Superior mirages are a class of mirages that can be very hard to find in nature, as I know now from my (little)
experience observing them.
These mirages are the images of objects visible in the sky above the object (as opposed to below the object, as with
inferior mirages). You may have seen photos or drawings of inverted ships in the sky over an ocean; these are formed by
superior miraging. In fact all objects that you see are images. Everything we see is an image, so it isn't really
sensible to talk about mirages of objects and objects themselves. All mirages and objects as we see them are images.
I have been looking for these kinds of mirages for years and finally, on March 30 and 31, 2004, I spotted a few, and it
immediately became clear to me that they are not rare in the place where I currently live, but I had been looking in the wrong
places. I think you can imagine my feeling of enjoyment and satisfaction when I finally did observe them! Below I
will attempt to explain where to look for these stunning mirages.
The familiar wet-road effect is the best example of an inferior mirage.
How mirages form in general
Any mirage is due to abnormal (i.e. steep) density gradients in the atmosphere. Light travels faster in less
dense air, so if the density of air in the atmosphere changes, light is deviated (refracted) in the
direction of denser air. This is because the wavefronts that make up the light travel slower on the side of the ray that
is closer to the denser air, so all wavefronts are angling slightly and the ray of light is refracted in a lateral
direction. (Compare this with the case
of two people walking side by side in a stairwell: whenever they need to change direction, the person on the outer side
of the curve needs to walk faster than the other person to keep up. Or, if two persons are walking side by side
through a swamp with equal effort but one of them has to walk in a slightly deeper part, that person will move slower and
they will gradually turn toward the deeper part.)
The density of air at some pressure changes as its temperature changes: warm air is less dense than cold air. Thus,
you may expect light to bend upward if the air near the ground (say a street) is warmer than the air above. Light that
comes from the sky near the horizon is bend upward to you from the road, and so the road appears to reflect the sky: it looks
as if there is a puddle of water on the road. This is the well-known wet-road mirage, which is an inferior mirage. In fact
there is no reflection but a refraction, but we can't tell the difference by looking at it from one location.
A superior mirage at work on a distant mountain range (the Sandia Mountains in New Mexico, USA) that
would normally appear gradually sloped. The mirage is both inverting part of the range as well as looming the crest.
Notice the brown band of smog with clearly defined upper boundary; this band marks the temperature inversion.
How superior mirages form
If the temperature near ground level is colder than the temperature aloft, there is a so-called temperature
inversion.1 This is the situation in which superior mirages may form. However, it is unusual for the atmosphere
to have a steep enough temperature gradient to actually act as a miraging lens. More likely, with a not too steep
temperature gradient, all objects on the horizon will appear to be raised (looming) by a very small amount
(of the order of arcminutes), something which will not be obvious to the casual observer.2
If the temperature gradient is steep enough, at least about 11oC per 100 meter in altitude, and if you are at
exactly the right elevation, you may see superior (mirrored or towering) images of objects near the horizon.
For this you should
be with your eye within or just under the temperature inversion; if you are above it, you will not see anything unusual, while if
you are too far under it, the miraging layer will be too high in the sky and likely too weak to mirage anything at the horizon and
you will not see anything unusual either.
The angles over which light is bent are really small, so you should be very close below or within the inversion
layer and looking through it horizontally.
Note: inferior and superior do not refer to the quality of the mirage, but to its location: inferior
meaning under or below and superior meaning over or above.
Suitable circumstances for formation of superior mirages
Superior mirages may be quite rare, as I noted above. However, there are a few types of places on Earth that favor
these mirages. These are:
- Dry desert regions, with exceptionally clear air. If there are no clouds and little water vapor in the sky during the
night, the desert floor can cool off substantially by thermal (infrared) radiation to space. The result is a quite steep
temperature inversion in the early morning that can be a change in temperature of up to 20oC in a layer of maybe
a few hundred meters high. Also, deserts usually are near mountains, and distant mountain ranges provide for suitable
background objects to be miraged.
- Arctic regions, where the cold icy surface cools off warmer air above it. Spectacular superior mirages and the Fata
Morgana are likely in regions such as the arctic seas and Antarctica.
- High-latitude regions close to a lake or sea, during spring-time, that have had severe frost during the winter. The
sea or lake water will be very cold or frozen, while the spring brings warm air that flows out over the cold water and
forms a suitable temperature inversion.
- Coastal regions bordering a desert. If the prevailing winds bring hot desert air over the cool ocean a temperature
inversion may form over the water, suitable for superior miraging.
In general, for any type of mirage to form, there should be little or no wind. Wind will mix the air in the boundary
layer and is likely to remove a temperature inversion.
Natural superior mirages are hard to discover, unless maybe you live near an arctic sea. The
mirage in this telescopic photo (taken in New Mexico, USA) was only a few arcminutes in size and barely
resolvable with the unaided eye.
Superior mirages are very dependent on observer altitude. If you move your head only a few meters, the mirage may
change drastically or disappear. Also, even if the atmosphere favors superior miraging, but there are no objects near
the horizon that are of the proper elevation, you won't see anything either! Lastly, if there is even a little bit
of wind, these mirages may be highly transient and disappear or reappear over the course of seconds.
