Driving down the highway on a recent hot day, I saw the pavement ahead of me appeared to be covered with water. I never ever reached that wet pavement because what I observed was a true mirage commonly known today as a highway mirage.
Before highways spider-webbed the national landscape, people would likely have called such a sight a desert mirage. By either name, the vision is an example of a mirage type known as the inferior mirage. The adjective inferior refers to the position of the perceived image in relation to the actual position of the object -- seen below or inferior to it -- rather than a commentary on the mirage 's quality. An inferior mirage may be either a normal or an inverted image of the original and greatly distorted.
The term mirage generally conjures up images of thirsty, lost cowboys or French Legionnaires crawling across desert sands toward pools of "cool, clear water" to quote a popular country/western song. Many believe mirages to be delusions of an overstressed mind, a pure figment of the imagination, but the mirage is a real image that can be photographed. They are optical illusions, images seen by our eyes but incorrectly interpreted by our mind. What our mind interprets as "water" are actually images formed by light rays from clouds and blue sky above and ahead of us that are refracted by large variations in air density near the surface. Since these rays are bent upward toward our eyes, our brain interprets them as having come up from the ground.
Inferior mirages form when light rays passing through a relatively hot layer of air are bent or refracted upward from their descending straight-line path. [Note: many references incorrectly call mirages reflected images.] In general, when light waves travel from one medium into another, they are bent from their straight-line path to a degree and direction that depends on the density difference between the two media. Refractions in the atmosphere occur whenever light passes through air layers of different densities. Light rays traveling through air layers of differing temperature will bend toward the colder air.
Strong solar heating usually produces large air density variations near the surface. Highway mirages will form when the air near the ground is much hotter than that above it -- the hotter the air, the greater the effect. To give some idea of how much hotter the air can be above a paved road in the full sun, measurements have shown surface temperatures 11 to 17 C degrees (20-30 F degrees) hotter than the temperature measured 1 cm (less than half an inch) above the surface. The most important factor for inferior mirage formation is the temperature difference between surface air layers rather than their absolute temperatures. Thus, highway mirages can be as commonly seen over dry pavement on sunny winter days as during the summer months.