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9 ominous images of roll clouds

Aug. 5, 2011, 11:50 a.m.
Roll cloud in Uruguay

Photo: Daniela Mirner Eberl/NASA

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Maldonado, Uruguay

This eerie scene, captured in January 2009 at Uruguay's Las Olas beach, hints at how far roll clouds can stretch out. It also reveals another quirk: They work alone, often snaking through the sky without a storm in sight.

Roll clouds are a type of "arcus cloud," formed when updrafts and downdrafts churn the front edge of a thunderstorm (or cold front) into a sideways cylinder. But unlike shelf clouds, the other arcus variety, roll clouds are detached from their parent storms — sort of like if a car's front axle broke off and rolled away.