Why does the asphalt seem to shimmer and appear to have water on it in the heat of the summer?

Answer 1

Here's a technical answer. The black surface absorbs the radiant heat of the sun efficiently. the asphalt in turn heats the air. Hot air expands, reducing its density. The air's index of refraction to light reduces proportionately. This bends light rays from the sky and other background objects creating a highly distorted image of them. It shimmers due to convective air currents as the lower density hot air rises.

Answer 2

Another way to call it is "vapor effect" when the heat of the sun, once absorbed by the asphalt, gives off radiant heat, which is invisible, just like the suns rays. You only see it due to the angle at which you look straight forward from a far distance, hence the "optical illusion" of the radiant heat "waves" coming off the surface of the road. Also can happen in the desert.

Answer 3

It's called an "inferior mirage" in which light rays are bent to produce a displaced image of the sky.

http://en.wikipedia.org/wiki/Mirage