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A striated shelf cloud, in Massachusetts in July 2022
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An arcus cloud is a low, horizontal cloud formation, usually appearing as an accessory cloud to a cumulonimbus. 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 thunderstorms; some of the most dramatic arcus formations mark the gust fronts of derecho-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 fronts.

Types

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Shelf cloud

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A shelf cloud is a low, horizontal, wedge-shaped arcus cloud attached to the base of the parent cloud, 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 and wind-torn. Cool, sinking air from a storm cloud's downdraft 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).

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 is approaching. An extreme example of this phenomenon looks almost like a tornado and is known as a gustnado.[1]

Roll cloud

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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. This rolling is due to the variation in speed and direction of the winds with altitude (wind shear).[2]

One of the most famous frequent occurrences is the Morning Glory cloud in Queensland, Australia, which can occur up to four out of ten days in October.[3] 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 and the Gulf of Carpentaria. Such coastal roll clouds have been seen in many places, including California, the English Channel, Shetland Islands, 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.[2] They have been reported at different locations inland, including Kansas.[4]

Roll clouds have not been associated with funnel clouds or tornadoes, as they are a horizontal vortex.[2]

See also

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References

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Arcus cloud

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An arcus cloud is a low, horizontal, and often dense formation characterized by a sharp, well-defined lower boundary and a more ragged upper edge, typically appearing as an elongated roll or wedge shape along the leading edge of a thunderstorm's gust front or occasionally a . It is classified as a supplementary feature of cumulonimbus or by the , distinguishing it from the main cloud body due to its horizontal orientation and association with outflow winds. Arcus clouds are notable for their ominous appearance, which can resemble a wall or shelf advancing across the sky, often signaling the imminent arrival of including strong gusts exceeding 50 knots (93 km/h), heavy , and turbulent conditions. Arcus clouds form when cool, moist air from a thunderstorm's downdraft spreads out horizontally, undercutting warmer air aloft and creating a rolling motion due to ; this process lifts the air along the gust front, condensing moisture into the visible structure. They are subdivided into two primary types: the roll cloud, a detached, tube-like formation that appears to rotate about its horizontal axis with ragged, tattered edges, and the shelf cloud, a wedge-shaped extension attached to the base of the parent thunderstorm , featuring a smoother upper surface and turbulent undersides. While most commonly linked to severe thunderstorms, arcus clouds can occasionally develop without or thunder, purely from cold frontal passages, though they rarely produce on their own. These clouds play a critical role in storm spotting and forecasting, as their presence often precedes the most intense phases of a , including , damaging winds, and tornadoes, prompting warnings from meteorological agencies like the . Observations of arcus clouds have been documented globally, with notable occurrences in regions prone to thunderstorms, such as the of the , underscoring their importance in understanding convective weather dynamics.

Overview

Definition

An arcus cloud is a low, horizontal cloud formation characterized by a dense, horizontal roll with more or less tattered edges, situated on the lower front part of certain clouds and having, when extensive, the appearance of a shelf or vault. It typically appears as an accessory cloud to a cumulonimbus, though less frequently to cumulus congestus, often manifesting as a dark, menacing arch with a well-defined, sometimes turbulent lower boundary. In meteorological classification, arcus is recognized as a supplementary feature rather than a standalone genus, as detailed in the World Meteorological Organization's International Cloud Atlas. The term "arcus" derives from the Latin word for "bow" or "arch," reflecting its distinctive arched or rolled shape. Unlike principal cloud genera, arcus is not classified independently but serves as an accessory form attached to or partly merged with convective clouds, usually occurring at low altitudes in the low étages (from the surface up to approximately 2 km or 6,500 ft). It is commonly linked to thunderstorm outflows, signaling the approach of strong winds.

