THE AIRLINE PILOTS FORUM & RESOURCE
Mirage |
| Source: Excerpt from The Book " Weather " |
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Mirages are produced when light is refracted as it passes through air layers of different temperatures and densities. The air acts like a lens, bending light and presenting a distorted, inverted, or enlarged image in a different position. There are two basic types of mirage --- inferior and superior --- although combinations of these can create a variety of effects. The more common type is the inferior mirage. Normally, air density decreases with height, but when the surface of the ground is heated strongly, air density may actually increase for the first 10 to 20 feet (3 to 6 m). This often occurs in deserts on calm, clear summer days when surface temperatures rise sharply. Under these conditions, light moving downward will be refracted back upward, producing a false image (usually of an object on or near the horizon) just above the surface. Because the image actually forms nearer the viewer, it may appear to be situated below ground level. Thus, when an observer looks at a tree on the horizon, he or she will see both the true image and a shimmering, false image beneath it. Similarly, light from the sky may appear just below the horizon, creating an image like a lake. A superior mirage, on the other hand, is produced when the air near the surface is colder (and denser) than the air immediately above it. Light heading upward will be refracted back down by the warm layer. This occurs most often over cold water and produces a false image above the level of (but nearer) the observer. A person looking at a ship on the water may therefore see an inverted image of it floating in the sky above. |
Acknowledgement: John W. Zillman, William J. Burroughs,
Bob Crowder, Ted Robertson, Eleanor Vallier-Talbot and Richard Whitaker.
