Mirages (from the French "mirage") are an optical phenomenon in the atmosphere, due to which images of objects appear in the visibility zone, which under normal conditions are hidden from observation. Miracles of this kind happen because in an optically inhomogeneous atmosphere, the rays of light are bent, as if looking beyond the horizon. Most often, irregularities arise due to uneven heating of air at different heights. In ancient Egypt, it was believed that a mirage is the ghost of a country that no longer exists. Legend says that every place on our planet has its own soul.
More often, mirages can be seen in the desert. This can be explained by the fact that hot air acts like a mirror. For example, in the Sahara, about 160,000 mirages are observed each year; they are stable and wandering, vertical and horizontal.
In particular, caravans in the Erg-er-Ravi desert in North Africa often fall victim to mirages. Oases appear before people "with their own eyes" at a distance of 2-3 km, to which, in fact, not less than 700 km. Mirage is capable of misleading experienced people.
In ancient times, nomads, to make sure they see a mirage or real objects, kindled a fire. If there was even a slight movement of air in the desert, then the smoke spreading along the ground quickly dispersed the mirage. For many caravan routes, maps have been drawn up, which indicate the places of common mirages. These maps even indicate where wells, oases, palm groves, mountain ranges, etc. are seen.
Atmospheric mirages can be divided into three classes, and the reasons that cause them are quite varied.
• First-class mirages - the so-called lake or lower mirages. They are the most common and simple ones. For example, water seen on desert sand or hot asphalt is a mirage of the sky over hot sand or asphalt. Plane landings in movies or car races on television are often filmed fairly close to the surface of hot asphalt. Then below the car or plane you can see their mirror image (inferior mirage), as well as the mirage of the sky.
The higher you are on land or at sea, the less air density. Under normal conditions, air density decreases with increasing altitude. When light passes over the surface of the earth, the air is denser below the light beam than above. A typical property of light is that it refracts towards a denser medium, so that a ray that travels along the surface of the earth is in fact always slightly refracted downward and travels along the slightly curved surface of the earth instead of heading straight towards the sky.
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The denser air, as it were, slows down the lower end of the beam and pulls it towards itself. On the other hand, it seems to a person that the object is in the direction from which the light reaches his eyes. Thus, when you look at the distant horizon, you see objects that are actually partially below the horizon. Light from these objects is refracted along a curved, curved surface of the land or sea, and therefore it only seems that the light reaches the eye of the observer from the horizon.
Many are familiar with the phrase that says that when we look at the sun as it sets, it is actually over the horizon. In astronomy, this phenomenon is known as refraction: the refraction of light in the atmosphere lifts celestial bodies on the horizon by about half a degree.
Very often, air density does not change evenly with altitude, and colder, denser air and warmer air form layers of different temperatures at different altitudes. The movement of light in such air can be quite erratic, thus creating a distorted image of the landscape.
The lower mirage is identical in structure: there is always only one inverted, more or less flattened mirage below the object. If the landscape itself is beautiful, then its mirage is also beautiful, and they can spread together on the horizon as a line of buildings and treetops.
If it happens in a desert, the surface of which and the adjacent layers of air are hot by the sun, the air pressure at the top can be high, the rays will begin to bend in the other direction. And then interesting phenomena will begin to occur with those rays that should, having reflected from the object, immediately buried themselves in the ground. But no, they will turn upwards and, having passed the perigee somewhere near the very surface, they will go into it.
Let us now imagine that such a ray, already bent, enters the pupil of a traveler walking through the desert. But to subjective perception, the object (say, a palm tree) will be located in the place where the tangent to the ray path points. Accordingly, the image of a palm tree will be reversed, as if reflected in water. And a lot of water will spill around. Such an insidious joke will play with the thirsty traveler, the sky that has moved into the sands.
French scientist Gaspard Monge, who took part in Napoleon's Egyptian campaign, described his impressions of the lake mirage as follows: “When the earth's surface is very hot by the Sun and just begins to cool down before dusk, the familiar terrain no longer extends to the horizon as in the daytime, but passes, as it seems, about one league in a continuous flood.
The villages further away look like islands among a lost lake. Under each village is its overturned image, only it is not sharp, small details are not visible, like a reflection in the water, swayed by the wind. If you begin to approach a village that seems to be surrounded by flooding, the coast of the imaginary water keeps moving away, the water arm that separated us from the village gradually narrows until it disappears altogether, and the lake now begins behind this village, reflecting the villages located further away."
Anyone can observe the lower mirage. If on a hot summer day you stand on a railway track or a mound above it, when the sun is slightly to the side or side and slightly ahead of the railway track, you can see how the rails 2-3 km ahead seem to plunge into a sparkling lake - as if the track was flooded with a flood. If we try to get closer to the "lake" - it will move away, and no matter how much we go towards it, it will invariably be at the same deceptive distance.
• Mirages of the second class - whose rays are bent beyond the horizon line, are called upper, or distant vision mirages. They arise right in the sky. If warm air, heated somewhere above the desert, invades the upper layers of the atmosphere, and cold dense air of the anticyclone is below, then the rays that have undergone refraction can look very deep beyond the horizon. Light reflected from a distant object (for example, an island) finds two paths to the eyes of the observer: the first passes almost directly from the island to the observer, and the second rises slightly upward to the warm air layer, where the ray refracts downward at a slight angle to the colder air and reaches the eye of the observer from above.
