A Highway Mirage
A highway mirage is an inferior mirage which can be seen very commonly on roadways by day or by night. This hot-road mirage is usually most noticeable on hot sunny days. Warm air is less dense than cool air, and the variation between the hot air at the surface of the road and the denser cool air above it creates a gradient in the refractive index of the air. Light from the sky at a shallow angle to the road is refracted by the index gradient, making it appear as if the sky is reflected by the road's surface. The mind interprets this as a pool of water on the road, since water also reflects the sky.