Noctilucent Clouds

Abstract

Noctilucent clouds (abbreviation: NLC) are clouds situated high in the atmosphere, seen against the background of a dark sky, while they are still illuminated by sunlight, when the sun is well below the horizon, at angles of depression between 6° and nearly 18° (Fig. 1). They generally resemble cirrus or cirrostratus, often having a delicate filigree pattern, with marked wave structure. They are very thin, and stars shine through them almost undimmed.