An arcus cloud is a low, horizontal

cloud In meteorology, a cloud is an aerosol consisting of a visible mass of miniature liquid droplets, frozen crystals, or other particles, suspended in the atmosphere of a planetary body or similar space. Water or various other chemicals may ...

formation, usually appearing as an accessory cloud to a

cumulonimbus Cumulonimbus () is a dense, towering, vertical cloud, typically forming from water vapor condensing in the lower troposphere that builds upward carried by powerful buoyant air currents. Above the lower portions of the cumulonimbus the water va ...

. Roll clouds and shelf clouds are the two main types of arcus clouds. They most frequently form along the leading edge or

gust front An outflow boundary, also known as a gust front, is a storm-scale or mesoscale boundary separating thunderstorm-cooled air ( outflow) from the surrounding air; similar in effect to a cold front, with passage marked by a wind shift and usually ...

s of

thunderstorm A thunderstorm, also known as an electrical storm or a lightning storm, is a storm characterized by the presence of lightning and its acoustics, acoustic effect on the Earth's atmosphere, known as thunder. Relatively weak thunderstorm ...

s; some of the most dramatic arcus formations mark the gust fronts of

derecho A ''derecho'' (, from , 'straight') is a widespread, long-lived, straight-line wind storm that is associated with a fast-moving group of Severe weather#Categories, severe thunderstorms known as a mesoscale convective system. Derechos cause Bea ...

-producing convective systems. Roll clouds may also arise in the absence of thunderstorms, forming along the shallow cold air currents of some

sea breeze A sea breeze or onshore breeze is a wind that blows in the afternoon from a large body of water toward or onto a landmass. By contrast, a land breeze or offshore breeze is a wind that blows in the night from a landmass toward or onto a large ...

boundaries and

cold front A cold front is the leading edge of a cooler mass of air at ground level that replaces a warmer mass of air and lies within a pronounced surface Trough (meteorology), trough of Low-pressure area, low pressure. It often forms behind an extratropica ...

Types

Shelf cloud

A shelf cloud is a low, horizontal, wedge-shaped arcus cloud attached to the base of the parent

, 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 as

turbulent In fluid dynamics, turbulence or turbulent flow is fluid motion characterized by chaotic changes in pressure and flow velocity. It is in contrast to laminar flow, which occurs when a fluid flows in parallel layers with no disruption between ...

and wind-torn. Cool, sinking air from a storm cloud's

downdraft In meteorology, an updraft (British English: ''up-draught'') is a small-scale current of rising air, often within a cloud. Overview Vertical drafts, known as updrafts or downdrafts, are localized regions of warm or cool air that move vertically ...

spreads out across the land surface, with the leading edge called a

. This outflow cuts under warm air being drawn into the storm's

updraft In meteorology, an updraft (British English: ''up-draught'') is a small-scale air current, current of rising air, often within a cloud. Overview Vertical drafts, known as updrafts or downdrafts, are localized regions of warm or cool air that mov ...

. 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 Wind shear (; also written windshear), sometimes referred to as wind gradient, is a difference in wind speed and/or direction over a relatively short distance in the atmosphere. Atmospheric wind shear is normally described as either vertical ...

). People seeing a shelf cloud may believe they have seen a

wall cloud A wall cloud (murus or pedestal cloud) is a large, localized, persistent, and often abrupt lowering of cloud that develops beneath the surrounding base of a cumulonimbus cloud and from which tornadoes sometimes form. It is typically beneath the ...

. 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 rising

fractus cloud Fractus clouds, also called fractostratus or fractocumulus, are small, ragged cloud fragments that are usually found under an ambient cloud base. They form or have broken off from a larger cloud, and are generally sheared by strong winds, givin ...

s. In a severe case there will be

vortices In fluid dynamics, a vortex (: vortices or vortexes) is a region in a fluid in which the flow revolves around an axis line, which may be straight or curved. Vortices form in stirred fluids, and may be observed in smoke rings, whirlpools in th ...

along the edge, with twisting masses of

scud A Scud missile is one of a series of tactical ballistic missiles developed by the Soviet Union during the Cold War. It was exported widely to both Second and Third World countries. The term comes from the NATO reporting name attached to the m ...

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 A squall is a sudden, sharp increase in wind speed lasting minutes, as opposed to a wind gust, which lasts for only seconds. They are usually associated with active weather, such as rain showers, thunderstorms, or heavy snow. Squalls refer to the ...

is approaching. An extreme example of this phenomenon looks almost like a tornado and is known as a

gustnado A gustnado is a brief, shallow surface-based vortex which forms within the downburst emanating from a thunderstorm. The name is a portmanteau by elision of "gust front tornado", as gustnadoes form due to non-tornadic straight-line wind features ...

. File:Shelf cloud pano oct07 ver4.jpg, Underside of a weak shelf cloud. File:Shelf clouds gif.gif, A

time-lapse photography Time-lapse photography is a technique in which the frequency at which film frames are captured (the frame rate) is much lower than the frequency used to view the sequence. When played at normal speed, time appears to be moving faster and th ...

of shelf cloud just before a

Pondicherry Pondicherry, officially known as Puducherry, is the Capital city, capital and most populous city of the Puducherry (union territory), Union Territory of Puducherry in India. The city is in the Puducherry district on the southeast coast of Indi ...

India India, officially the Republic of India, is a country in South Asia. It is the List of countries and dependencies by area, seventh-largest country by area; the List of countries by population (United Nations), most populous country since ...

