Arcus clouds develop on a thunderstorm’s gust front, the leading edge of the downdraft. As the cool air in the dowdraft undercuts the warm moist air ahead of the storm, forcing it upwards. At the same time, the rising air entrains (draws along) cooler air from the downdraft, cooling it. As the air cools, it condenses, forming a cloud along the outflow gust front. Typically, a gust front will have a convex shape (as seen from above), producing an arching cloud – therefore the arcus name.
In environments with low clouds the exact position of a thunderstorm is often revealed by arcus clouds, as the updraft may be obscured.
Arcus clouds are outflow based features, forming on the outflow gust front. In general there are two distinct types of arcus clouds: shelf cloud and roll cloud.