Etymology

The term "arcus cloud" originates from the Latin word arcus, meaning "arch" or "bow," a nomenclature that directly reflects the cloud's distinctive curved, arch-like formation often observed along the leading edge of a thunderstorm. This Latin-based terminology emerged as part of the broader evolution in cloud classification during the early 20th century, building on the foundational system established by British pharmacist and meteorologist Luke Howard in 1803, who introduced standardized Latin names for principal cloud genera such as cirrus, cumulus, and stratus to facilitate international communication among scientists. The specific term "arcus" was formally adopted in 1930 by the International Commission for the Study of Clouds (CEN) within the International Atlas of Clouds and of States of the Sky, Abridged Edition, where it was defined as a supplementary feature—a dark, roll-shaped cloud arch associated with cumulonimbus formations. This marked a significant step in codifying accessory cloud types beyond Howard's initial genera. The (WMO) further refined and adopted the term in subsequent editions of the , including the 1956 extended version, ensuring its integration into global meteorological practice and distinguishing "arcus" from related features like the detached roll cloud species volutus in later revisions such as the 2017 update.

Formation and Characteristics

Formation Mechanism

Arcus clouds primarily form along the of a thunderstorm's outflow, known as the gust front, where cool downdraft air interacts with the surrounding atmosphere. This downdraft originates from the of within the , producing a pool of cooler, air that sinks rapidly to the surface and spreads horizontally as a density current. The density contrast between this cool air and the overlying warmer air drives the process, with the denser cool air undercutting and displacing the less dense warm air ahead of the storm. As the cool air advances, it forces the warm, moist air upward along the gust front, creating a zone of enhanced lift at the . This ascent causes the warm air to cool adiabatically, reaching saturation and leading to the of into droplets, which form the characteristic horizontal cloud band. The rolling motion often observed in arcus clouds arises from horizontal generated by the interaction, particularly under conditions of between the surface and aloft. In secondary scenarios, arcus clouds can develop along cold fronts outside of thunderstorms through analogous undercutting dynamics, where advancing cool air from the front lifts and overlying moist air, albeit without the intense evaporative cooling from cores. Formation typically requires high relative near the surface to support rapid , strong to sustain the horizontal rolling structure, and environments with warm surface temperatures (e.g., above 20°C or 68°F) contrasting with the incoming cold air to enable efficient cooling and uplift.

Visual and Physical Characteristics

Arcus clouds are characterized by their low-altitude positioning, with bases typically forming between 300 and 2,000 meters (1,000 to 6,500 feet) above the surface, often near the ground in association with gust fronts. They exhibit a predominantly horizontal orientation, extending parallel to the ground and creating a striking, elongated appearance that can resemble a , arch, or tube due to their curved upper surface and defined boundaries. This structure results in sharp, often crisp edges along the leading margin, contrasting with the more diffuse boundaries of surrounding cloud formations. Visually, arcus clouds present a dense, gray to dark coloration attributable to their high content and thickness, which scatters effectively and obscures underlying features. The lower edges frequently appear tattered or ragged, a result of turbulent air flows eroding the and producing irregular fringes that enhance their dramatic, rolling . In terms of scale, these clouds can span widths of tens to hundreds of kilometers, forming extensive horizontal bands that dwarf nearby landscapes, though their vertical thickness remains relatively shallow compared to towering cumulonimbus systems. They typically persist for 30 minutes to a few hours before dissipating as the associated outflow weakens, allowing observers to witness their evolution from formation to fragmentation. Physically, arcus clouds consist primarily of water droplets, with ice crystals possible in colder environments where temperatures drop below freezing within the cloud layer. Internally, they feature turbulent airflow that drives billowing motions, contributing to their dynamic, undulating form and the turbulent conditions beneath. This composition and motion underscore their role as dense, moisture-laden features detached or appended to parent storm clouds, emphasizing their brevity and intensity in the atmospheric .

