n. pl. cu·mu·lo·nim·bus·es or cu·mu·lo·nim·bi (-bī)
An extremely dense, vertically developed cumulus with a low dark base and fluffy masses that extend to great heights, usually producing heavy rains, thunderstorms, or hailstorms.
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Indo-European & Semitic Roots Appendices
Thousands of entries in the dictionary include etymologies that trace their origins back to reconstructed proto-languages. You can obtain more information about these forms in our online appendices:
The Indo-European appendix covers nearly half of the Indo-European roots that have left their mark on English words. A more complete treatment of Indo-European roots and the English words derived from them is available in our Dictionary of Indo-European Roots.