Black Desert Inferior Mirage
October 08, 2009
Photographer: Aymen Ibrahem
Summary Author and Editor: Aymen Ibrahem, Jim Foster
A mirage is simply any atmospheric image, not normally present, caused by refraction in abnormal temperature gradients. The index of refraction for air is proportional to its density and the density of air depends inversely on the temperature. Cooler air is denser than warmer air. The greater the temperature gradient between the surface and our eyes, the greater the curvature (distortion) of the rays will be.
The photo sequence above shows an inferior mirage over the Black Desert of Egypt as observed on August 11, 2008. In this type of mirage, the refracted image is below the objects actual position. The distance to the dark mesa or hill (about 100 ft or 30 m in elevation) in the background from the camera was about 0.6 mi (1 km) and to the automobile about 325 ft (100 m). Each photo was snapped within a few seconds of each other – at approximately 4:14 p.m. local time. As this is the middle of summer and near the peak of afternoon heating the surface temperature was blazing hot. Thus light rays arriving from objects positioned over heated surfaces first pass through relatively cooler, denser air to the hotter air at the surface and then back up to our eyes. Note that as a result of refracted skylight on the road surface (inferior mirage) the mesa appears to be floating in frames one and two. Extended objects like hills, trees or trucks can appear differently during a mirage. An example of this is shown in frame two where the mesa is somewhat stooped while it towers in the third frame. A “wet strip” in frame three between the camera and the auto is also refracted skylight – the refracted image is obviously below its true position.