Mammatus clouds

Mammatus clouds
Mammatus are pouch-like cloud structures. They're also a rare example of clouds in sinking air-- most clouds form in rising air.
For a mammatus to form, the sinking air must be cooler than the air around it and have high liquid water or ice content. They derive their name from their appearance, like the bag-like sacs that hang beneath the cloud resemble cow's udders.

Mammatus are long-lived if the sinking air contains large drops and snow crystals since larger particles require greater amounts of energy for evaporation to occur. Over time, the cloud droplets do eventually evaporate and the mammatus dissolve.

Despite popular misconception, mammatus clouds are not a sign that a tornado is about to form. While associated with thunderstorms, mammatus clouds are not necessarily an indicator of severe weather. Mammatus result from the sinking of moist air into dry air. They are in essence upside-down clouds. The sharp boundary of mammatus is much like the sharp boundary of a rising cumulonimbus cloud before an anvil has formed.

Reference:
https://www.newscientist.com/article/mg23130910-100-upsidedown-clouds-grow-down-and-cascade-across-the-kansas-sky/

http://www.accuweather.com/en/weather-glossary/what-are-mammatus-clouds/5506130

Photo credit: Mitch Dobrowner

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