Arcus clouds

Arcus clouds are low-level clouds typically associated with thunderstorms. They are low, horizontal cloud formations below the thunderstorm’s rain-free base. The name ‘arcus’ comes from the latin word for ‘arch’.

The World Meteorological Organization defines arcus clouds as: “dense, horizontal roll with more or less tattered edges, situated on the lower front part of certain clouds and having, when extensive, the appearance of a dark, menacing arch.”