Why Does the Highway Look Wet on a Hot Day?
On a summer day you may have noticed a pool of water ahead of you in the road. When you get closer, the pool of water disappears. What you saw was a mirage. On a hot day, the warm pavement heats the air just above it. The layer of hot air acts as a mirror. It reflects the light coming from the sky; you see a reflection of the blue sky, which looks like water.
Light refracts not just when it moves through two different mediums like air and water, but also when it moves through different layers of the same medium that have different densities. As the sun beats down on the blacktop, it heats it up. The road, in turn, heats the air immediately surrounding it, keeping the air just above it warmer and less dense than the air farther up.
As light from the sky travels downward toward the hot road, it moves through these increasingly warm and less dense layers of air, changing speed and refracting as it moves through each one. It winds up taking a sort of u-shaped path down toward the road, then parallel to it and finally back up into the sky—where it may meet the eye of someone standing up the road.
When this refracted light reaches you, your brain and eye—don’t account for all the bending it did along the way. They trace it back along a straight line and interpret that point as its origin and the location of the object. What you see, then, is a little bit of sky that appears to be sitting on the ground—an inferior mirage where the mirage is under the real object.
Even as your brain and eye try to quickly make sense of what you’re seeing, the brain knows that sky on the ground doesn’t make sense, so you often wind up perceiving the mirage as water on the road reflecting the sky. Turbulence of the air also distorts the mirage, strengthening the effect.
Some of the most deceptive mirages are seen in the desert. The layer of hot air floating just above the sand reflects the sky, and a thirsty traveler thinks he sees a cool oasis in the distance.