Heat haze, also called heat shimmer, refers to the inferior mirage observed when viewing objects through a mass of heated air. Common instances when heat haze occurs include images of objects viewed across asphalt concrete (also known as tarmac) roads and over masonry rooftops on hot days, above and behind fire (as in burning candles, patio heaters, and campfires), and through exhaust gases from jet engines. When appearing on roads due to the hot asphalt, it is often referred to as a “highway mirage”.
Convection causes the temperature of the air to vary, and the variation between the hot air at the surface of the road and the denser cool air above it creates a gradient in the refractive index of the air. This produces a blurred shimmering effect, which hinders the ability to resolve the image and increases when the image is magnified through a telescope or telephoto lens.
Light from the sky at a shallow angle to the road is refracted by the index gradient, making it appear as if the sky is reflected by the road’s surface. The mind interprets this as a pool of water on the road, since water also reflects the sky. The illusion fades as the observer approaches the miraged object.
On tarmac roads, it may appear as if water, or even oil, has been spilled. This kind of inferior mirage is often called a “desert mirage” or “highway mirage”. Both sand and tarmac can become very hot when exposed to the sun, easily being more than 10 °C (18 °F) higher than the air a meter above, enough to make conditions suitable to cause the mirage.
Heat haze is not related to the atmospheric phenomenon of haze.