Roll cloud off coast of Brazil was taken aboard ship

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

A cloud feature like this isn’t something you would want to run into on the open sea or on a crowded highway, but roll clouds (arcus cloud) aren’t necessarily a presage of severe weather. They most often form along the leading edge of a mature thunderstorm in the relatively cool air associated with the storm’s downdraft; however, they’re detached from the storm itself and may be seen to roll or tumble along their horizontal axis. Photo taken on February 6, 2012.

via EPOD