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Mirages come in two major families: inferior and superior. Mirage is a refraction phenomenon and is born when two air layers have large temperature difference. I.e. temperature changes a lot in a short (vertical) distance or equally "there is a strong temperature gradient". If warm layer of air is covered by a cold layer, an inferior mirage is born below the "true" image; if cold layer of air is covered by a warm layer a superior mirage is born above the "true" image. Clearly, inferior mirage condition is not stable because warm air rises and mixes with cold layer and thus temperature gradient is smoothened. Likewise, superior mirage condition can be pretty stable when cold air is trapped by an inversion layer.
Inferior mirage is frequently seen at open sea or at large lakes especially in autumn, when cold night air rolls over hot water. Distant islands look as if they were reflected in calm water. However, there is an easy way to find out whether question is of mirage or of an ordinary rellected image: lower your vantage point and see if islands vanish the more the lower you observe. If they vanish you are observing mirage. Superior mirage is far less frequent than inferior mirage. Favourable conditions are born f.ex. at cold sea, when warm air from land is blown above it creating an inversion layer a few meters above the cold water surface. In my northern latitude this kind of situation is most common in spring just after sea is freed from ice cover. Occasionally more than one inversion layer is born giving rise to several miraging images. These images are above the "true" object: the first miraging image is upside down, the second (usually greatly flattened) is upright, then again (greatly flattened) third miraging image is upside down and so on. An inferior mirage is visible from different heights of the miraging surface; only the strength of the phenomenon differs. With superior mirages things are different: one has to find correct vantage point to see it at all. Typically there is a "height zone" of only 1-2 meter, where superiors are seen. This height may be changing rapidly as inversion layer alters in height. To hunt for superior mirages one has to find a place where it is easy to hike up and down. Complicated displays may be hard to explain, because there are islands at different distances and details at different heights mirage differently. Mirage videofiles (RM-files) are typically between 500-800 kB, a few of them larger (up to 1.8 MB).
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