Description

A dense, horizontal roll with more or less tattered edges, situated on the lower front part of certain primary clouds and having, when extensive, the appearance of a dark, menacing arch. Technicall an "accessory cloud" which is in association with other cloud types, it occurs with Cumulonimbus (Cb) and, less often, with Cumulus.

Roll clouds and shelf clouds are the two main types of arcus clouds. They most frequently form along the leading edge or gust fronts of thunderstorms. Roll clouds may also arise in the absence of thunderstorms, forming along the shallow cold air currents of some sea breeze boundaries and cold fronts.