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In 1959, the Smithsonian Institution Annual Report carried the story of strange mirages seen near Yuma, Arizona. On hot, unusually still days, a clear image of a city appeared in the desert to the west of Yuma. It was no phantom either – the shimmering image was unmistakably that of San Diego, California, 150 miles west on the Pacific Coast, beyond several mountain ranges.
Mirages are tricks of the atmosphere, optical illusions caused when a layer of air next to the ground becomes superheated from heat stored in the soil or in dark pavement. The boundary between this hot – and therefore less dense – air and the cooler, denser air above it bends the light rays that strike it, acting like a giant mirror or lens held parallel to the ground. Depending on the altitude and extent of the layer, or layers, many kinds of mirages are produced.
Water mirages paint lakes across parched desert sands, deluding desert travelers. (The flat expanse of the Lordsburg Playa, west of Lordsburg, New Mexico, is well-known for the realism of its sparkling blue water mirages.) Like puddles stretching across dry highway pavement, these mirages are produced when the boundary between superheated and normal air reflects the sky, looking like water on the ground. More complicated mirages result when viewers can see across the abrupt hot air/cold air boundary – essentially, looking across the surface of a mirror – at the distant horizon. Mesas seem to float free, separated from the land by a layer of blue sky. Mountain ranges clone themselves, growing upside-down mirror images attached at the peaks.
More complicated – and rarer – mirages include Fata Morganas, cities appearing where none exist at all, named for the Arthurian witch Morgan le Fay. Lateral mirages reflect the image side-by-side with the actual scene, rather than above it. Several inverted pairs of images, one above the other, are called multiple mirages. One especially deceptive mirage is the Novaya Zemlya effect, named for the location in Arctic Russia where it was first seen. In the dark days of the northern winter, the sun appears to rise over the horizon weeks before it is due to return. The sun is actually below the horizon, but its image is reflected back and forth between the upper and lower boundaries of the atmospheric layer and projected above the horizon.
The clear images of the city of San Diego seen at Yuma were telescopic mirages, sharp, stable and magnified images reflected at least several hundred miles away from the source image. Telescopic mirages are rare because they require very stable air layers without the slightest whisper of wind covering many miles of landscape. Several giant lenses – abrupt hot air/cold air boundaries – transmit and magnify the image. In the case of the Yuma mirages, at least two lenses were required, to pick the image up at sea level, carry it over a 4,000 foot-high mountain mass, and reflect it again near sea level some 150 miles away.
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