Types of Mirages

Two-image mirages (these are the classical types):

1. Inferior mirage

So called because the inverted image is below the erect one. This is the familiar hot-road mirage seen every sunny day on smooth paving. Caused by the thin layer of hot air below eye level, at the surface. (For simulations, see here.)

2. Superior mirage

Here the inverted image is above the erect one. Caused by a layer of hot air not far overhead (a "thermal inversion".) Like the inferior mirage, this looks much like a simple reflection. Astronomical objects cannot appear in a simple superior mirage. (For simulations, see here.)

Three-image mirages:

An inverted image lies between two erect ones. The top image is often strongly compressed. These purely refractive phenomena are also caused by inversion layers; they are of at least two kinds:

3. The “mock mirage”

Caused by looking down into an inversion below eye level, and then (thanks to the curvature of the Earth) out through it again beyond the horizon. The miraged objects may be about the same height above sea level as the eye, or may be considerably higher. (Cf. the simulations.)

4. Wegener's “late mirage”

Caused by looking up through an inversion above the observer. The miraged objects are always higher than eye level (e.g., distant mountains; astronomical objects). A true superior mirage of objects below the inversion may also be present, if the inversion is strong enough.

Mirages of higher multiplicity:

There are also distinct 5-image mirages. These have not been analyzed; they are certainly associated with strong thermal inversions, but the optical details are obscure. Multiple quasi-reflections at an inversion may be involved; cf. the simulations — e.g., here and here.

Complex mirages:

5. The Fata Morgana is the general name for these; but the phenomena are so varied that two or more mirage types must really be involved. The image is marked by repeated vertical and horizontal features, due to repeated alternations of erect and inverted images of some object. Often the mirage shows considerable internal motion, producing an illusion that people or animals appear in the scene. Almost certainly, strong inversion layers are responsible; but the detailed explanation of these rare displays has yet to be found.

Other uses of the term “mirage”:

Sometimes, you will come across the term “lateral mirage,” which is used in two senses. The first is a supposed sideways displacement of a miraged image, often by many degrees along the horizon. Such displacements are physically impossible; refraction is almost entirely in the vertical direction. (The largest measured refractive displacements in the horizontal direction are a few seconds of arc, dozens of times too small to be seen by the naked eye.)

Reports of large lateral displacements are the result of mis-identifications. (For example, a distant mountain that is normally hidden by closer ones may become visible by looming. As many mountains in any small region have similar shapes, the unfamiliar object is often incorrectly “identified” as a strongly displaced image of some familiar feature of the normal landscape.)

A more legitimate use of the term refers to mirages seen on sunlit walls. However, these mirages are simply the familiar inferior (hot-surface) mirage, turned through 90°. In this case, to avoid confusion with the erroneous reports mentioned above, I would prefer the term “mural mirage” to be used instead of “lateral mirage.”

Of course, “mirage” is often used metaphorically, to denote a false hope like that of the thirsty traveler in the desert, who imagines that water lies in the distance on seeing an inferior mirage. If you search for the word “mirage” on the Web, most of the pages you turn up contain such rhetorical uses of the term, and do not refer to real mirages.

Other refraction phenomena:

Besides mirages, there are other phenomena due to atmospheric refraction. These include looming (the appearance of objects normally hidden by the horizon) and towering (objects greatly elongated vertically), and the opposite effects, sinking and stooping. These, together with mirages and the displacements of astronomical and terrestrial objects from their geometric directions that are serious problems for astronomers and surveyors, are all classified as "refraction phenomena".

© 1999, 2000, 2005 – 2008, 2010, 2012, 2014 Andrew T. Young

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