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No, that’s not a mountain behind Madison...

It’s an optical illusion that tricks all of our eyes - one that’s exceptionally rare in southern Wisconsin.
Published: Dec. 4, 2020 at 6:51 PM CST|Updated: Dec. 5, 2020 at 3:17 PM CST
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MADISON, Wis. (WMTV) - It was a clear & cool Wednesday morning in Wisconsin’s capitol city. There weren’t clouds in the sky or rain on radar. But, for a short time, it appeared as though a mountain rose up in the western sky - towering over the Madison skyline.

Researcher Tim Wagner at the University of Wisconsin-Madison’s Space, Science, & Engineering Center (SSEC) knew exactly what it was. All he had to utter was “Superior Mirage” and his wife grabbed his camera for him. He snapped a photo shortly before 8:30 Wednesday morning - showing what appeared to be a mountain behind the capitol city.

Normally, the Blue Mound hill (roughly 30 miles southwest of Madison) appears as a bump on the horizon, but given the atmospheric conditions, it appeared 3-4 times as large on Wednesday morning. So, what happened?

Overnight temperatures & a clear sky allowed the surface to rapidly cool (as it does every night like this). But, the air aloft was much warmer. Since there were calm winds & the air is a poor conductor of heat, a large temperature difference set up in the lowest level of the atmosphere. This is also known as a temperature *inversion*.

Light bends & refracts in air of different temperatures. Think of a hot asphalt road on a summer day. As you drive down the road, it appears as though there are puddles. Since the surface of the road is hot & the air above is much cooler, light bends up - showing the sky on the road.

The exact opposite happened here.

Due to the cold surface and warmer air aloft, the light bent downward - along the curvature of the Earth - showing a much larger Blue Mound hill.

Tim says the only time he’s seen a Superior Mirage this close to Wisconsin was near Lake Superior. They’re more common near the Earth’s Poles - where the ground is extremely cold.

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