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What causes a highway mirage?

A classic highway mirage: the view forward, along a highway, with a car ahead appearing to be surrounded by water.
View at EarthSky Community Photos. | Sheryl R. Garrison captured this highway mirage on July 1, 2021, in the midst of the recent dramatic heatwave in the Pacific Northwest. She wrote: “Shot an inferior mirage through the windshield of my car (passenger) on the highway as record-breaking temperatures continue across Southern Alberta, Canada.” Thank you, Sheryl.

When you reach it, it’s gone

On a long car trip, you sometimes see what looks like water on the road ahead. As you move toward it, the “water” disappears. That’s a mirage.

People sometimes label a mirage as an illusion or as a hallucination. But, a mirage is neither one of those. Illusions and hallucinations are products of the mind. But the physics of Earth’s atmosphere causes a mirage.

You know how a spoon in a glass of water can appear broken or bent? Our atmosphere can cause some distant images to undergo a similar effect. Atmospheric refraction is what happens when light (or another electromagnetic wave) deviates from a straight line. It typically happens as light passes through the atmosphere at times when our planet’s air may be more or less dense, depending on its height above the ground.

Highway mirage in summer

So think about a warm summer day, the heat of the asphalt highway, and the heat of the air above the highway. If the temperature goes up as you get higher above the ground, you might see what’s known as a ‘superior mirage.’ That would mean an object would look higher above the ground than it truly is.

But if the temperature goes down as you go up in the atmosphere, you might get an inferior mirage. The object looks closer to the ground than it really is.

That’s what happens when you see a highway mirage, like the one depicted in the photo above. When you see a highway mirage, you’re seeing an “inferior mirage.” It means the asphalt surface of the road is much hotter than the air above it. You’d realize that if you tried to walk across it barefoot. The very hot road and the cooler air above create the mirage. The image of something higher up is refracted downward, to create what looks like a pool of water on the road ahead.

It’s the sky

What’s being refracted, exactly, to cause the appearance of water? The highway mirage is due to refracted light from the blue sky just above your horizon.

Bottom line: A highway mirage is an inferior mirage, caused by the fact that the road is hotter than the air above.

July 3, 2021

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