. File:Rolling-thunder-cloud.jpg, A shelf cloud over

Enschede Enschede (; local ) is a list of cities in the Netherlands by province, city and municipalities of the Netherlands, municipality in the province of Overijssel and the Twente region of the eastern Netherlands. The east of the urban area reaches ...

Netherlands , Terminology of the Low Countries, informally Holland, is a country in Northwestern Europe, with Caribbean Netherlands, overseas territories in the Caribbean. It is the largest of the four constituent countries of the Kingdom of the Nether ...

File:Shelf cloud 1.jpg, A shelf cloud in Durango, Mexico

Roll cloud

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 In mathematics and physics, a soliton is a nonlinear, self-reinforcing, localized wave packet that is , in that it preserves its shape while propagating freely, at constant velocity, and recovers it even after collisions with other such local ...

, 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 (

).. One of the most famous frequent occurrences is the

Morning Glory cloud The Morning Glory cloud is a rare List of meteorological phenomena, meteorological phenomenon consisting of a low-level atmospheric solitary wave and associated cloud, occasionally observed in different locations around the world. The wave o ...

Queensland Queensland ( , commonly abbreviated as Qld) is a States and territories of Australia, state in northeastern Australia, and is the second-largest and third-most populous state in Australia. It is bordered by the Northern Territory, South Austr ...

Australia Australia, officially the Commonwealth of Australia, is a country comprising mainland Australia, the mainland of the Australia (continent), Australian continent, the island of Tasmania and list of islands of Australia, numerous smaller isl ...

, which can occur up to four out of ten 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 The Cape York Peninsula is a peninsula located in Far North Queensland, Australia. It is the largest wilderness in northern Australia.Mittermeier, R.E. et al. (2002). Wilderness: Earth's last wild places. Mexico City: Agrupación Sierra Madre, ...

and the

Gulf of Carpentaria The Gulf of Carpentaria is a sea off the northern coast of Australia. It is enclosed on three sides by northern Australia and bounded on the north by the eastern Arafura Sea, which separates Australia and New Guinea. The northern boundary ...

. Such coastal roll clouds have been seen in many places, including

California California () is a U.S. state, state in the Western United States that lies on the West Coast of the United States, Pacific Coast. It borders Oregon to the north, Nevada and Arizona to the east, and shares Mexico–United States border, an ...

, the

English Channel The English Channel, also known as the Channel, is an arm of the Atlantic Ocean that separates Southern England from northern France. It links to the southern part of the North Sea by the Strait of Dover at its northeastern end. It is the busi ...

Shetland Islands Shetland (until 1975 spelled Zetland), also called the Shetland Islands, is an archipelago in Scotland Scotland is a Countries of the United Kingdom, country that is part of the United Kingdom. It contains nearly one-third of the Uni ...

, the North Sea coast, coastal regions of Australia, and Nome, Alaska. However, similar features can be created by downdrafts from thunderstorms or advancing cold front, and are not exclusively associated with coastal regions. They have been reported at different locations inland, including

Kansas Kansas ( ) is a landlocked U.S. state, state in the Midwestern United States, Midwestern region of the United States. It borders Nebraska to the north; Missouri to the east; Oklahoma to the south; and Colorado to the west. Kansas is named a ...

. Roll clouds have not been associated with

funnel cloud A funnel cloud is a funnel-shaped cloud of condensed water droplets, associated with a rotating column of wind and extending from the base of a cloud (usually a cumulonimbus or towering cumulus cloud) but not reaching the ground or a water su ...

s or

tornado A tornado is a violently rotating column of air that is in contact with the surface of Earth and a cumulonimbus cloud or, in rare cases, the base of a cumulus cloud. It is often referred to as a twister, whirlwind or cyclone, although the ...

es as they are a horizontal vortex. File:Roll-Cloud-Racine.jpg, A roll cloud associated with a severe thunderstorm over

Racine, Wisconsin Racine ( ) is a city in Racine County, Wisconsin, United States, and its county seat. It is located on the shore of Lake Michigan at the mouth of the Root River (Wisconsin), Root River, south of Milwaukee and north of Chicago. It is the List ...

, United States File:Roll-cloud.JPG, Coastal roll cloud in

Punta del Este Punta del Este () is a seaside city and peninsula on the Atlantic Coast in the Maldonado Department of southeastern Uruguay. Starting as a small town, Punta del Este grew to become a resort for the Latin and North American jet set and tourists. T ...

, Maldonado,

Uruguay Uruguay, officially the Oriental Republic of Uruguay, is a country in South America. It shares borders with Argentina to its west and southwest and Brazil to its north and northeast, while bordering the Río de la Plata to the south and the A ...

, a type known as ''Volutus'' File:Volutus Southern Ocean.jpg, A sequence of volutus clouds at sea in the

Drake Passage The Drake Passage is the body of water between South America's Cape Horn, Chile, Argentina, and the South Shetland Islands of Antarctica. It connects the southwestern part of the Atlantic Ocean (Scotia Sea) with the southeastern part of the Pa ...

of the

Southern Ocean The Southern Ocean, also known as the Antarctic Ocean, comprises the southernmost waters of the world ocean, generally taken to be south of 60th parallel south, 60° S latitude and encircling Antarctica. With a size of , it is the seco ...

File:Rollclouds.jpg, Roll clouds over southern Brazil.

References

External links

* * {{Cloud types Cumulus Accessory clouds

Types

Shelf cloud

Roll cloud

See also

References

External links