Mammatus clouds are an intriguing enigma of atmospheric fluid dynamics cloud physics. They’re most commonly observed beneath anvil-shaped thunderclouds. They are formed when ice crystals fall from the cumulonimbus cloud’s anvil. “The ice crystals sublimate, or change from ice to water vapour as they fall, causing the surrounding air to thermodynamically cool. The cooled air becomes negatively buoyant and begins to sink, producing the punched-out look indicative of the mammatus cloud.