10 September 2004

The highway mirage

Forrest M. Mims III

Black asphalt highways form a perfect substrate for the production of mirages. As the sun illuminates the pavement, it's temperature rises significantly. The hot asphalt then warms a thin layer of air over the pavement. Warm air has a lower index of refraction than cool air. Thus, the interface between the warm and cool air forms a mirror when viewed at a high angle. The mirror reflects an image of the sky to an observer. Because the sky is blue, and because of the shimmering effect caused by slight variations in the heating of the air over the pavement, the reflected view of the sky closely resembles standing water.

This mirage photograph was made on Highway 285 near Pecos, Texas, on 3 September 2004. Notice that the headlights of the oncoming car are visible at two points in the mirage. This is caused by the change in the angle of the road, which can be seen by referring to the white stripe along the road at the right side of the image. To better show the mirage, a small portion of the actual image has been greatly enlarged.

The web has many good articles about mirages. See, for example, "Hot Road Mirage" for details about making and studying laboratory mirages. This site nicely explains the tiny differences in the index of refraction of air than can cause mirages.