thunderhead

thunder +‎ head

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thunderhead (plural thunderheads)

1. The top portion of a cumulonimbus cloud, which tends to be flattened or fibery in appearance, and may be indicative of thunderstorm activity.
   • 1918, Willa Cather, chapter 19, in My Ántonia‎, Boston: Houghton Mifflin, page 158:

     Half the sky was checkered with black thunderheads, but all the west was luminous and clear:

   • 1947, Kenneth Roberts, chapter 28, in Lydia Bailey‎, Garden City, NY: Doubleday:

     The wisps of smoke above Léogane became dark and thick, and flowered into a towering thunderhead—an ominous cloud that drifted slowly to the westward and was constantly replenished at its base by uprushes of blacker smoke.

   • 2000, Philip Pullman, chapter 29, in The Amber Spyglass‎, New York: Del Rey, page 349:

     The sky ahead was huge with storm: all the whiteness had gone from the thunderheads, and they rolled and swirled with sulphur yellow, sea green, smoke gray, oil black, a queasy churning miles high and as wide as the horizon.