highway mirage

   Also known as highway inferior mirage, asphalt mirage, hot-road mirage, and road mirage. All five terms are indebted to the French verb se mirer, which means to reflect or to be reflected. They are used interchangeably to denote a "physical illusion typically depicting distant pools of water or oil or blue sky over a hot surface such as a road, a runway or a parking lot, or over hot objects such as a toaster, a heater, or the hood of a car. When they appear over a desert, the term desert mirage is traditionally used. The desert mirage should not be confused with the "desert hallucination, which typically occurs during the night. If the light is reflected not by the sky but by a mountain or some other distant object, then that object is reflected in the mirage. The mediation of highway mirages is attributed to differences in the refractive index of the atmosphere, which are in turn attributed to differences in temperature between adjacent layers of air. Highway mirages are classified as "inferior mirages. Like other inferior mirages, they are perceived beneath the horizon or some distant object. They arebelievedtostemfromlayersofcoldairover-lying layers of hot air, with differences in temperature between the adjacent layers of 10°Cor more per metre. Uniform temperature gradients tend to produce undistorted images. Increasing temperature gradients, such as those of 10°C/m near the ground, and 20°C/m higher up, tend to yield inverted images. Temperature gradients ofa higher complexity may yield even more complex distortions.

   References

   Lynch, D.K., Livingston, W. (1995). Color and light in nature. Cambridge: Cambridge University Press.