Known as asperitas, the cloud structures form on the bottom of a cloud, creating a wave-like ceiling when viewed from below, according to the UK Met Office.
"Although they appear dark and storm-like, they tend to dissipate without a storm forming," the National Weather Service said. "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."
The cloud formation was added to the International Cloud Atlas in 2017.
In Wisconsin, asperitas were seen hovering over apartment complexes in New Berlin.
Asperitas clouds were also seen over DeKalb in northern Illinois, where they appeared in a more tight-knit fashion.
About 30 miles north, near the Wisconsin-Illinois border, soft, rolling asperitas hovered over a field in Caledonia.