It’s been over 50 years since a new cloud has been added to the International Cloud Atlas. The undulatus asperatus (turbulent undulation) shown above just might make the cut.

Reported over hilly country, the formation comes with a theory as to how it is formed. Gavin Pretor-Pinney of the UK-based Cloud Appreciation Society says: “It’s warmer, moister air above and colder, drier air below, with an abrupt boundary in between” giving “the same wavy effect as on the surface of water.”

Whatever causes it, that’s one ominous looking cloud.