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 (from Latin ''cumulus'', "heaped" and ''nimbus'', "rainstorm") 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. ...

. 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 acoustic effect on the Earth's atmosphere, known as thunder. Relatively weak thunderstorms are some ...

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

derecho A ''derecho'' (, from es, derecho, link=no , 'straight') is a widespread, long-lived, straight-line wind storm that is associated with a fast-moving group of severe thunderstorms known as a mesoscale convective system. Derechos can cause hurri ...

-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 any wind that blows from a large body of water toward or onto a landmass; it develops due to differences in air pressure created by the differing heat capacities of water and dry land. As such, sea breezes ar ...

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 of low pressure. It often forms behind an extratropical cyclone (to the west in the Northern ...

Types

Shelf cloud

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

, which is usually a thunderstorm cumulonimbus, but could form on any type of convective clouds. Rising cloud 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 a 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 is a small-scale current of rising air, often within a cloud. Overview Localized regions of warm or cool air will exhibit vertical movement. A mass of warm air will typically be less dense than the surrounding region, ...

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 is a small-scale current of rising air, often within a cloud. Overview Localized regions of warm or cool air will exhibit vertical movement. A mass of warm air will typically be less dense than the surrounding regi ...

. 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 (or 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 or horizont ...

). 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 r ...

. 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 (''scuds'') also known as Fractostratus or Fracto-Cumulus 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 ...

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 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. 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 thus ...

of shelf cloud just before a

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Puducherry Puducherry or Pondicherry may refer to: * Puducherry (union territory), a union territory of India ** Pondicherry, capital of the union territory of Puducherry ** Puducherry district, a district of the union territory of Puducherry ** Puducherry t ...

India India, officially the Republic of India (Hindi: ), is a country in South Asia. It is the seventh-largest country by area, the second-most populous country, and the most populous democracy in the world. Bounded by the Indian Ocean on the so ...

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

Netherlands ) , anthem = ( en, "William of Nassau") , image_map = , map_caption = , subdivision_type = Sovereign state , subdivision_name = Kingdom of the Netherlands , established_title = Before independence , established_date = Spanish Netherl ...

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 or solitary wave is a self-reinforcing wave packet that maintains its shape while it propagates at a constant velocity. Solitons are caused by a cancellation of nonlinear and dispersive effects in the medi ...

, which is a wave that has a single crest and moves without changing speed or shape. One of the most famous frequent occurrences is the Morning Glory cloud in

Queensland ) , nickname = Sunshine State , image_map = Queensland in Australia.svg , map_caption = Location of Queensland in Australia , subdivision_type = Country , subdivision_name = Australia , established_title = Before federation , establishe ...

, Australia, 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 Cape York Peninsula is a large peninsula located in Far North Queensland, Australia. It is the largest unspoiled wilderness in northern Australia.Mittermeier, R.E. et al. (2002). Wilderness: Earth’s last wild places. Mexico City: Agrupació ...

and the Gulf of Carpentaria. However, similar features can be created by

downdrafts In meteorology, an updraft is a small-scale current of rising air, often within a cloud. Overview Localized regions of warm or cool air will exhibit vertical movement. A mass of warm air will typically be less dense than the surrounding region, ...

from thunderstorms and are not exclusively associated with coastal regions. Coastal roll clouds have been seen in many places, including

California California is a state in the Western United States, located along the Pacific Coast. With nearly 39.2million residents across a total area of approximately , it is the most populous U.S. state and the 3rd largest by area. It is also the m ...

, the

English Channel The English Channel, "The Sleeve"; nrf, la Maunche, "The Sleeve" (Cotentinais) or ( Jèrriais), (Guernésiais), "The Channel"; br, Mor Breizh, "Sea of Brittany"; cy, Môr Udd, "Lord's Sea"; kw, Mor Bretannek, "British Sea"; nl, Het Kana ...

, Shetland Islands, the North Sea coast, coastal regions of Australia, and Nome, Alaska. File:Roll-Cloud-Racine.jpg, A roll cloud associated with a severe thunderstorm over Racine, Wisconsin, 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 later became internationally known as a resort for the Latin and North American j ...

, Maldonado,

Uruguay Uruguay (; ), officially the Oriental Republic of Uruguay ( es, República Oriental del 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 ...

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

Drake Passage The Drake Passage (referred to as Mar de Hoces Hoces Sea"in Spanish-speaking countries) is the body of water between South America's Cape Horn, Chile and the South Shetland Islands of Antarctica. It connects the southwestern part of the Atla ...

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 60° S latitude and encircling Antarctica. With a size of , it is regarded as the second-small ...

File:Rollclouds.jpg, Roll clouds over the south of Brazil.

References

External links

*
Roll Cloud vs. Shelf Cloud
* {{Cloud types Cumulus Accessory clouds

Types

Shelf cloud

Roll cloud

See also

References

External links