Arcus
From Wikimedia Commons, the free media repository
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 |
 Approaching thunderstorm with lead gust front and shelf clouds. |
 Approaching thunderstorm with lead gust front. |
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- From the Roll cloud gallery
English: A roll cloud is one type of arcus cloud.
 Roll cloud in racine, Wisconsin |
 A roll cloud just outside Chicago, in Mokena (Illinois) |
