Stephanie Pappas – LiveScience, 03/05/2012
ROLLING THUNDER: A roll cloud, associated with thunderstorm downdrafts and strange sea winds, tumbles across the sky off the coast of Brazil. (Photo: Capt. Andreas M. van der Wurff)
A photo taken near Brazil captures a rare and beautiful “roll cloud,” a tubular cloud that seems to tumble across the sky.
Roll clouds are a type of arcus cloud, which is a category of low cloud formations. Their more common cousin is the shelf cloud, often seen on the leading edges of thunderstorms. Roll clouds sometimes form along with storms, too, born out of the storm’s downdraft. Sinking cold air causes warm, moist air on the planet’s surface to climb to higher altitudes, where the moisture condenses into cloud form. Winds from the storm “roll” the cloud parallel to the horizon, creating an effect that looks much like a horizontal tornado. Unlike shelf clouds, rolls clouds are completely detached from the bulk of the storm.
