Types of Mirages

Mirages

The term mirage implies an inverted image. Refraction phenomena without image inversion are discussed below.

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.)

To produce a superior mirage, the inversion must be steep enough to produce a duct.

NOTE: The textbook account of superior mirages usually includes only two images. But in the real world, superior mirages usually involve three images: an inverted image between two erect ones.

However, the upper erect image of a 3-image superior mirage often is so extremely compressed as to escape notice. Then the casual observer will report only two images. This happens often enough that I have put superior mirages here in the 2-image group.

It's also possible for the lower erect image in a 3-image mirage to be hidden below the apparent horizon — particularly if the observer is close to the sea or ground surface. Then the display will again be reported as a 2-image superior mirage; or even a single inverted image, if the top image is too compressed to notice.

Three-image mirages:

An inverted image lies between two erect ones. The top image is often strongly compressed. These purely refractive phenomena are caused by inversion layers. In addition to the complete, 3-image version of a superior mirage, there are 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.)

The inversion need not be strong enough to form a duct; the mirage is due to the decrease in lapse rate at the base of the inversion.

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 included in this term. 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. Certainly, strong inversion layers are responsible.

At least one type of Fata Morgana display is attributable to strong, low-lying inversions of considerable thickness, such as are produced in valley circulations — especially those with cold water at the bottom, such as the Strait of Messina, Lake Geneva, etc. See the Fata Morgana page for details and references. The optical mechanism is focusing by a quasi-reflection in the inversion.

Another similar kind of display seems to be produced by looking up through an inversion, and is purely refractive; but the detailed explanation of these rare mirages 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".

Mirages and Dip

In particular, atmospheric refraction alters the dip (sometimes called depression) of the apparent horizon. This is of great importance in celestial navigation, because observations of celestial objects at sea, made with a sextant, are referred to the sea horizon. So we have a special page devoted to dip.

Dip is intimately related to mirages: the classical inferior mirage is always associated with increased dip; and the classical superior mirage, with decreased dip. Looming can be so large as to produce negative dip: an apparent horizon above the astronomical one. Very large looming can even make the normal sea horizon disappear, replacing it with an elevated, false horizon.

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