Introduction

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 11^oC 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 20^oC 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.

Observing

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 equipment

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 length lens, 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 soundings.

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 there yet.²

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.

1. The normal state of the atmosphere has decreasing temperature (and pressure) with increasing altitude, so an increase in temperature with altitude is called an inversion. Inversions are very stable, not allowing for any free convection.

2. I have noticed a "bowl effect" in The Netherlands a few times - the whole horizon being raised upward a bit due to a strong temperature inversion, and the sounds of civilization being heard from far distances. But that effect is only noticeable if you live at a certain place for years and know exactly how the landscape looks. I could easily see it because the city of Amsterdam would be visible over the lake dike from my home, which usually is blocked by it. Being out in the open, completely flat polders of The Netherlands, it is as if you are at the bottom of a very large bowl. The effect is small (maybe about 5 arc-minutes in elevation), but quite noticeable, and happens a few times per year.