by Steuart Campbell

From (http://www.bufora.org.uk/archive/mirages.htm) on April 29, 2002.

What are mirages and how do they appear?

A mirage is usually defined as a phenomenon where light is reflected from a shallow layer of very hot air in contact with the ground, the appearance being that of pools of water in which inverted images of more distant objects are seen. This is the inferior mirage, which occurs where a very hot plane surface, such as a desert or a roadway, heats a layer of air very close to it. The temperature gradient in the thermocline (the region of rapidly changing temperature) between this hot layer and cooler air above it is so steep as to constitute a discontinuity. This discontinuity acts as a mirror (or caustic) for light striking it above a critical (large) angle to the normal. In this way one can see distant objects such as the sky or vehicles reflected in the surface.

How can this explain UFO reports? It is not well known that these discontinuities can form in the upper air as the result of a temperature inversion - that is where a layer of warm air lies over cold air. Temperature inversions form almost every clear night when the ground cools by radiation more rapidly than the air above. Strong inversions are more likely to form a discontinuity and lead to mirages. These are called superior mirages, that is a mirage seen above the source or object being reflected (see Figure 1). In this way an inverted image of some bright but distant source may be seen in the sky. The definition of a superior mirage needs to be extended to cover one or more displaced images of a very distant but bright light source, usually distorted and brightened. Naturally this must be considered a major alternative to the ETH and a strong contender for explaining UFO reports. Figure 1: How the rays from a source (S) are reflected by the caustic in the thermocline of a temperature inversion if they strike it at or above the critical angle (c). Ray 4 enters at below the critical angle and so penetrates the caustic and undergoes normal gradual refraction.

Where the source is already in the sky, for example, an astronomical object, the image may be elevated, considerably so where the thermocline is curved. Non?horizontal thermoclines may displace the image laterally, and moving thermoclines may produce a moving image. Because an inversion forms in a fluid (air), the image can take various shapes and alter its shape with time. Consequently superior mirages can be unusual and protean.

Not all mirages are reflections; some are caused by abnormal refraction. If a temperature inversion forms over a very wide area, say over a cold ocean or ice field, and the temperature gradient is strong enough, light can be ducted around the curvature of the Earth, so allowing one to see an image of an astronomical object that is actually below the horizon. This is the 'Novaya Zemlya' mirage. The light in such a mirage can be ducted for hundreds of kilometres and the image may be distorted. It may also change shape and/or colour and be very bright. Light striking the discontinuity below a critical (large) angle to the normal, will not be reflected, but will pass through it and be refracted (Figure 1). An observer above the thermocline may then see a bright source elevated above its normal position.

Mirage images can consist of double images, with an upright image above the inverted one. This may be due to light penetrating the thermocline and being bent back down towards the observer (as shown in Figure 1). Where the thermocline is low over the source, the separation of the two images will be large. However, as the height of the thermocline increases, the two images can merge, making it difficult to recognize the image (see Figure 2). There is some reason to believe that each mirage image can split in the plane of the inversion, creating two separate images; if this occurs when there are already two images, the result will be four images of the same object!

Figure 2: One means by which the twin images of a mirage can be formed. Image Y1 is formed by reflection from the discontinuity in the thermocline (T) of the inversion. At P reflection ceases because the critical angle is not exceeded and the observer sees a refracted (upright) image (Y2). It can be seen that, as the height between the object (X) and the inversion increases, the two images will merge, eventually disappearing. Conversely, as the height decreases, Y1 and Y2 separate. If T is very shallow, Y2 will not appear. Drawn with exaggerated vertical scale for clarity.

