When the weather is hot, the ground will appear fuzzy and undulating. What is the principle?

It is related to water vapor, but it is not only related to water vapor. This phenomenon is related to all gases. Because the gas near the ground absorbs solar radiation and ground thermal radiation, even the heat transfer temperature is high and the density is low. In the process of rising, it will lead to uneven gas density in a certain area, leading to different refractive indexes, which will lead to the distortion of light propagation direction and the scene we see will bend accordingly.