These amazing clouds are one of a few photographed examples of the rare Undulatus Asperatus cloud. Proposed as a new cloud type in 2009, the source of these clouds is still a mystery.
The name "undulatus asperatus" is latin, and translates approximately "roughened or agitated waves". They are related to the standard undulatus (wave-like) clouds, but because of their dark and stormy appearance, may be a distinct type.
The Royal Meteorological Society is currently studying the way that they form in order to determine if they are in fact distinct from the standard undulatus clouds. Standard undulatus clouds have a much tighter wave pattern, and are usually caused by destabilizing return flows.