What are heatwaves? And why do they happen?

Question

You know when you're walking down the road on a hot day and there are heatwaves above the road. What are they (In a really physicsy type of way)? And why do they appear just above the road?

Answer 1

There are heated layers of atmosphere that appear to shimmer as they rise above the hot road surface.

Answer 2

The light coming to you from the other side doesn't travel in a straight line to get to you because it is bent slightly on entering air at different temperature.  The angle it bends at is a function of temperature.  The air over the road is mixing and is a very complicated roiling mix of air at different temperature.

Just think of putting a yardstick in water and look at the angle it bends at.  What you would first expect doesn't happen - the yardstick doesn't go straight through, but the yardstick doesn't physically bend.  It's just the light moves at an angle when it enters or leaves the boundry between the water and air.

Over the road, there are many many boundries between each small region of air which is at a different temperature.

It appears only just above the road because by the time the air reaches a higher altitude, it's cooled off.

The angle light bends at is related to its "refractive index" which is the ratio of the speed of light in a vacuum to the speed of light in the material it travels through.  The refractive index of air at room temperature is very close to 1 which is why you don't notice this angle change very much in air everyday.

Water has a much higher refractive index.  Glass has an index of 1.5