on rare occasions, a shelf cloud may turn into a roll cloud. The motions of the warm air riding up and over the cool air moving down and under creates a swirling of air or an eddy. The cloud takes on the shape of a horizontal tube that appears to be rolling. It is detached from the thunderstorm on its leading edge.
1. The popular term for arcus. 2. A low-level, horizontal, tube-shaped arcus cloud associated with a gust front of a convective storm (or occasionally a cold front). Roll clouds are relatively rare; they are completely detached from the convective storm's cloud base, thus differentiating them from the more familiar shelf clouds. Roll clouds appear to be rolling about a horizontal axis because of the shearing effects and horizontal vorticity provided by the differing air masses. See also rotor cloud, morning glory.
A turbulent altocumulus-type cloud formation found in the lee of some large mountain barriers. The air in the cloud rotates around an axis parallel to the range. Also sometimes refers to part of the cloud base along the leading edge of a cumulonimbus cloud; it is formed by rolling action in the wind shear region between cool downdrafts within the cloud and warm updrafts outside the cloud. Also called rotor cloud.