One summer a few years ago a pilot flying a seaplane from the Gulf of Mexico to the West Coast developed engine trouble over Arizona. When a quick check of his maps revealed that he was in range of Cochise County's Hado Flats, he breathed a sigh of relief: only the winter before he had passed this way on the Southern Pacific railroad, and he remembered seeing along the right-of-way a lake about 10 miles long, with an island in the middle, and the surrounding mountains clearly reflected in its waters. Sure enough, when he altered course and let down through the bumpy air, there was the same lake, beckoning him toward the safety of its smooth, glistening waters. He swung the nose of his plane into the wind, throttled back, and touched down, completely demolishing his plane on the brick-hard surface of a lake that had abruptly disappeared.
A split second too late, the pilot discovered that an optical illusion had induced him to land in the middle of a bone-dry lake bed. As an example of nature's whimsy, this particular mirage would be hard to improve upon, for ironically enough when the pilot had observed the same area from his train window the winter before, it had been a lake and the water in it had been real. Brimming with water from mountain runoff following winter rains, the lake regularly dries up under the hot summer sun, but the smooth baked-mud bottom provides a perfect reflector above which the sun's shimmering rays create a fictitious lake, complete with upside-down images of adjacent mountains mirrored in its surface.
Nature's mirages are seldom so neat, but they are invariably persuasive, taking in even such hard-headed explorer-scientists as Roy Chapman Andrews and Admiral Robert Peary, discoverer of the North Pole. Peary, in fact, was an unwitting participant in a comedy of errors that was to lure a scientific institution into exploring a wholly non-existent range of mountains:
Act I: Seeking a Northwest Passage across the top of the American continent, Sir James Ross and his uncle Sir John sail across. Baffin Bay into Lancaster Sound, only to find it blocked on the west by an enormous range of mountains. They return to England, bitterly acknowledging defeat. The year is 1818.
Act II: In 1906, Admiral Robert Peary confirms the Ross discovery, but ice floes forestall his attempt to map the mountains at close range. He labels the unexplored area "Crocker Land" on his maps and reports his findings to the American Museum of Natural History.
Act III: In 1913, Arctic expert Donald MacMillan is dispatched by the American Museum of Natural History to explore and map Crocker Land. At last, after a trip dogged by misfortune, the elusive mountains turn up, far to the west of their mapped position and blocked by ice floes. MacMillan leads a picked crew of volunteers across the jagged ice in hot pursuit of the mountains, which now seem to retreat ahead of them. And then, as the sun slowly sinks below the horizon, so do the mountains and their hopes as nothing lies before them but a vast, flat, unbroken ice floe, extending out apparently to infinity. Curtain.
Considering how commonplace mirages are—nearly everybody has seen "puddles of water" on a hot road receding as his automobile advances—scientists know surprisingly little about them. Normally, air nearer the surface of the earth, compressed by the air above it, is correspondingly denser. The light rays which transmit images of objects to an observer thus refract, bend downward, as they pass through the increasingly dense layers of air, slowing down just as a man walking slows as he passes from ankle-deep water to the viscous mud of a bog. The refraction is usually so little as to be scarcely noticed, unlike the refraction of a soda straw in a glass of water, where the differential densities of the air and water apparently bend the straw to a quite visible degree.
The slight distortion of light waves in air through normal refraction becomes immediately noticeable, however, if for any reason the usual density pattern is disrupted. This most commonly occurs when a radiating surface—the heated earth, for example, or relatively warm water between two ice floes—heats a layer of surface air which in expanding, becomes less dense in exact proportion to its rising temperature. Now, instead of refracting the light rays down, it deflects them upward, lifting and carrying them far beyond their natural destination. The result: a mirage.
Optimum conditions for the creation of a mirage are a stretch of absolutely level landscape, still air, and great contrasts between the layers of air overlaying the surface. Few mirages are therefore perfect, yet their very imperfections often reinforce their semblance of reality. In a brisk breeze, for instance, the "waters" of a mirage lake may appear to form combers and break on the shoreline, while the lake's shoreline itself unaccountably stays in place. Nor is flat land really essential. One of the most fearsome mirages is the "Brocken Specter," named for Germany's Brocken Mountain on which it was first observed. Alpinists sometimes are uncomfortably aware that they are being watched and, looking up, see an enormous climber ascending an identically steep rock face. The sudden apparition so frightened some climbers that they lost their balance and fell to their death—victims of nothing more than a mirage of themselves, projected and enlarged against a distant cloud bank.
