Noctilucent Clouds

Abstract

Noctilucent clouds are immediately recognizable, even when being seen for the first time. The name suggests it all: they are night-shining clouds. From mid-latitudes(ø > 50°), they can be seen during the summer in the twilight arch which moves around the north (or south, in the southern hemisphere) horizon as the night progresses. In form much like cirrostratus clouds, they are usually silvery-white or pale blue in colour and they stand out clearly behind the darker twilight sky. Ordinary (i.e. tropospheric) clouds are dark silhouettes under these conditions; noctilucent clouds shine. The reason for this is that noctilucent clouds are very high in the atmosphere and remain in sunlight long after the Sun has set at ground level.