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Breakdown: Why do we see highway mirages?

Published: Aug. 6, 2021 at 11:12 AM CDT
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MEMPHIS, Tenn. (WMC) - Ever been on driving in the car and you see what looks like water on the road up ahead? But as you get closer 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. Mirages, however, are a result of the physics of Earth’s atmosphere.

You know how a pencil or utensil in a glass of water can appear broken or bent? Our atmosphere can cause some distant images to undergo a similar effect.

Refraction is the bending of light.
Refraction is the bending of light.(Florida Center for Instructional Technology)

Atmospheric refraction is the result of light deviating from a straight line. It usually 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.

On a hot, sunny day, the sun heats the road, and because roads are generally black, they absorb a lot of heat and become hotter than light-colored objects. This increases the air temperature just above the surface of the road.

This creates an uneven medium, as the air just above the road becomes somewhat less dense than the rest of the air.

Now the sun’s rays pass through the air in a straight line, but when they reach the relatively warmer and less dense layer just above the road, their speed increases slightly, and they change course, being refracted to reach the eyes of the observer.

The water you see on the road is not really water but a reflection of the sky. Mirages are...
The water you see on the road is not really water but a reflection of the sky. Mirages are often observed on sunny days when the sweltering heat of the sun warms flat surfaces like roads, and thus the air above these muggy asphalt tracks.(ScienceABC)

In other words, a highway mirage is an inferior mirage, caused by the fact that the road is hotter than the air above.

One of the most commonly seen optical illusions is the highway mirage in which shimmering pools...
One of the most commonly seen optical illusions is the highway mirage in which shimmering pools of water seem to cover the roadway far ahead. The effect is caused by a thin layer of hot air just above the ground.(Joe Orman | Universities Space Research Association)

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

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