Superior mirages over vast water expanses on which ships are sailing are easiest to detect, because the mirages are
typically a few arcminutes over the horizon, and the location of the (astronomical) horizon may be difficult to
figure out unless you are near a lake or sea. Distant ships are likely to show miraging, if the conditions are right.
Superior mirages over desert regions are most likely at dawn until about 2 hours after sunrise, at which time
the desert floor starts heating up and the radiative temperature inversion disappears. You need to look out over a
large flat area - uneven terrain is not good. Look for a very distant mountain region that is
at least 75 to 100 km far.
Note: natural mirages like these are quite inconspicuous and require binoculars or a telescope to be seen clearly. The
mirage will be very close to the astronomical horizon, being only a few arcminutes above it.
Photography of mirages in general requires a long telephoto lens, with a focal length of at least 400mm to 500mm. A
telescopic lens of 1000mm is better; you could also reach this by using a doubler (2x teleconverter) with a shorter focal
but be aware of vignetting! You also need a very sturdy tripod, preferably wooden (which damps vibrations better
than metal), and a cable release.
Ofcourse you can try photography with less exotic equipment than this, like a normal
200mm lens on a hand-held camera, provided you keep it steady. But the above stuff you need if you want to do a good job
on mirage photography.
Exposure and film speed
Expose the frame according to your camera's light meter. For film I suggest 100, 200 or 400 ISO, or any faster film to
make exposures not longer than about 1/30 second. Long exposures with telescopic telephoto lenses are not good,
because the air turbulence (shimmering) will smear out any details and the photo will be blurry. In fact, the unsharpness that
is associated with fast, coarse-grained films such as 800 ISO is actually less bad (yields higher resolution) than
the air turbulence at long exposure times, at focal lengths above 1000mm!
So, if there is so much light that you can use
fast shutter speeds with slow, fine-grained film, do so; otherwise, choose a fast shutter speed with faster film.
Increasing contrast using polarizing filter
Since you will be photographing mirages in the far distance, usually over 50km, there will be a lot of haze and
Rayleigh (blue sky) scattering between the mirage and you. If you have a polarizing filter, this can be of great
help in reducing that scattering (not the haze), if the angle between the sun, the mirage and your camera is suitable
(around 90 degrees, i.e. the sun should be to the side or overhead in the sky). This is because the air is maximally
polarized 90 degrees away from the sun in any direction, so you can improve contrast drastically if you block the polarized
light from the air with a filter.
Directions for observing a superior mirage in New Mexico (USA)
If you become frustrated looking for a superior mirage and would like to see one, you should visit New Mexico or Arizona,
in the USA, if you can / live close. I am pretty sure that the superior mirages I see in New Mexico are quite common
because of the following reasons:
- a steep radiative temperature inversion is likely to form during the night, because it is usually exceptionally clear
(no clouds and little water vapor in the atmosphere) at night
- vast desert plains with distant mountain ridges providing for a good background
- high altitude, thus the air is less dense in addition to having very low humidity, increasing visibility
- sparse population, allowing for undisturbed photography along roads, and a scenery that is not too much spoiled
by commercial advertising signs and such
Now for specific directions. At the following vantage point I have seen a nice
superior mirage and suspect they can be seen often from this point:
- From Albuquerque, take the interstate highway I-25 south for about 50 miles. At exit 169, you turn to the east, pass
under the interstate overpass, and turn right (south) directly after. You are then on a small service road running parallel
to the highway; about 100m from the junction the road goes over a low hill. On top of this hill, you have a good
view to the Sandia Mountain Range just east of Albuquerque, which are likely to show a superior mirage at the lower
peaks (depending on the thickness of the inversion layer). There is a powerline running across your view, but
on top of that hill the view is acceptable.
You should be there around sunrise, and not later than 2 hours after sunrise, since at around that time the miraging
will disappear due to solar heating. Also, the night should have been totally clear and dry with good visibility,
and the days before should have been sunny and warm with little wind. This is not too uncommon in New Mexico. If you
are seeing a brownish
layer of smog over the Albuquerque area, and this layer has a well-defined sharp upper boundary, superior mirages are
very likely (in fact, the sharp upper boundary of the smog layer is a superior mirage of itself).
You may also see superior mirages over mountain ranges way south of where you are driving. The whole Rio Grande
Valley in New Mexico is rather flat if you look north-south. You may also see superior mirages at particular places
in e.g. Arizona, USA, however I have no observation experience there. I have noticed, though, that quite frequently
there is an extreme temperature inversion near the Flagstaff area in Arizona, by looking at morning atmospheric
Some other possibly suitable locations (that I know of)
In Europe, you may see superior mirages over waters like the North Sea or Atlantic during springtime. They are most frequently
reported and photographed in Finland. They should be visible sometimes in The Netherlands (my home country) as well,
especially from the beaches looking west over the North Sea during spring, although I haven't seen any textbook examples
Other favorable places on the North American continent are Alaska, Canada, and maybe the northernmost states of the USA.
At other continents I would not know preferred locations, but the section
on how superior mirages form should give you some information on whether they are possible at your location.