Watching the races this year on tv, it looks to me like a lot heat coming from the back of the cars. I can see for fraction of a second the air looks "wavey" like looking at a distance in a desert.
Sorry, I don't know how to describe it. You could definitely see the effect at the end of the back straight in China.
It must be driving the engineers crazy. First they had to deal with turbulence coming from the car in front and now heat.