This apparently new class of clouds is still a mystery. But experts suspect asperatus clouds' choppy undersides may be due to strong winds disturbing previously stable layers of warm and cold air.

Asperatus clouds may spur the first new classification in the World Meteorological Organization's International Cloud Atlas since the 1950s, Gavin Pretor-Pinney said.

Since the last addition to the atlas, the emergence of satellite imagery has pushed meteorologists to take a much broader view on w...