|MadSci Network: Earth Sciences|
When we see the distant objects, we are seeing the light (from the sun) that reflects off those objects. Light bends when it goes from air with more density to air with less density. (think about how your pencil looks like it is bending if you put it in a glass of water. The water has a different density from the air, so the light rays are bending as they go from water to air.) Hot air is less dense than cold air. When there is a hot day, air density is going to change (air over a road is hotter than air over green grass for instance) depending on several things. So if you see a ripple you can intrepet that the light rays are bending from the object to your eye, and that looks like a ripple to your eye.
You might also find these answers from our archives of interest:
Why are we able to see reflections in a mirage?
How do heat waves cast shadows on objects?
Try the links in the MadSci Library for more information on Earth Sciences.