Undulatus means wavy and asperatus translates as agitated or roughed, so the name is Latin for “agitated waves.” (MORE: 10 Amazing Clouds)

Why are Asperatus clouds rare?

Asperitas clouds are rare and cherished by skywatchers and cloudspotters. They form when the low- or mid-levels of the atmosphere are stable, allowing air to settle in horizontal layers. A nearby perturbation disturbs those layers, sending wavelike ripples through them.

Where are Asperitas clouds most common?

The ominous-looking clouds have been particularly common in the Plains states of the United States, often during the morning or midday hours following convective thunderstorm activity.