What causes highway mirage?

On hot Summer days in the Louisville area while you are out driving, you may look ahead of you down the road and see what looks like a puddle of water in the road, but when you finally get up to where it was, it disappears and isn't even there. Have you ever wondered what this is and what causes it?

This is known as highway mirage. The physics behind Earth's atmosphere is the reason behind why this phenomenon occurs during hot afternoons. Highway mirages are extremely convincing optical illusions caused by the interactions of light and hot air. You know how a spoon in a glass of water can appear broken or bent? This is known as atmospheric refraction, and it can occur within the atmosphere as well. This is what happens when light steers away 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.

Now picture yourself driving down that road on the hot asphalt and you see this "puddle" occurring. if the temperature drops as you increase altitude in the atmosphere, you would see what is called inferior mirage. That basically means that the surface of the asphalt is hotter than the temperature of the air above it. This is the type of mirage that you see on the highway because as we all know from walking across asphalt barefoot as a child, the road is typically much hotter than the actual air above it during the Summer time. The very hot road and the cooler air above create the mirage.

Courtesy: ScienceABC

When the rays from the sun reach the cooler air just above the warmer road, the speed of the particle of light increases just a tad, causing its path to change, or bend from your point of view. This makes something that looks like a puddle of water appear on the road. However, due to the bending of the light, this water is actually just a reflected image of the sky. Because the reflection occurs solely at very shallow angles, the mirage appears only in the distance and continually recedes as one moves towards it.