A mammatus cloud formation over Swifts Creek, Victoria, Australia. The name "mammatus" derives from the Latin mamma, or breast, due to their rounded, hanging shape. Mammatus clouds only occur where cumulonimbus are present and are often the byproduct of strong storm activity. Detailed observations of mammatus have been meager and usually occur only by chance, since they do not pose a meteorological threat to society.Photo credit: Fir0002