Hi, Dave!
This is a fine example of the supplementary feature cloud known as asperitas: the cloud which Gavin Pretor-Pinney of this very society named!
I’m not entirely sure what the mechanisms behind the flowing waves (Gavin will hopefully post his hypothesis/hypotheses here); however, one thing is for certain: atmospheric instability is needed for these clouds to form in the wavy manner that they do… This explains why you saw them accompanying nearby thunderstorms!
Check out this utterly amazing time-lapse showing the fluid dynamics of asperitas in action!
As a side note, the main cloud genus is, IMO, Altocumulus, the species of which is stratiformis and the variety opacus – so, altogether, the cloud is Altocumulus stratiformis opacus asperitas. If anybody thinks differently (perhaps the genus is Stratocumulus), please let me know. :D
Great photographs, though. You’re very lucky to have seen such a stunning display of asperitas, especially one associated with nearby thunderstorm activity. Next time you see them, try to take a time-lapse; I’m sure your awe would be accentuated *even more*. :)
Dan
