Why Does the Road Look Wet on a Hot Summer Day?
On a hot summer day, while driving along the highway, you may see something that looks like a pool of water ahead of you in the road.
But when you get closer, the water disappears. So you know you must be seeing something called a mirage.
On a hot day, the warm pavement heats the air above it. The hot air acts as a kind of mirror. Light rays from the sky strike the layers of hot air just above the pavement and are reflected toward your eyes.
You see a reflection of the sky, which looks like water, and you don’t see the surface of the road at all.
In particular it is caused by hot air near the pavement and warm air above it which creates a gradient in the refractive index of the air, making a virtual image of the sky appear to be on or below the road.
Air currents make this shimmer, similar to a reflection of the sky on water, hence causing the illusion of wetness.