thunderhead

noun

1. The top portion of a cumulonimbus cloud, which tends to be flattened or fibery in appearance, and may be indicative of thunderstorm activity.

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

   “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.”

   “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.”