Astronomy Picture of the Day 

Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.
2016 June 12
A Roll Cloud Over Uruguay 
Credit & Licence: Daniela Mirner Eberl

Explanation: What kind of cloud is this? A type of arcus cloud called a roll cloud. These rare long clouds may form near advancing cold fronts. In particular, a downdraft from an advancing storm front can cause moist warm air to rise, cool below its dew point, and so form a cloud. When this happens uniformly along an extended front, a roll cloud may form.  Roll clouds may actually have air circulating along the long horizontal axis of the cloud. A roll cloud is not thought to be able to morph into atornado. Unlike a similar shelf cloud, a roll cloud is completely detached from their parent  cumulonimbus cloud.  Pictured above, a roll cloud extends far into the distance in 2009 January above Las Olas Beach in Maldonado, Uruguay.

Tomorrow's picture: unexpected dimmings

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