Inferior Mirage on a Desert Road
October 08, 2012
We’ve all seen that “patch of water” on the road ahead that miraculously retreats as we drive, as though evaporating at our approach. This is an inferior mirage, and what we see as “water on the road” is actually skylight that’s been refracted by hot air near the surface. Heated by sunlight, the pavement gets hot. The air in contact with the pavement also heats up. When air is heated, it expands, causing the index of refraction to decrease. The rapid decrease in temperature makes the air behave like a lens, refracting distant light upward, creating an inferior mirage. The term “inferior” refers to the location of the mirage below the object that’s being miraged. Note that in the diagram the observer is looking down, below the horizon.
This photograph taken in Imperial County California reveals a number of interesting aspects of such mirages. The first is blue color of the mirage, which we associate with water. In this case, the blue color of the mirage is coming from the distant mountains, or rather the blue airlight. Airlight is simply the blue sky between the mountain and us. The more distant the mountain, the more air there is and thus the bluer and brighter it is. We know this is intervening airlight because we see it immediately to the left and right of the miraged green palm tree (and its mirage), and similarly its brown trunk, and the white cloud from an industrial facility.
Also, notice the horizontal bands in the mirage. These are caused by slight undulations in the road. Since we are viewing them at such shallow angles, even minor height changes alter the path of the reflected light rays. In some instances, refracted skylight and air light reach our eyes; in other cases, we see directly to the road surface, and thus we see horizontal blue and gray bands.
Finally, zoom in and look at the telephone poles on the left (not their mirage). The further away they are, the more they are distorted. This is the result of turbulence in the atmosphere, the result of heating and convection of the air. Slight changes in the refractive index lead to refraction that degrades the image quality. The farther away the pole is the more turbulent air there is between the pole and us and thus the more distortion we see.