During summer noon, why do the trees and houses on the other side of an open ground appear to be shaking?

During summer noon, the ground becomes very hot and heats the layer of air in contact with it. This lower layer of air becomes rarer than the cooler air above it. Since these layers of air have different densities, their refractive indices are also different.

Light coming from trees and houses on the other side of the ground passes through these layers and gets refracted repeatedly. As a result, the light rays keep changing their path, so the image formed appears wavering or shaking.