Arcus Clouds
An arcus cloud is a low, horizontal cloud formation associated with the leading edge of thunderstorm outflow, or occasionally with a cold front even in the absence of thunderstorms. Roll clouds and shelf clouds are the two types of arcus clouds, slight variations in their generation and appearance being the difference.
Formation
Cool, sinking air from a storm cloud's downdraft spreads out across the surface with the leading edge called a gust front. This outflow undercuts warm air being drawn into the storm's updraft. As the cool air lifts the warm moist air, water condenses creating a cloud which often rolls with the different winds above and below (wind shear).
A sharp, strong gust front will cause the lowest part of the leading edge of an arcus 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 roll cloud is a low, horizontal, tube-shaped, and relatively rare type of arcus cloud. They differ from shelf clouds by being completely detached from the thunderstorm base or other cloud features. Roll clouds usually appear to be "rolling" about a horizontal axis. They can be a sign of possible microburst activity.
Morning Glory Clouds Over Australia
NASA - August 24, 2009What causes these long, strange clouds? No one is sure. A rare type of cloud known as a Morning Glory cloud can stretch 1,000 kilometers long and occur at altitudes up to two kilometers high. Although similar roll clouds have been seen at specific places across the world, the ones over Burketown, Queensland Australia occur predictably every spring. Long, horizontal, circulating tubes of air might form when flowing, moist, cooling air encounters an inversion layer, an atmospheric layer where air temperature atypically increases with height. These tubes and surrounding air could cause dangerous turbulence for airplanes when clear. Morning Glory clouds can reportedly achieve an airspeed of 60 kilometers per hour over a surface with little discernible wind. Pictured above, photographer Mick Petroff photographed some Morning Glory clouds from his airplane near the Gulf of Carpentaria, Australia.
A shelf cloud is a low, horizontal wedge-shaped arcus cloud. Unlike a roll cloud, a shelf cloud is attached to the base of the parent cloud (usually a thunderstorm). 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.
Occasionally people seeing a shelf cloud may believe they have seen a wall cloud. This is a common mistake, since an approaching shelf cloud appears to form a wall made of cloud. Generally speaking, a shelf cloud appears on the leading edge of a storm, and a wall cloud will usually be at the rear of the storm.
Shelf Cloud Over Saskatchewan
NASA - January 22, 2008
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