South Florida is no stranger to thunderstorms, but it's still hard to shrug off a sight like this. A graduate student from MIT shot this shelf cloud as it swept across Miami Beach on Dec. 4, 2010.
Shelf clouds are a type of arcus cloud, formed by colliding updrafts and downdrafts. As a storm vacuums up warm air from below, it also pumps out cooler air at the top, which can spill forward, slip below the warm updrafts and condense into a horizontal "shelf." While some arcus clouds float away on their own — they're known as "roll clouds" — shelf clouds like this one remain attached to their parent storms.