[dualvariable]

Cumulonimbus has been separated from its etymological roots. It now refers to the physics that can produce deep convection, high clouds, and the formation of an anvil shape (along with lightning, etc). The convection that produces a cumulonimbus rain cloud is present in a pyrocumulonimbus cloud. The word isn't wrong.

[falloon]

Well, I was confidently wrong about that. I stand corrected, thanks for the pointers.

[runeks]

Comment threads like these are why I read HN comments.

Thank you for the insights, both of you.

[energy123]

People need to learn what compound words are.