Mirage images can be greatly enlarged and/or distorted by atmospheric lens effects: the more distant the object, the greater the magnification (because of the greater size of the atmospheric lens). Sources outside the atmosphere may be subject to the greatest magnification; among these, the commonest are astronomical sources. It may be expected therefore that the largest and most common mirages will be those of astronomical objects at low altitude. Magnification also increases as the source aligns with the thermocline. This means that, as the disc of an astronomical object approaches the thermocline, the two images enlarge and merge until they form a classic 'flying saucer' shape (see Figure 3). The two images may not always be the same size. Figure 3: A diagram showing how the two images of an astronomical body in a mirage can appear with different separation. As the images merge and enlarge, they form a classic 'flying saucer'.

Some mirage images of astronomical objects may display clusters of lights, perhaps multiple images of the object, and it is common for mirage images to shimmer. The enlargement of an astronomical object in a mirage will make its intrinsic colour more apparent, although differential refraction may produce several different colours at once, spatially separated. In a statement submitted to a symposium on UFOs organized by a committee of the US House of Representatives in 1968, astronomer Donald Menzel explained how strange an astronomical mirage could appear: "Sometimes a layer of warm air, sandwiched between two layers of cold air, can act as a lens, projecting a pulsating, spinning, vividly colored, saucer?like image of a planet. Pilots, thinking they were dealing with a nearby flying object, have often tried to intercept the image, which evades all attempts to cut it off. The distances may seem to change rapidly, as the star fades or increases in brightness. Actual 'dog fights' have been recorded between confused military pilots and a planet. I myself have observed this phenomenon of star mirage. It is both realistic and frightening." This is a reference to Menzel's own observation of a 'flying saucer' when he was flying over Alaska on a military mission in 1955. The object, which appeared to be flying alongside his aircraft, was complete with flashing red and green lights, a 'lighted propeller' on top and with a silvery metallic sheen. Later he identified it as a mirage of the bright star Sirius although it appears that it was actually a mirage of the planet Saturn.

UFO reports explained by mirages Surprisingly, and significantly, the very first 'flying saucer' report, that by Kenneth Arnold in 1947, can be explained in this way. He reported seeing a chain of nine peculiar 'aircraft' flying near Mount Ranier in Washington state (USA). They all moved together and occasionally flashed very brightly. However analysis shows that the apparent movement was entirely due to his own, just as a low moon will appear to follow you across a stationary landscape. All very distant objects at low altitude will appear to move because their direction does not change as that of a nearer object would. In this case, the source was nine snow-capped peaks in the Cascade Range over 100 kilometres away. In the bright sunlight, mirages of them were formed by temperature inversions over two deep river valleys between Arnold and the mountains. Where the inversions were strong, the mirages of the peaks flashed brightly. It appears that Arnold was not familiar with mirages, but this is true of almost all pilots.

In the right circumstances, any bright surface object can produce a mirage. On 17 November 1986, a Japanese freighter aircraft had crossed the North Pole and was heading SW toward its next stop, Anchorage in Alaska. Suddenly the crew were confronted by clusters of lights just ahead of them. They assumed that the lights were the exhausts of some unidentified aircraft and tried in vain to evade them. Gradually the mysterious lights shifted to port and the captain was sure he could make out the shape of a huge UFO alongside them. The incident was reported to the (US) Federal Aviation Administration (FAA), who issued a report on the incident, but without any explanation.

Because the object's direction appeared to move aft with time, it was obvious that the source lay on the ground only a few hundred kilometres away, and because the crew gave good descriptions and bearings to the lights at various times on their route, it was possible to locate its source. This turned out to be the US Army airfield at Delta Junction. The crew's description of the lights exactly matched that of typical runway lights and the FAA reported that a temperature inversion had existed over the area at the time. The 'UFO' was a mirage of the runway lights.

