Undular Bores

Sounds like a descrptive term for myself tossing and turning late at night. But these big gravity waves, extending across the horizon, look interesting!:

Those giant waves—"undular bore waves"—were photographed Oct. 3rd flowing across the skies of Des Moines, Iowa. (Credit: Iowa Environmental Mesonet Skycam.)

..."These waves were created by a cluster of thunderstorms approaching Des Moines from the west," he explains. "At the time, a layer of cold, stable air was sitting on top of Des Moines. The approaching storms disturbed this air, creating a ripple akin to what we see when we toss a stone into a pond."