The rarest of all clouds is the altocumulenticulostratonimbulocirruslenticulomammanoctilucent cloud, caused by an interaction between warm moist air, cool dry air, cold slippery air, cursed air, and a cloud of nanobots.

"Cumulonimbus", by Randall Munroe, licensed under Creative Commons Attribution-NonCommercial 2.5 License

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The rarest of all clouds is the altocumulenticulostratonimbulocirruslenticulomammanoctilucent cloud, caused by an interaction between warm moist air, cool dry air, cold slippery air, cursed air, and a cloud of nanobots.