Types of Mirages
The term mirage implies an inverted image. Refraction phenomena
without image inversion are discussed
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
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
appear in a simple superior mirage. (For simulations, see
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.
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.
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
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.,
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
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
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.
Besides mirages, there are other phenomena due to atmospheric
refraction. These include
(the appearance of objects normally
hidden by the horizon) and
(objects greatly elongated
vertically), and the opposite effects,
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
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
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.
© 1999, 2000, 2005 – 2008, 2010, 2012, 2014, 2017,
2020 Andrew T. Young
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