Mammatus clouds (from the Latin for 'udders') are formed when pockets of cold, saturated air sink rapidly from the top of a storm cloud, forming downward bulges like these seen over a sports stadium in Hastings, Nebraska in June 2004
An eerie rippling cloud formation hanging low over Grand Rapids, Iowa. This kind of low-hanging altostratus layer results from the lifting of large masses of humid air ahead of an incoming warm front
Roll cloud heading inland over Shark Bay, Western Australia: A roll cloud is a horizontal tubular formation that forms in the cold downdraughts that spread out ahead of an approaching storm
Wave clouds over Amsterdam Island, a tiny volcanic peak in the southern Indian Ocean. A layer of humid air, rising and falling as it passes over the island, has formed narrow lenticular clouds
Close up of the beautiful breaking wave formations of a Kelvin-Helmholtz wave cloud, seen along the eastern side of the Rocky Mountains, over the town of Monument, Colorado