Cause

• Mirages form when light rays bend and distort as they cross different layers of air density. The technical term for this phenomenon is refraction. The extent to which light is affected by refraction is proportional to the pressure, temperature and water content of the air. Cool air has a greater density than warm air; and when light from an object passes through different air densities in close proximity, light is refracted in different ways, causing a mirage to appear to the viewer.
  
  

Inferior Mirage

• In an inferior mirage the subject is seen under the real object--the real subject can be any distant object, including the sky. Inferior mirages appear to show patches of water in the desert by refracting light from the sky so that it appears inverted and shimmering on the ground. Highway mirages work in the same way and can make road surfaces appear wet or covered in oil on a hot day.
  
  

Superior Mirage

• A superior mirage occurs when the air below the line of sight is colder than the air above, the opposite to an inferior mirage. As a result, the subject of the mirage appears above the real object and does not shimmer, because cold air is more stable than hot air. Superior mirages are less common than inferior mirages and usually appear in polar regions, where the surface temperature of the land is cooler than the air above.
  
  

Fata Morgana

• Fata Morgana mirages are extremely complex and occur only when the inversion in temperature from the ground to the air above is able to bend light rays to a level greater than the curvature of the earth. These types of mirage can go from inferior to superior in a matter of minutes, depending on the atmospheric conditions, and cause the subject to appear as though it is moving up and down or stretching from side to side. Fata Morgana are usually observed in arctic regions on cold days and in warmer climates on warm days.
  
  

Create Your Own Refraction

• It is easy to quickly illustrate refraction, the cause of mirages, by inserting a pencil or straw in a clear glass of water. When viewed from the side, the straw or pencil appears to kink as it enters the water. The degree to which the straw or pencil appears to bend is an indication of its "index of refractivity," which varies depending on the density of the straw, pencil or object inserted in the water. Refraction is also affected by the color of the object and is responsible for many other optical phenomena, including rainbows.