Types

Shelf Cloud

A shelf cloud is a subtype of arcus cloud characterized by its low, horizontal, wedge-shaped structure attached to the base of a parent , protruding forward in a broad, ledge-like formation. This attachment distinguishes it from other arcus forms, as it remains connected to the thunderstorm's base rather than detaching. Shelf clouds typically develop along the leading edge of a , serving as a visual indicator of the outflow boundary from the storm. The formation of a shelf cloud is directly tied to the dynamics of gust fronts, where downdraft-generated cool air spreads outward beneath the warmer, moist air ahead of the . This cool air undercuts and lifts the warm air, causing rapid ascent, cooling, and into a visible layer that stays linked to the cumulonimbus base. The process is most pronounced in intense , where strong downdrafts enhance the gust front's propagation, sustaining the cloud's wedge shape as it advances. Visually, shelf clouds present a dramatic, broad horizontal ledge with a turbulent, boiling lower edge due to rising air motion and associated gusty, straight-line winds along the leading part. The upper surface often appears smooth, contrasting with the chaotic underside, and the cloud is typically followed by from the parent storm. These features create an ominous, advancing wall-like appearance, with the cloud's concave slope directing away from the rain area. Shelf clouds are commonly observed along the leading edges of lines and in the U.S. during the peak season of spring and early summer, where favorable conditions for produce prominent gust fronts.

Roll Cloud

A roll cloud is a detached, horizontal, cylindrical subtype of that rolls along its horizontal axis, remaining unconnected to any parent system. Unlike attached formations, it develops independently as a solitary feature, often signaling the passage of a density current. Roll clouds typically form along the leading edge of cold fronts or fronts in environments lacking thunderstorms, where cooler, denser air undercuts warmer air aloft, generating waves that propagate through stable atmospheric layers. These waves create a rolling motion as moist air is lifted and condenses at the wave's head, sustaining the cloud's tube-like structure without reliance on convective outflows from storms. Visually, a roll cloud presents as a smooth, elongated tube with an undulating, rotating appearance, spanning the horizon like a solitary arch and evoking the motion of a frozen in the sky. It is characteristically low-lying. A prominent variant is the observed in , where diurnal convergences over the produce multiple parallel roll clouds, first documented in the 1940s. These formations arise from the interaction of and sea breezes in stable, moist boundary layers, creating spectacular, wave-like disturbances up to 1 kilometer deep and over 100 kilometers long.

Significance

Meteorological Importance

Arcus clouds serve as critical visual indicators of an approaching gust front, the leading edge of cool outflow air from thunderstorms that signals the onset of , reaching up to 60 mph (100 km/h), and potential severe thunderstorms. These formations, particularly shelf clouds, mark the boundary where denser cool air undercuts warmer surface air, often preceding damaging downdrafts and that can trigger new convective cells or intensify existing storms. In weather forecasting, arcus clouds enhance radar and satellite interpretation by appearing as prominent linear reflectivity echoes ahead of main storm cores, facilitating short-term nowcasting of gust front propagation and associated hazards. Doppler radar detects these features through thin-line echoes with reflectivities of 7–57 dBZ and high radial shear, allowing meteorologists to track outflow boundaries up to 100 km away and predict wind shifts or new storm initiation. This capability is essential for issuing timely warnings, as collisions between multiple gust fronts can spawn additional thunderstorms. Research on arcus cloud dynamics, particularly through observations since the 1980s, has significantly advanced models for outflow prediction by providing empirical data on gust front structure, including lengths of 6–200 km and maximum radial winds of 43 m/s. Case studies from this era, such as those compiled in the 1985 Gust Front Case Studies , analyzed eight events from 1978–1984 to develop automated detection algorithms, improving the accuracy of low-level forecasts and hazard mitigation strategies. These contributions underscore arcus clouds' role in refining systems for severe convective events.

Safety Considerations

Arcus clouds, particularly shelf and roll varieties, pose significant safety risks due to their association with powerful outflows, including gust fronts that can generate sudden straight-line exceeding 100 mph (160 km/h). These , often linked to microbursts, can reach speeds up to 150 mph (240 km/h) in extreme cases, causing widespread damage such as uprooted trees, snapped power lines, and structural failures to buildings and vehicles. In aviation, arcus clouds indicate hazardous low-level wind shear and turbulence, where rapid changes in wind speed and direction beneath the cloud can lead to loss of aircraft control, especially during takeoff or landing. The advises pilots to avoid flying near or under arcus formations during thunderstorms, as these zones frequently precede severe downdrafts capable of producing dangerous shear. For the public, the appearance of an arcus cloud serves as a visual signal of an approaching core, often preceding intense , , and ; individuals should immediately seek sturdy shelter indoors, away from windows, upon sighting one. The issues alerts emphasizing this cue, recommending evacuation to basements or interior rooms in response to such formations during severe warnings. Mitigation strategies include real-time monitoring through weather applications, radar updates, or community siren systems to detect arcus development early.

