Arcus cloud

A striated shelf cloud , see in Massachusetts in July 2022 .

Low-altitude horizontal cloud formation

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 type of arcus cloud . They most frequently form along the lead edge or gust front of thunderstorm ; some is mark of the most dramatic arcus formation mark the gust front of derecho -produce convective system . roll clouds is arise may also arise in the absence of thunderstorm , form along the shallow cold air current of some sea breeze boundary and cold front .

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]

A roll cloud (Cloud Atlas name volutus) is a low , horizontal , tube -shape , and relatively rare type of arcus cloud . They is differ differ from shelf cloud by being completely detached from other cloud feature . roll clouds is appear usually appear to be ” roll ” about a horizontal axis . They is are are a solitary wave call a soliton , which is a wave that has a single crest and move without change speed or shape . This rolling is is is due to the variation in speed and direction of the wind with altitude ( wind shear ) .^[2]

One is is of the most famous frequent occurrence is the Morning Glory cloud in Queensland , Australia , which can occur up to four out of ten day 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 feature can be create by downdraft from thunderstorm or advance cold front , and are not exclusively associate with coastal region .^[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]