When an unusual optical phenomenon is observed in the atmosphere, its positive identification as a mirage cannot be made without a physically meaningful description of what is seen and a complete set of meteorological and astronomical data. The required "hard" data are practically never available for the specific place and time of observation, so that the descriptive account remains the only basis for identification; in this case, successful identification depends on a process of education. Thus, the casual observer of an optical phenomenon can establish the likelihood that his observation is a mirage only by being aware of the basic characteristics of mirage and the physical principles that govern its appearance and behavior.
The conditions required for mirage formation and the principal characteristics of mirage images, as described in this report, are summarized below. The summary presents a set of standards by which to interpret the nature of an optical observation in terms of a specific natural atmospheric phenomenon.
Optical mirages arise from abnormal temperature gradients in the atmosphere. A temperature decrease with height (temperature lapse) exceeding 3.4°C per 100 m or a temperature increase with height (temperature inversion) is most commonly responsible for a mirage sighting.
Large temperature lapses are found in the first 10 meters above the ground during daytime. They occur when ground surfaces are heated by solar radiation, while during nighttime they can occur when cool air flows over a relatively warm surface such as a lake. When the temperature decreases with height more than 3.4° per 100 m over a horizontal distance of 1 kilometer or more, an observer located within the area of temperature lapse can sight an inferior mirage near the ground (e.g., road mirage, "water" on the desert)
Layers of temperature inversion ranging in thickness from a few meters to several hundred meters Mai be located on the ground or at various levels above it. In areas where they are horizontally extensive, an observer can sight a superior mirage that usually appears far away (beyond 1 kilometer) and "low in the sky." The strength of the inversion determines the degree of image-elevation; the stronger the inversion, the higher the image appears above the horizon. Layers of maximum temperature inversion (30°C) are usually found adjacent to the ground.
Calm, clear-weather conditions (no precipitation or high winds) and good horizontal visibility are favorable for mirage formation. Warm days or warm nights during the summer are most likely to produce the required temperature gradients.
Geometry of Illumination and Viewing
The geometry of illumination and viewing in the case of optical mirage is determined by the spatial variations of refraction index that occur in the cloud-free atmosphere, and by Snell's law of refraction, which relates these variations to changes in the direction of propagating wavefronts. The spatial variations in refractive index are associated with layers of temperature inversion or temperature lapse. Variations of 3x10-5, corresponding to temperature changes of 30°C, are considered near maximum. As a consequence of Snell's law and the small changes in the atmospheric refractive index, an optical mirage develops only when a temperature inversion layer or a layer of large temperature lapse is illuminated at grazing incidence. The requirement of grazing incidence implies that the source of illumination must be either far away, i.e., near the horizon, or very close to or within the layer of temperature gradient. Therefore, both terrestrial and extraterrestrial sources can be involved. Because of the distance factor, the actual source of illumination Mai not be visible. Its location, however, must always be in the direction in which the mirage image is observed, i.e., observer, image and "mirrored" source are located in the same vertical plane.
Another consequence of Snell's law and the small spatial changes in refractive index is that noticeable refractive effects are not likely beyond an angular distance of approximately 14 degrees above the horizon and that a superior mirage image is not likely beyond an angular distance of 1 to 2 degrees above the horizon. Hence, mirages appear "low in the sky" and near the horizontal plane of view. An optical image seen near the zenith is not attributable to mirage.
Because of the restricted geometry between observer, mirage image, and source of illumination, the observed image can often be made to disappear abruptly by moving to higher or lower ground. Furthermore, when mirage observations are made from a continuously moving position, the image can move also, or can move for a while and then abruptly disappear.
Forme et couleur
A mirage can involve more than one image of a single object. Observations of up to four separate images, some inverted and some upright, are encountered in the literature. When multiple images occur they all lie in a single vertical plane or very close to it.
The apparent shape of a mirage can vary from clearly outlined images of an identifiable object such as a distant ship, landscape, or the sun or moon, to distorted images that defy any description in terms of known objects (e.g., Fata Morgana). Apparent stretching either in the vertical or in the horizontal plane is common.
During daytime, a mirage can appear silvery white ("water" on the ground), or dark when projected against a bright sky background, or it can reflect the general color of the land or seascape. Distinctly colored images ranging from red and yellow to green and blue are observed when unusual conditions of mirage occur near sunrise or sunset (e.g., Red and Green Flash) or, at night, during rising or setting of the moon or of a planet such as Venus.
In the presence of atmospheric turbulance and convection, the effects of scintillation become superimposed on the large-scale mirage image. When scintillation occurs, extended mirage images appear in constant motion by changing their shape and brightness. When the image is small and bright, as Mai be the case at night, large fluctuations in brightness and under unusual conditions in color can give an illusion of blinking, flashing, side to side oscillation, or motion toward and away from the observer. The effects associated with scintillation can dominate the visual appearance of any bright point-object in the area between the horizon and approximately 14 degrees above the horizon.
The theory of ray optics adequately explains such observed large-scale aspects of the mirage as the number of images, image inversion, and apparent vertical stretching and shrinking. However, if the interference and focussing of wavefronts within the refracting layer are as fundamental in mirage formation as purported by Sir C.V. Raman, the ray-tracing technique Mai have to be replaced by the theory of wave-optics.
Sir C. V. Raman's application of wave-optics to mirage suggests that under special conditions of illumination, the upper boundary of an atmospheric temperature inversion could exhibit a large concentration of radiant energy due to focussing of wavefronts. Also, interference of wavefronts could produce alternating layers of high and low brightness. Under what conditions and to what extent these brightness effects can be observed in the atmosphere is not known. Relevant observations have not been encountered in the literature, although some unusual observations of the green flash made under mirage conditions (O'Connel, 1958) could possibly have been caused by the enhancement of brightness in an inversion. The visual effects from focussing and interference of wavefronts must be considered as the least explored aspect of mirage.