A dense, horizontal roll with more or less tattered edges, situated on the lower front part of certain primary clouds and having, when extensive, the appearance of a dark, menacing arch. Technicall an "accessory cloud" which is in association with other cloud types, it occurs with Cumulonimbus (Cb) and, less often, with Cumulus.
Roll clouds and shelf clouds are the two main types of arcus clouds. They most frequently form along the leading edge or gust fronts of thunderstorms. Roll clouds may also arise in the absence of thunderstorms, forming along the shallow cold air currents of some sea breeze boundaries and cold fronts.
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 cumulonimbus, but could form on any type of convective cumulus clouds. Rising cloud motion can often 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 convective cloud's downdraft spreads out across the land surface, with the leading edge called a gust front. This outflow cuts under warm, stable air being drawn into the storm's updraft. As the lower and cooler air lifts the warm moist air, its water condenses, creating a cloud which often rolls with the different winds above and below (wind shear).
People seeing a shelf cloud may believe they have seen a wall cloud often associated with severe weather. This is a mistake, because although an approaching shelf cloud appears to form a wall made of cloud, shelf clouds usually appear on the leading edge of a storm, while wall clouds are usually 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 gustnado, a small, usually weak vortex.
Shelf cloud seen over Enschede, The Netherlands, July 2004. Source: Wikicommons
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. They occur with strong inversion conditions with a shallow layer of colder air moving under warm air. The warm air is lifted up ahead of the leading edge of the cold air, and with enough moisture present, a cloud is formed. The air then rolls down behind the leading edge giving the roll appearance. They can last for several hours and be hundreds of miles long. One of the most famous frequent occurrences is the Morning Glory cloud in Queensland, Australia, which can occur up to four out of 10 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 or ahead of cold fronts and are not exclusively associated with coastal regions.
Coastal roll clouds have been seen in many places, including California, the English Channel, Shetland Islands, the North Sea coast, coastal regions of Australia, and Nome, Alaska.
Roll cloud seen over Punta del Este, Uruguay, 25 January 2009. Source: Wikicommons. Author: Daniela Mirner Eberl