Arcus

An arcus cloud is a low, horizontal cloud formation, usually appearing as an accessory cloud for a cumulonimbus. Roll clouds and shelf clouds are the two main types of arcus. Arcus clouds are most often formed along the leading edge or "gust fronts" of the storm exit; some of the most dramatic arcus formations mark the lightning fronts of convective systems that produce right. Roller clouds can also arise in the absence of storms, forming along the cold surface air curves of some sea breeze boundaries and cold fronts.

Subtypes
Shelf cloud

Roll cloud