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mammatus

American  
[muh-mah-tuhs, muh-mat-uhs, mam-uh-tuhs] / məˈmɑ təs, məˈmæt əs, ˈmæm ə təs /

noun

Meteorology.

plural

mammatus

  1. a formation of pouchlike clouds created within a cumulonimbus and hanging from its underside, typically as a short-lived phenomenon associated with severe weather.

    The spectacle of mammatus is the opposite of fluffy, floating clouds—it’s like “thundercloud udders” filled with downward-pulling cold moisture.

Etymology

Origin of mammatus

Coined in 1894 by English meteorologist William Clement Ley (1840–1896); from Latin: “breastlike, having breasts,” adjective derivative of mamma mamma 2 ( def. )

Example Sentences

Examples are provided to illustrate real-world usage of words in context. Any opinions expressed do not reflect the views of Dictionary.com.

Unusual mammatus cloud spotted in Warwickshire on Wednesday.

From BBC • Apr. 17, 2026

If you see mammatus cloud like this, it is a sure sign of impending heavy rain, hail or a big thunderstorm.

From BBC • Feb. 26, 2025

Unusual mammatus clouds were spotted on Wednesday morning in parts of the United Kingdom.

From BBC • Feb. 26, 2025

The name mammatus comes from the Latin mamma which means 'udder' or 'breast'.

From BBC • Feb. 26, 2025

These mammatus clouds, underneath a cumulonimbus cloud, were spotted by Patsy Hampson in Douglas, Isle of Man.

From BBC • Jul. 30, 2011

Definitions and idiom definitions from Dictionary.com Unabridged, based on the Random House Unabridged Dictionary, © Random House, Inc. 2023

Idioms from The American Heritage® Idioms Dictionary copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.