Arcus cloud

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A striated shelf cloud, seen in Massachusetts in July 2022.
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An arcus cloud is a low, horizontal

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

Shelf cloud

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

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

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

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

References

  1. Environment Canada. Archived from the original
    on June 23, 2006. Retrieved 2008-06-22.
  2. ^
    NASA
    . Retrieved June 1, 2014..
  3. doi:10.1175/1520-0493(1981)109<1726:TMGOTG>2.0.CO;2
    .
  4. ^ "Roll clouds slide over Wichita". ktvb.com. 28 April 2020. Retrieved 2023-07-24.
  5. ^ Sutherland, Scott (March 23, 2017). "Cloud Atlas leaps into 21st century with 12 new cloud types". The Weather Network. Pelmorex Media. Retrieved 24 March 2017.

External links

Wikimedia Commons has media related to Arcus clouds.
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