Cumulus congestus clouds form when invisible updrafts of heated air cool as they rise and form towers of puffy cloud. Usually, the updrafts are created by the Sun heating the land. But on rarer occasions, the heating comes from something else, like flames from a forest fire or the lava of a volcano. Cumulus congestus clouds formed by a localised heat source like this are given the moniker flammagenitus.

Dan Chaney (Member 63,013) rose early enough on the island of Hawai’i, US to catch an eruption of Kīlauea volcano producing such a cloud at sunrise. Its fountain of lava was lifting moist air way above and mixing fine ash particles into it. The result, directly over the volcano, was a towering Cumulus congestus flammagenitus cloud aglow in the golden morning sun.

https://cloudappreciationsociety.org/wednesday-21st-may-2025/