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Eerie round, orange clouds were spotted over a Michigan town, making the sky appear "on fire" and leading residents to worry that wild weather was coming.

The bizarre sight formed in the skies over the Michigan town of Iron Mountain at around 8:30 p.m. local time, and led to worries that severe thunderstorms or tornadoes were approaching.

National Weather Service Warning Coordination meteorologist Jeff Last, who posted images of the curved, tinted clouds to Twitter, said they were a rare phenomenon called Mammatus, which means "breast cloud."

Mammatus, or mammatocumulus, clouds are often associated with severe thunderstorms, said Iron Mountain Daily News reporter Chris Tomassucci.

"The pictures don’t really capture how eerie the whole experience was," he wrote.

"The mammatocumulus that formed over Iron Mountain made everything take on strange coloration. Greens looked more green, blues more blue, and so on."

Resident Jason Asselin captured the "incredible and crazy sky" on video, which he posted to YouTube.

"All of a sudden it got very yellow outside, it felt strange and mysterious," he wrote. "Then it slowly looked very orange, it was the craziest thing I have ever witnessed over my head. I almost expected to see a tornado or something!"

"They are extremely rare in this part of the country and many people have never seen anything like it before," Mr Asselin said.

The clouds were part of a cold front that spread across Wisconsin and Iowa, bringing hail, heavy rain and a tornado with them, the Times-Press reports.

Fears for more savage weather were unfounded, although police and fire fighters did have to rescue a group of people caught in a river when the strong winds struck.

"The people were on a sand bar, and their canoe blew away," Sauk County Sheriff Chip Meister told the Times-Press.