
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 fronts 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 somet ...
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 hu ...
-producing
convective systems. Roll clouds may also arise in the absence of thunderstorms, forming along the shallow cold air currents of some
sea breeze 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 ...
s.
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
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 ...
, 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 and wind-torn. Cool, sinking air from a
storm cloud
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. ...
'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 regi ...
spreads out across the land surface, with the leading edge called a
gust front. This outflow cuts under warm air being drawn into the storm's
updraft. 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 horizon ...
).
People seeing a shelf cloud may believe they have seen a
wall cloud. 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 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
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 t ...
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 of shelf cloud just before a 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 somet ...
in Pondicherry
Pondicherry (), now known as Puducherry ( French: Pondichéry ʊdʊˈtʃɛɹi(listen), on-dicherry, is the capital and the most populous city of the Union Territory of Puducherry in India. The city is in the Puducherry district on the sout ...
, Puducherry, 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 ...
.
File:Rolling-thunder-cloud.jpg, A shelf cloud over Enschede, Netherlands
)
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, subdivision_name = Kingdom of the Netherlands
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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, 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
)
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, map_caption = Location of Queensland in Australia
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, subdivision_name = Australia
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,
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: Agrupación ...
and the
Gulf of Carpentaria
The Gulf of Carpentaria (, ) is a large, shallow sea enclosed on three sides by northern Australia and bounded on the north by the eastern Arafura Sea (the body of water that lies between Australia and New Guinea). The northern boundary ...
. However, similar features can be created by
downdrafts 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 ...
, 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 Kan ...
,
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
Racine ( ) is a city in and the county seat of Racine County, Wisconsin, United States. It is located on the shore of Lake Michigan at the mouth of the Root River. Racine is situated 22 miles (35 km) south of Milwaukee and approximately 60 ...
, 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 ...
, 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 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-smal ...
File:Rollclouds.jpg, Roll clouds over the south of Brazil.
See also
*
Atmospheric convection
*
Horizontal convective rolls (cloud streets)
*
International Cloud Atlas
*
Morning Glory cloud, an extremely long variety of roll cloud
*
World Meteorological Organization
The World Meteorological Organization (WMO) is a specialized agency of the United Nations responsible for promoting international cooperation on atmospheric science, climatology, hydrology and geophysics.
The WMO originated from the Intern ...
References
External links
*
Roll Cloud vs. Shelf Cloud*
{{Cloud types
Cumulus
Accessory clouds