Superior mirages

From Glossary of Meteorology
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superior mirage

A mirage in which the image or images are displaced upward from the position of the object.

If only a single image of distant objects is seen, then the term looming is often applied: A horizontal surface appears to curve upward with increasing distance and terminate in a relatively distant optical horizon. The looming might be accompanied by either stooping or towering. The superior mirage is most striking when it exhibits three or more images. The upper and lower images are always erect, while a single middle image will be inverted. No matter the number, images will alternate between erect and inverted, although sometimes a pair will appear back to back and might be interpreted as a single image. Although textbooks sometimes suggest that what is seen is an object and an even number of images, all are images, and have positions and magnifications that differ from that of the object. What is seen is dependent upon both the distance to objects and the height of the eye. A change of either can produce markedly different image characteristics. Superior mirages occur over a surface when (molecular number) density decreases with height, but are always most striking when temperature increases with height. Then, what is seen (for a particular distance and observing height) depends critically on the shape of the temperature (and thus, the refractive index) profile. Everything from stooping to towering to multiple images is a possible result of fairly simple profiles. A common profile, a lifted temperature inversion, can not only produce the three-image mirage but, since internal gravity waves often occur on such an inversion, the resulting periodic horizontal inhomogeneity can also produce a higher number of images. Such lifted inversions are common over, but hardly confined to, enclosed bodies of water on warm afternoons when the warmer air from the surrounding land flows over the colder water.
Compare inferior mirage, sinking.


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