Roll cloud off coast of Brazil
March 5, 2012
The photo above showing a sinister looking roll cloud was taken aboard ship while en route from Paranagua, Brazil to Montevideo, Uruguay. Photographer: Capt. Andreas M. van der Wurff
Roll Cloud over Wisconsin
September 8, 2011
This is 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. Photographer Megan Hanrahan.
