What Causes A Mirage?

Driving down the road recently, I saw pavement ahead of me that appeared to be covered with water. I reached that spot, but found there was no water actually there. What I observed was a true mirage, commonly known today as a highway mirage.

Before roads covered much of the nation, people would probably have called this image a desert mirage. By either name, the vision is an example of a mirage type known as the inferior mirage. The word inferior refers to the position of the image in relation to the actual position of the object. The mirage is seen inferior, or below, the actual object that creates the mirage.

Inferior mirages form when light rays pass through a relatively hot layer of air and are refracted, or bent, upward from their downward path. In general, when light moves from one medium to another, the rays are refracted. Put a pencil in a clear glass of water, and you will see that the pencil looks bent near the surface. This is refraction. Refractions in the atmosphere occur whenever light passes through air layers of different densities. Light rays traveling through layers of differing temperature will bend toward the cooler air.

Solar heating usually causes large air temperature variations near the surface. Highway mirages form when the air near the ground is hotter than the air above it, and the hotter the air, the greater the effect. When the light rays pass through the hot air near the road surface, the rays refract upwards toward the cooler air above. The image I saw on the road was not a puddle of water, but really a refraction of the sky.

Highway mirages can easily been seen over dry pavement on sunny winter days and during the summer months.