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The Weirdest Clouds: Arcus Clouds (Roll Clouds and Shelf Clouds)

Updated on March 10, 2012
A unique sight: solo roll cloud over Uruguay's Las Olas beach captured in 2009.              Photo: Daniela Mirner Eberl
A unique sight: solo roll cloud over Uruguay's Las Olas beach captured in 2009. Photo: Daniela Mirner Eberl
Photo: Tim101
Photo: Tim101

Arcus Clouds: what are they?

Arcus clouds are low, horizontally spread out cloud formations. Arcus clouds contain two different types of cloud - Roll clouds and Shelf clouds.

Roll clouds (pictured above and below) are long horizontal pipe shaped clouds which lie at a low altitude. Their appearance can be very dominating although they are somewhat rare.

Roll clouds indicate a microburst, or strong downdraft of wind, may be happening in the atmosphere. Contrary to popular myth, roll clouds are not capable of causing tornadoes.

Image by eazydp
Image by eazydp

Arcus Clouds: Where and when do they occur?

Arcus clouds develop at low altitudes of around 6,500 feet, and bring thunderstorms and rain. Although roll clouds are pretty rare, shelf clouds are common in storms among cumulonimbus clouds. Arcus clouds are more common in coastal areas but are not limited to that environment.

Shelf clouds captured in Warsaw. Photo: Dariusz Wierzbicki
Shelf clouds captured in Warsaw. Photo: Dariusz Wierzbicki

Arcus Clouds: How do they form?

Arcus clouds are formed by a downdraft of cool air in a storm which spreads out to create the horizontal cloud formation due to water condensing. The cloud also forms in part due to wind shear. In extreme cases, an arcus cloud may be large and low enough to reach the ground, bringing strong winds with it.

Photo: John Kerstholt
Photo: John Kerstholt
Photo: NebraskaSC
Photo: NebraskaSC
Moscow 2006
Moscow 2006

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