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.
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.
Photo: Giant Tubular Cloud Rolling Across the SeaLive Science - March 5, 2012
This roll cloud photo was taken by Capt. Andreas M. van der Wurff from a ship en route from Paranagua, Brazil, to Montevideo, Uruguay, on Feb. 6. A photo taken near Brazil captures a rare and beautiful "roll cloud," a tubular cloud that seems to tumble across the sky. The spectacular photo, captured last month, was just released by NASA's Earth Science Picture of the Day blog.Roll clouds are a type of arcus cloud, which is a category of low cloud formations. Their more common cousin is the shelf cloud, often seen on the leading edges of thunderstorms. Roll clouds sometimes form along with storms, too, born out of the storm's downdraft. Sinking cold air causes warm, moist air on the planet's surface to climb to higher altitudes, where the moisture condenses into cloud form. Winds from the storm "roll" the cloud parallel to the horizon, creating an effect that looks much like a horizontal tornado. Unlike shelf clouds, rolls clouds are completely detached from the bulk of the storm.
Roll Cloud Over WisconsinNASA - August 31, 2011
What kind of cloud is this? A type of arcus cloud called a roll cloud. These rare long clouds may form near advancing cold fronts. In particular, a downdraft from an advancing storm front can cause moist warm air to rise, cool below its dew point, and so form a cloud. When this happens uniformly along an extended front, a roll cloud may form. Roll clouds may actually have air circulating along the long horizontal axis of the cloud. A roll cloud is not thought to be able to morph into a tornado. Unlike a similar shelf cloud, a roll cloud is completely detached from their parent cumulonimbus cloud. Pictured above, a roll cloud extends far into the distance as a storm approached in 2007 in Racine, Wisconsin, USA.
Morning Glory Clouds Over AustraliaNASA - August 24, 2009
What 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.
New phenomenon found in internal waves PhysOrg - April 28, 2010
Internal waves -- huge but nearly invisible ripples that occur in the oceans, the atmosphere and stars ...
Morning Glory clouds in Australia are one example of internal waves in the atmosphere.
Internal waves -- huge but nearly invisible ripples that occur in the oceans, the atmosphere and stars -- can play an important role in climate change and other processes, but there is plenty about them that remains to be understood. In a significant new finding, however, MIT researchers have discovered that such waves can be analyzed using the same techniques physicists have developed to study the passage of light.
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 SaskatchewanNASA - January 22, 2008
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