Two images of the same island are created - one is normal, and the second is an inverted image above the island, that is, the upper mirage. In turn, the specific type of atmospheric phenomenon that creates such a mirage is called thermal inversion. Then on the surface of the cold mass of air lies a clearly defined, lighter and less dense layer of warm air. Severe thermal inversion also causes occasional interference to radio, TV reception, and cell phones.
2006, May 8 - Thousands of tourists and locals watched a mirage that lasted for 4 hours in the city of Penglai off the east coast of China on Sunday. The fogs created an image of a city with modern high-rise buildings, wide city streets and noisy cars. It rained in Penglai for 2 days before this rare weather event occurred. Inhabitants of the French Riviera on a clear morning more than once saw how on the horizon of the Mediterranean Sea, where water merges with the sky, a chain of Corsican mountains rises from the sea, to which about 200 km from the French Riviera.
The upper mirage is described in one of the works of N. V. Gogol:
“A great miracle seemed beyond Kiev! Suddenly it became visible far to all parts of the world. In the distance, the Liman turned blue, the Black Sea was overflowing beyond the Liman. Experienced people recognized the Crimea, rising like a mountain from the sea, and the swamp Sivash. On the right hand, the Galician land was visible.
- What is it? - interrogated the assembled people, pointing to the gray and white tops that seemed far away in the sky and looked more like clouds.
- The Carpathian Mountains! - said the old people."
Lateral mirages can occur when layers of air of the same density are located in the atmosphere not horizontally, as usual, but obliquely or even vertically. Similar conditions are created in the summer, in the morning shortly after sunrise at the rocky shores of the sea or lake, when the coast is already illuminated by the sun, and the surface of the water and the air above it is still cold. Lateral mirages have been repeatedly observed on Lake Geneva. For example, people saw a boat approaching the shore, and next to it, exactly the same boat was moving away from the shore. A side mirage can appear at a stone wall of a house heated by the sun, and even on the side of a heated stove.
Thanks to the side mirage, silent foggy ghosts appear, blocking the path of the traveler in the mountains. Usually, a frightened person sees himself. Strongly heated rocks cause such rarefaction of air around them that the rays reflected from the observer and directed towards the rocks are bent near them to such an extent that, like a boomerang, they return back.
Images in side mirages are almost always equal in size to reflected objects, but they can double, triple, etc. There is a hypothesis that the famous ghosts, who have chosen some castles, are nothing more than a side mirage. In winter, as you know, damp damp walls must be intensively heated. The stones from which the stoves are built are much more hot than boulders in the midday sun, and high vaulted ceilings allow the beam to loop and return to the observer.
• Mirages of the third class are amazing mirages called ultra-long-range mirages. For them, distances of thousands of kilometers are not a hindrance. Here is the case described in the book "Optical Phenomena in Nature":
Class III mirages have no reliable scientific explanation. In order to somehow justify their appearance, it is suggested that giant aerial lenses are formed in the atmosphere or secondary, tertiary - multiple mirages that relay the same image along a complex chain appear. Some even try to prove that there is a special "mirror" in the ionosphere, from which the sunbeam, like a radio signal, is reflected and, simultaneously self-focusing, is carried away to another part of the light.
An interesting version is expressed by Victor Loisha: “Why not admit that under some very successful coincidences of many physical circumstances in the air, natural superconducting light guides can form, linearly oriented channels of anomalous ionization, along which light beams are transmitted over very long distances - so, that the sunrise over Japan suddenly becomes visible, for example, in the Azores ….
• Fata Morgana is a complex optical phenomenon in the atmosphere, which consists of several forms of mirages, in which distant objects are seen many times and with various distortions. Fata Morgana appears when several alternating layers of air of different densities are formed in the lower layers of the atmosphere, capable of producing specular reflections.
As a result of reflection, as well as refraction of rays, real-life objects give on the horizon or above it several distorted images, partially overlapping and rapidly changing in time, which creates a bizarre picture of this complex mirage. This phenomenon was named after the heroine of legends - Fata Morgana. They say that she was the half-sister of King Arthur, but after the knight Lancelot rejected her love, she settled out of grief at the bottom of the sea, in a crystal palace, and since that time has deceived seafarers with ghostly visions.
Anyone who spends a lot of time in polar waters will certainly see mirages. For example, seasoned Finnish sailors and fairway connoisseurs know full well that there are conditions that make it extremely difficult to find a familiar route among the confusing mirages on the rocky coastline. In Finland, conditions for mirages are especially favorable in the spring when the sea ice melts. A water temperature of 0 ° C with a spring wave of warm air of 15 ° C can create incredible mirages in the sky.
This kind of deception of mirages can also be explained by the deviation of light from rectilinear advance, in which the object is seen in the wrong direction or is distorted. Ghost mirages are usually visible on the horizon. The angle of mirages is very low, but they can be very different in shape. Bushes and stones on a small island can be perceived as towers in the sky; low rocky shores stretch vertically, and they resemble precipices; the ship and its deck superstructures can be distorted into unidentified square shapes, and the islands themselves seem to swirl in the air.
E. Gurnakova