Arcus

English: An arcus cloud is a low, horizontal cloud formation associated with the leading edge of thunderstorm outflow, or occasionally with a cold front even in the absence of thunderstorms. Roll clouds and shelf clouds are the two types of arcus clouds, slight variations in their generation and look being the difference.

From the Shelf cloud gallery

English: A shelf cloud is one type of arcus cloud.

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  supercell shelf cloud
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  Approaching thunderstorm with lead gust front and shelf clouds.
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  Approaching thunderstorm with lead gust front.
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  shelf cloud in the Netherlands, Zwolle
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  Shelf cloud
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  Shelf cloud

From the Roll cloud gallery

English: A roll cloud is one type of arcus cloud.

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  Roll cloud in racine, Wisconsin
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  A roll cloud just outside Chicago, in Mokena (Illinois)
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  Roll cloud in the Nederlands