known informally as asperatus clouds, this atmospheric phenomenon gets its name from the latin aspero, which roman poets used to describe the sea as it was roughened by the cold north wind.
though the cause of their formation remains unknown, it is likely that the undulating and lumpy underside is a result of warmer, moister air from above and colder, dryer air from below meeting at the boundary between the lower and middle atmosphere.
when high level wind passes over rolling terrain, you get the same wavy effect as on the surface of water. but despite their ominous appearance, asperatus clouds tend to dissipate without a storm forming.
photos by (click pic) ken prior and allan gathman in perthshire, scotland; bryan and cherry alexander in qaanaaq, greenland; ti cranium in ohio; robert lurie in cape town, south africa; witta priester in new zealand; jesse klein in wisconsin