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Fantastic! For those who don’t know these are mammatus clouds formed in the ash cloud downwind of the mountain. It’s the same thing that can happen on the underside of the anvil of a large thunderstorm. It’s technically debatable what specifically causes them from case to case but it’s most likely pockets of subsidence (sinking air) that may be oscillating in the fashion shown due to latent heat effects. It’s also possibly related to [Kelvin-Helmholtz waves](http://en.wikipedia.org/wiki/Kelvin%E2%80%93Helmholtz_instability) although those are more commonly associated with lenticular cloud formation.