A Roll Cloud Over Uruguay.

What kind of cloud is this? 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, a type of Arcus 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 .

Note: An APOD editor will review astronomy images of 2009 , hosted by the Amateur Astronomers Association of New York on Friday, January 8 at the American Museum of Natural History, NYC. digg_url = 'http://apod.nasa.gov/apod/ap100105.html'; digg_skin = 'compact';