Aircraft headlights are a typical source of mirages. In May 1996, BBC Scotland showed me a video of mysterious lights seen over Inverness a few months earlier. It turned out that they were multiple mirages of the lights of a Nimrod aircraft which regularly trains from RAF Kinloss on the Moray Firth. This phenomenon explains the lights filmed in 1950 over Great Falls (Montana); two jet aircraft were flying about the area at the time but no one seems to have asked if they had their lights on. It also explains the many lights filmed over Tremonton (Utah) in 1952. In that case, there is evidence of several inversions, one on top of the other. A mirage of aircraft lights also explains a report investigated by physicist Bruce Maccabee in 1975: two bright objects 'like bright stars' were seen to the NE of Cheverly (Maryland), just east of Washington DC. They were seen in the general direction of Baltimore?Washington Airport about 34 kilometres away where a Boeing 707 was due to take off about the time of the sighting. Maccabee never considered mirages as an explanation and so failed to explain the report. Given that distant bright objects are often the source of mirages, astronomical objects at low altitude must be strong candidates.

Although the moon has sometimes been responsible, Venus, the brightest planet is the commonest source of such mirages. Indeed it was the object filmed as a UFO by a film crew in an aircraft off New Zealand in December 1978. In the new year, the film was shown on TV all over the world. Although Venus itself was below the horizon, its mirage image was visible via a Novaya Zemlya effect in which the light was ducted several hundred kilometres around the earth due to a temperature inversion over the cold Southern Ocean. It was also the object seen in daylight by forester Robert Taylor at Livingston (Scotland) in November 1979, a case I investigated on the ground. Mirages of Venus explain very many strange UFO reports, including the 1952 Nash/Fortenberry report (USA), the egg-shaped object seen over Anglesey (Wales) in September 1978 and the object seen and report in Todmorden (England) by policeman Alan Godfrey.

Other bright planets at low altitude have also been the source of UFO reports. The most sensational was the mirage of Jupiter reported and photographed by Almiro Barauna from a Brazilian research ship at Ilha da Trindade in the south Atlantic Ocean in January 1958. These are unique photographs, clearly showing the double image which results from the merging of two mirage images (see photo). A mirage of Jupiter was also the object which Capt. Thomas Mantell followed to his death over Kentucky (USA) in January 1948 and which Lt George Gorman tried to catch over Fargo (N. Dakota) in October the same year.

Two enlargements of the mirage of Jupiter photographed by Almiro Barauna at Ilha da Trindade (APRO). A mirage of Saturn was the object which scared young Ronald Johnson at his parents' farm near Delphos (Kansas) in November 1971. Mirages of Mars and Mercury have also produced strange UFO reports. Sometimes several planets together have been involved, as in the 1959 Gill case from Papua-New Guinea. Bright stars at low altitude can also stimulate mirages, but not necessarily only at night.

Sirius, the brightest star, is often responsible, as at Kirtland AFB in New Mexico in November 1957, when it was thought to be an object trying to land at the base. But it is the second-brightest star, Canopus, which has caused more reports. A mirage of Canopus was the object reported by police patrolman Lonnie Zamora over Socorro (New Mexico) in April 1964. This appears to have been caused by an inversion over the Rio Grande valley, south of the town. Astronomer Allen Hynek frequently challenged sceptics to explain this report, which he regarded as the epitome of the UFO phenomenon, apparently unaware that it had an astronomical explanation. A mirage of Canopus was also responsible for the sensational Cash/Landrum report from Huffman (Texas) in December 1980. The witnesses were convinced that a UFO had landed on the road ahead of them. A mirage of Canopus appears to have been the object which led to the death of pilot Frederick Valentich over the Bass Strait in October 1978. Disorientated by the mirage and convinced that it was on top of him, he seems to have crashed into the sea. There are 20 first magnitude stars, almost all of which at various times and in various places either directly or via mirage have been responsible for UFO reports.

In Conclusion Not only are mirages an 'alternative to the ETH', they explain reports which are otherwise inexplicable, especially the core reports which remain when all other reports have found an explanation. The result is that no UFO report remains unexplained and there is no mysterious phenomenon behind the reports. Furthermore UFO reports have nothing to do with extraterrestrial intelligence.

Steuart Campbell, 2000 References The UFO Mystery Solved Campbell, S. Explicit Books, 1994