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Anarcus cloud is a low, horizontalcloud formation, usually appearing as an accessory cloud to acumulonimbus.Roll clouds andshelf clouds are the two main types of arcus clouds. They most frequently form along the leading edge orgust fronts ofthunderstorms; some of the most dramatic arcus formations mark the gust fronts ofderecho-producingconvective systems. Roll clouds may also arise in the absence of thunderstorms, forming along the shallow cold air currents of somesea breeze boundaries andcold fronts.
Ashelf cloud is a low, horizontal, wedge-shaped arcus cloud attached to the base of the parent cloud, which is usually a thunderstorm cumulonimbus, but could form on any type of convective clouds. Rising air motion can often be seen in the leading (outer) part of the shelf cloud, while the underside can often appear asturbulent and wind-torn. Cool, sinking air from astorm cloud'sdowndraft spreads out across the land surface, with the leading edge called agust front. This outflow cuts under warm air being drawn into the storm'supdraft. 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 awall cloud. This is likely to be a mistake, since 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 risingfractus clouds. In a severe case there will bevortices along the edge, with twisting masses ofscud 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 windsquall is approaching. An extreme example of this phenomenon looks almost like a tornado and is known as agustnado.[1]
Aroll cloud (Cloud Atlas namevolutus) 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 asoliton, which is a wave that has a single crest and moves without changing speed or shape. This rolling is due to the variation in speed and direction of the winds with altitude (wind shear).[2]
One of the most famous frequent occurrences is theMorning Glory cloud inQueensland,Australia, which can occur up to four out of ten days in October.[3] One of the main causes of the Morning Glory cloud is themesoscale circulation associated with sea breezes that develop over theCape York Peninsula and theGulf of Carpentaria. Such coastal roll clouds have been seen in many places, includingCalifornia, theEnglish Channel,Shetland Islands, the North Sea coast, coastal regions of Australia, and Nome, Alaska.
However, similar features can be created bydowndrafts from thunderstorms or advancing cold front, and are not exclusively associated with coastal regions.[2] They have been reported at different locations inland, includingKansas.[4]
Roll clouds have not been associated withfunnel clouds ortornadoes, as they are a horizontal vortex.[2]
![Coastal roll cloud in Punta del Este, Maldonado, Uruguay, a type known as Volutus[5]](/mt/?noimg=&dark=on&url=https%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aRoll-cloud.JPG)
