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APOD: Roll Cloud Over Wisconsin (2013 Aug 08) 
Image Credit: Megan Hanrahan (Pierre cb), Wikipedia 
http://apod.nasa.gov/apod/fap/ap130908.html 

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 a tornado. 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 as a storm approached in 2007 in Racine, Wisconsin, USA. 

http://en.wikipedia.org/wiki/File:Roll-Cloud-Racine.jpg 

Starship Asterisk* • APOD Discussion Page 
http://asterisk.apod.com/discuss_apod.php?date=130908 

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APOD: Roll Cloud Over Wisconsin (2013 Aug 08) Image Credit: Megan Hanrahan (Pierre cb), Wikipedia 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 a tornado. 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 as a storm approached in 2007 in Racine, Wisconsin, USA. Starship Asterisk* • APOD Discussion Page #APOD

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