Zones of the visibility of a noctilucent cloud

Abstract

For a noctilucent cloud (NLC) with its typical features the brightness contrast k of such a cloud against the sky is computed as observed from any point within a circle of more than 800 km. This contrast is compared with the contrast threshold E of the eye. The quotient I k I : E is a measure of the degree of visibility. Its distribution over the earth's surface in the vicinity of an NLC at 5 different times during twilight reveals zones of differential visibility (Figs. 4a-e).