Mirages, it should be emphasized, are representations, however distorted, of objects that do exist. The cliche, beloved of cartoonists, in which a thirsty desert traveler sights an inviting pool surrounded by palm trees and thinly-clad beauties and dives in, only to surface with a mouthful of sand, is a confusion of two distinct phenomena—mirage and hallucination. The image of palm trees bordering a body of water is a frequent desert mirage. Tree images are transported by freak heat waves over scores, even hundreds, of miles; or they may be merely clumps of grass or knee-high bushes, magnified and twisted by heat waves into giant palm groves. The "water," as is the case in almost all mirages, is the reflection of the clear blue sky above. The lissome females, sad to report, are manufactured by the fevered mind of the traveler to his own specifications.
The popular association of mirages with deserts goes back at least as far as Napoleon's abortive Egyptian campaign of 1798, during which his parched troops were plagued by water that wasn't there. In the remarkable company of scientists who accompanied Napoleon, was a physicist named Gaspard Monge who was driven by the soldiers' suffering to discover the secret of the tantalizing visions. Monge usually gets credit for the invention of the word mirage—from French mirer, "to gaze at"—as well as for the first scientific description of the phenomenon; actually, the word was in current use long before the French savant was born. More significantly, Monge quickly came to the correct conclusion that differential densities of surface and near-surface air, caused by extremes in temperature, give rise to mirages. This view was immediately accepted by the scientific world, which has modified the theory only to the extent of classifying mirages into three types: superior, inferior, and double.
Inferior mirages include such illusions as the puddles-in-the-road and the palm-tree oasis, in which light waves are reflected upward, away from the surface of the earth. Superior mirages are the mirror twins of inferior mirages, except that in superior mirages the light waves first bend upward, then down, thus inverting the image. The Eiffel Tower has been observed numerous times, while not noticeably under the influence, standing on its own head. Similarly, one of the most common mirages at sea is that of a ship sailing serenely along under full sail, high in the sky, upside down. Double mirages are simply a combination of superior and inferior mirages, and are much less common. A rare type of optical illusion, called the lateral mirage, would come in very handy if a party of campers ever spotted a bear on the other side of the mountain coming their way.
Since mirages are natural phenomena, they can be charted and photographed as well as seen, but they do embody one basic contradiction that stubbornly defies explanation. A law of physics states that the apparent size of an object diminishes at a rate inversely proportional to the square of its distance from an observer. An object whose distance from an observer is doubled, therefore, should appear one-quarter its former size. This rule conspicuously fails to apply to "longdistance" mirages which carry images of trees, ships, buildings and other objects a thousand or more miles, and set them up in the sky or on dry land, with no diminution in size. No one knows why. In deserts, where day after day conditions most closely approximate the ideal, some mirages appear almost as regularly as the morning sun. In some places mirages occur seasonally, and in others infrequently or not at all. A village in Maryland has recorded only one mirage in its entire hundred-year history, but that one made up for what the town had been missing: a city of domed roofs appeared in the sky above it, perhaps brought all the way from North Africa or even the Middle East.
Perhaps not, for mirages have the habit of playing tricks. Cakes of ice are often transmuted into icebergs which frighten ships' crews into altering courses to avoid collision. Saplings loom as redwoods, riot-sized rocks as mountain peaks, a gravelly beach becomes a metropolis complete with industrial smoke manufactured by nature from sea spume.
When the oil geologists first landed in Saudi Arabia's Eastern Province, they had a terrible time with mirages. Dunes became cliffs, palmettos became forests, camels came over the horizon as tall as silos and the tiny village of Jubail, as Wallace Stegner wrote, "threw up a skyline like New York's, a vision of cloud-capped towers and gorgeous palaces."
In 1918, as British General Allenby pushed forward into Palestine, he met head-on a Turkish contingent which by sheer weight of numbers ground his forces to a standstill. Allenby was seriously contemplating breaking off the engagement when suddenly the Turks themselves decamped in haste, leaving the field to the baffled British. A Turkish outpost, sighting large British reinforcements hurrying toward the battleground, gave the alarm to their commanders who, after one of the briefest conferences on record, decided to fight another day. There were reinforcements, all right—slogging along 100 miles away.
Probably the eeriest of all mirages was that seen by the soldiers left behind at Fort Abraham Lincoln, South Dakota, when General George A. Custer and his 264 troopers marched out in 1876 to their appointment with immortality at the Battle of the Little Big Horn. Sentinels on the ramparts watched the little detachment pass out of sight behind the hills and then, as if nature had suddenly become prophet, a mirage formed revealing the silent column marching up into the sky. Shortly after, Custer and his men were annihilated.
Daniel da Cruz writes regularly for Aramco World Magazine.
