What you're seeing here is a cloud formation so rare that it defies classification. Meteorologists suspect that these are undulatus asperatus clouds — an entirely new cloud type that was proposed only three years ago by the Cloud Appreciation Society. The one featured here was photographed by Witta Priester in New Zealand.
Image credit and copyright Witta Priester.
NASA's APOD explains:
Whereas most low cloud decks are flat bottomed, asperatus clouds appear to have significant vertical structure underneath. Speculation therefore holds that asperatus clouds might be related to lenticular clouds that form near mountains, or mammatus clouds associated with thunderstorms, or perhaps a foehn wind — a type of dry downward wind that flows off mountains. Such a wind called the Canterbury arch streams toward the east coast of New Zealand's South Island. The above image, taken above Hanmer Springs in Canterbury, New Zealand, in 2005, shows great detail partly because sunlight illuminates the undulating clouds from the side.
Click here for a larger image.