Definition
A cumulonimbus cloud whose underside displays mamma — rounded, pouch-like protrusions hanging downward from the cloud base or anvil. The mamma form indicates extreme instability and severe turbulence within and beneath the cloud, and is associated with very strong thunderstorms.
Plain English
A thunderstorm cloud with strange-looking pouches or bumps hanging down from its underside. When you see those pouches, the storm is violent and the air around it is dangerous.
Context Anchor
Seen in weather study, cloud identification, and preflight weather decisions when recognizing thunderstorm clouds visually.
Derivation
From Latin cumulus (heap), nimbus (rain cloud), and mamma (breast or udder) — referring to the rounded pouches that hang beneath the cloud. The Latin roots together describe a heaped rain cloud with pouch-like bulges underneath.
Why Pilots Care
Indicates extreme atmospheric instability with high risk of severe turbulence, hail, and sudden wind shifts.
Analogy
The underside can look like a ceiling covered with soft, rounded bags hanging down. The shape is distinctive, but it is attached to a thunderstorm cloud, not a harmless decorative cloud.
Grounding Statement
If you look toward a thunderstorm and see rounded pouches hanging under the cloud, you are looking at cumulonimbus mamma and should treat the area as storm weather.
Intuition Check
Do not read “mamma” as a separate cloud layer or a harmless low cloud. Here it means the pouch-shaped underside feature of a cumulonimbus thunderstorm cloud.
Example Sentence 1
The pilot diverted around the storm cell after spotting cumulonimbus mamma hanging beneath the anvil.
Example Sentence 2
Visual sighting of cumulonimbus mamma during preflight led to delaying the flight until the storm passed.