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Panorama of a strong shelf cloud, a type of arcus cloud
Underside of a weak shelf cloud

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.

Contents

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).

Types

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Roll cloud

A roll cloud in Uruguay.

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.

Shelf cloud

A shelf cloud over Enschede, Netherlands.

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.

Sign of danger

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.[1]

See also

External links

Notes and references

  1. ^ (English)Meteorological Service of Canada (Decembre 19th, 2002). "Gust fronts and wind squalls". Severe Weather Watcher Handbook. Environment Canada. http://www.msc-smc.ec.gc.ca/education/severe_weather/page19_e.cfm?. Retrieved 2008-06-22.  

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