What Is a Mirage?

Puddles don't just evaporate instantly into thin air. Buildings don't shimmy like belly dancers. But sometimes, mirages make faraway objects look like they're rippling.

"A mirage is an inverted image produced by atmospheric refraction," says Andrew Young, an astronomer at San Diego State University.

Atmospheric refraction occurs when varying air densities cause the sun's rays to bend, moving in directions other than the normal straight lines.

"When refraction is strong enough to produce inverted images, it usually produces multiple images and distortions as well," Young explained, "and that refraction displaces images only in the vertical direction, not sideways. Everything we see appears slightly displaced from its geometric direction by refraction caused by density gradients in the air."

Mirages are often seen on the streets on sunny days. These are produced by warmer air developing at the surface, where dark asphalt is heated by solar energy. They're known as "inferior mirages" because the inverted image is below the erect one.

Superior mirages are produced when relatively warmer air lies over cold air, and light rays bend downward toward the colder, denser air. This causes an image of an object, such as a setting sun at the horizon, to appear above its actual position and sometimes even upside down. Superior mirages are "usually more spectacular," according to Young.

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