What causes a highway mirage?
When you reach it, its 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. Thats 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 Earths atmosphere causes a mirage. 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 planets 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 whats 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. Thats what happens when you see a highway mirage, like the one depicted in the photo above. When you see a highway mirage, youre seeing an inferior mirage. It means the asphalt surface of the road is much hotter than the air above it. Youd realize that if you tried to walk across it barefoot. The very hot road and the cooler air above creates the mirage. The image of something higher up is refracted downward, to create what looks like a pool of water on the road ahead. 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.