The sun heats the surface of the Earth intensively in the morning and at noon. What is more, the layer of air just over the ground is warmed up as well. However, the upper layers of air remain cool.
As a result of the difference in the density of the air, which is thin on the ground level and thickens in the upper layers, the light reflected by the objects above the observer’s eyes reaches the eye in two different ways – first directly from the object to the eye, and the second when the light gets through the layers of less dense air it is deflected over and over again and consequently, its path is not straight but rather curvy.
This kind of twofold rays that are reflected from the same place creates two images of the same object. The upper one is clear and sharp whereas the bottom one is a weaker mirror image.