Mammatus clouds are round, dark clouds that form when moisture-laden
air rapidly rises high in a cumulonimbus cloud during a storm. This rising air is warmer than the surrounding air and is pushed quickly outward, giving it the appearance of an upside-down cumulus cloud. The air begins to sink once its upward motion stops and it begins to cool. Mammatus clouds can precede or follow stormy weather events
such as tornadoes. Very large mammatus are a strong indicator of nearby severe weather.