A shelf cloud is a low, horizontal, wedge-shaped arcus cloud. A shelf cloud is attached to the base of the parent
cloud, which is usually a thunderstorm, but could form on any type of convective clouds. Rising cloud motion often can be seen in the leading (outer) part of the shelf cloud, while the underside often appears
turbulent and wind-torn. Cool, sinking air from a
downdraft spreads out across the land surface, with the leading edge called a
gust front. This outflow cuts under warm air being drawn into the storm's
updraft. As the lower cooler air lifts the warm moist air, its water condenses, creating a cloud which often rolls with the different winds above and below (
People seeing a shelf cloud may believe they have seen a
wall cloud. This is likely a mistake, since an approaching shelf cloud appears to form a wall made of cloud. A shelf cloud usually appears on the leading edge of a storm, and a wall cloud will usually be at the rear of the storm.
A sharp, strong gust front will cause the lowest part of the leading edge of a shelf cloud to be ragged and lined with rising
fractus clouds. In a severe case there will be
vortices along the edge, with twisting masses of
scud that may reach to the ground or be accompanied by rising dust. A very low shelf cloud accompanied by these signs is the best indicator that a potentially violent wind
squall is approaching. An extreme example of this phenomenon looks almost like a tornado and is known as a
A roll cloud (Cloud Atlas name volutus) is a low, horizontal, tube-shaped, and relatively rare type of arcus cloud. They differ from shelf clouds by being completely detached from other cloud features. Roll clouds usually appear to be "rolling" about a horizontal axis. They are a solitary wave called a
soliton, which is a wave that has a single crest and moves without changing speed or shape. One of the most famous frequent occurrences is the
Morning Glory cloud in
Australia, which can occur up to four out of ten days in October.
 One of the main causes of the Morning Glory cloud is the
mesoscale circulation associated with sea breezes that develop over the
Cape York Peninsula and the
Gulf of Carpentaria. However, similar features can be created by
downdrafts from thunderstorms and are not exclusively associated with coastal regions.
Coastal roll clouds have been seen in many places, including
Shetland Islands, the North Sea coast, coastal regions of Australia, and Nome, Alaska.
A roll cloud associated with a severe thunderstorm over
, United States