MADISON (WKOW) -- This week UW-Madison shared a video and photo of what experts are calling a rare rainbow imposter showing up in the sky on Thursday.
UW meteorologists say a 'fire rainbow' is an atmospheric phenomenon known as a circumhorizontal arc, often mistaken for a rainbow because of its colors.
"They're not all that common but they're not completely rare, either," says Steve Ackerman, director of the UW-Madison Cooperative Institute for Meteorological Satellite Studies. "I've only seen a half-dozen in my lifetime here in Madison."
Rainbows form when sunlight reflects off water droplets in the atmosphere, but circumhorizontal arcs require a certain number of ice crystals of a particular shape to form. Experts say as sunlight passes through a cloud full of ice crystals, the light bends through and separates into the bright spectrum of colors.
Fire rainbows occur only when the sun is 58 degrees or higher above the horizon, which means they're only visible in the U.S. in the six weeks before or after the summer solstice.
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