Though it’s not quite as dramatic as the clouds in my three previous posts in this series, I consider myself quite fortunate to have seen this formation. It was in (I’m not making this up) the Paul Bunyan State Forest in northern Minnesota in late September. Something had caused some turbulence in the steadily-moving low cloud cover and, for a short while, it was perpetuated in the form of something like upside-down waves. The phenomenon lasted only a minute or two, but I was able to make this shot after scrambling for the best foreground I could find, missing its peak by maybe half a minute. Just a very few minutes later the waves had disappeared and the clouds returned to smooth, uniform gray.
Note: Since publishing this post, I have come to think that this may have been an (albeit rather tame) example of asperatus clouds. Please see the comment below by Tintenfisch, which provides a very interesting link and direcction to some additional information.