References

  1. https://cloudatlas.wmo.int/clouds-supplementary-features-arcus.html
  2. https://www.weather.gov/oun/spotterglossary
  3. https://www.weather.gov/glossary/glossary_a
  4. https://cloudatlas.wmo.int/docs/wmo_407_en-v1.pdf
  5. https://cloudatlas.wmo.int/
  6. https://cloudatlas.wmo.int/appendix-1-etymology-of-latin-names-of-clouds.html
  7. https://cloudatlas.wmo.int/preface-to-the-1939-edition.html
  8. https://cloudatlas.wmo.int/appendix-3-history-of-cloud-nomenclature.html
  9. https://www.weather.gov/spotterguide/glossary_a
  10. https://cloudappreciationsociety.org/cloud-library/arcus/
  11. https://kids.weather.gov.hk/eBook/ebook_cloud/m_ebook_cloud09_e.htm
  12. https://skybrary.aero/articles/arcus
  13. https://www.metoffice.gov.uk/weather/learn-about/weather/types-of-weather/clouds/other-clouds/arcus
  14. https://www.weather.gov/media/lmk/soo/cloudchart.pdf
  15. https://cloudatlas.wmo.int/en/clouds-supplementary-features-arcus.html
  16. https://www.hko.gov.hk/en/education/weather/clouds/00464-arcus-cloud.html
  17. https://www.nationalgeographic.com/science/article/131115-rare-roll-cloud-video-rare-morning-glory
  18. https://www.noaa.gov/jetstream/appendix/weather-glossary-s
  19. https://forecast.weather.gov/glossary.php?word=shelf%20cloud
  20. https://www.noaa.gov/jetstream/tstrmtypes
  21. http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/svr/comp/out/gust/vis.rxml
  22. https://www.weather.gov/media/unr/SpottersGuide2011_fielduse.pdf
  23. https://www.weather.gov/lmk/shelfcloudversusawallcloud
  24. https://forecast.weather.gov/glossary.php?word=R
  25. https://journals.ametsoc.org/view/journals/mwre/126/1/1520-0493_1998_126_0167_gollmc_2.0.co_2.xml
  26. https://www.weatheronline.co.uk/reports/wxfacts/Roll-cloud.htm
  27. https://www.bom.gov.au/aviation/data/education/flying-tropics.pdf
  28. https://journals.ametsoc.org/view/journals/atsc/61/12/1520-0469_2004_061_1360_tgotmg_2.0.co_2.xml
  29. https://geo.libretexts.org/Bookshelves/Meteorology_and_Climate_Science/Atmospheric_Processes_and_Phenomena/14%3A_Thunderstorm_Hazards/14.03%3A_Gust_Fronts
  30. https://www.dtn.com/outflow-boundaries-and-gust-fronts/
  31. https://www.nesdis.noaa.gov/about/k-12-education/severe-weather/what-gust-front
  32. https://www.faa.gov/documentlibrary/media/advisory_circular/ac%2000-24c.pdf
  33. https://apps.dtic.mil/sti/tr/pdf/ADA163277.pdf
  34. https://www.noaa.gov/jetstream/wind_damage
  35. https://www.nssl.noaa.gov/education/svrwx101/wind/types/
  36. https://www.faasafety.gov/files/events/SO/SO15/2024/SO15129441/FAA-H-8083-28-Chpt22.pdf
  37. https://www.weather.gov/media/iwx/webpages/skywarn/spotterhandout.pdf
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