On a hot summer day, the air directly above the road surface is extremely hot, whereas the air at higher elevations is cool.

The refractive index of air increases with density, as we all know.

As a result, the colder air towards the top has a higher refractive index than the hotter air near the road.

The hotter, thinner air goes faster than the cooler, denser air above it.

The temperature drops with height on hot days.

As a result, the refractive index of air rises with altitude.