What produces the wavy effect in air over a hot stove

The reason that things look wavy in the heat over a hot grill or stove is that the air temperature is not stable. Hot air naturally rises, so as air comes off the hot surface, it rises for a bit before rapidly cooling and sinking to be heated again. This constant mixing of hot and cool air produces vibration. This vibration affects how we see the light moving through the air, making it look wavy.