Lake Marble Falls, Texas

Shelf clouds as this one formed over Marble Falls, Texas 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.