with a bluish or silverish color, although sometimes orange to red, standing out against a dark night sky. Also referred to as luminous clouds. These clouds have been observed rarely and then only during twilight in the summer months in both hemispheres and only between latitudes 50° to 75° N and 40° to 60° S.