Wim van Bochoven's Superior Mirage


Wim van Bochoven, who took these pictures, tells how he happened to see this display, on May 22, 2002:

We sailed the famous Hurtigruten ('the worlds most beautiful voyage') from Bergen to Kirkenes and back in 12 days.... On this particular and beautiful day we left Stamsund (Lofoten) heading for Bodø on the mainland of Norway.

[ATY remarks: That puts him about 68° North.]

I took, standing on the back of the boat, 10 to 15 meters above sealevel, some pictures of the sun, which was sinking behind the mountains. The weather was clear, almost no wind. It had been a very nice and warm day.

picture taken just before sunset Annotation: This image was taken at 22:44 local time (1 hour east of Greenwich). The focal length used on the digital camera was 85 mm. This picture and the others on this page are all © 2002 by Wim van Bochoven, the Netherlands, and are used here with his permission.

ATY remark: Note the haze layer hugging the sea. I have seen this effect here in San Diego as well, when there is a strong thermal inversion just above the sea surface. I believe the strong gradient in the opacity of this haze is due to the steep temperature gradient in the surface layer, combined with the rapid change in size of the aerosol droplets with relative humidity: near the cold water, the air is closer to saturation than a few meters higher up, where the temperature is several degrees warmer.

(Don't worry about the artifact in this picture: a very dim red disk centered near the waterline, just below the large mountain on the left. This is a “ghost” of the overexposed Sun, due to weak reflections between optical elements in the zoom lens.) Now the photographer continues:

And then... suddenly it was there: this ‘band of humidity’ I thought. Quite high from sealevel on, compared to the mountains. And in this band things happened: ‘shades’ were growing and growing and mirroring...

First picture of mirage Annotation: This picture was taken at 23:29 local time, with focal length 256 mm (3× optical zoom, compared to the wide-angle shot before sunset), plus about 2.5× enlargement of the original image.

ATY comments: A textbook example! Noteworthy (and very typical features) are: (1) the absolutely horizontal upper boundary of the duct that contains the miraged images. (2) Perfectly normal appearance of the mountains that stick up above the mirage. (3) Markedly hazy appearance of everything in the duct. (4) Everything seen in the upper part of the duct is an inverted image of corresponding features below; the “fold line” where the erect (lower) and inverted (upper) images meet is about 2/3 of the way up from the bottom of the duct, so that (4) the inverted image is compressed vertically, relative to the erect image.

Because of the motion of the ship, the distance to the miraged objects is uncertain. However, objects must be beyond the horizon to appear in a superior mirage like this. If the camera was 10 meters above the sea, the normal distance to the horizon would be about 12 km; but this is surely an under-estimate, given the strong refraction. The miraged objects are probably all more than 20 km away.

A second image, taken just 13 seconds later, shows another section of the mirage along the horizon to the right (closer to the Sun, which is why the clouds are brighter here):

Second picture of mirage

ATY comments: The fictitious “cliff” shown here is typical of these mirages, with an apparent overhang at the top. It's really just an inverted (and vertically stretched) image of a much gentler slope of the land.

Although the erect and inverted features that surround the “keyhole” in the middle of the image are almost mirror images (apart from the difference in vertical magnification), the correspondence is not perfect. That's because of perspective effects: some of these features are closer to the camera than others, but all sense of distance is lost because of the superimposed haze in the duct. But the erect and inverted images are seen from different directions: the eye has effectively a higher angle for the upper (inverted) image.

Notice how the duct appears as a lighter band in front of the shadowed ground at the right, but is seen dark against the (brighter) background sky at the left. These effects are both due to the scattering, or “optical thickness,” of the hazy air in the duct.

Third picture of mirage Here's a third image, taken just 17 seconds later. The scattering effects are more pronounced here. As these images were all taken within a minute, and the Sun is moving nearly parallel to the horizon, the decreasing illumination is due to looking farther and farther away from the Sun, into the part of the sky where the Earth's shadow is about to appear.

The Lofoten Islands angle off to the south-south-west, so as the photographer looks more to the left, away from the Sun (which is only a few degrees west of straight north), the islands seen are farther and farther from the camera. This increase in distance affects the mirage: the vertical stretching of features becomes more marked, and vertical features become more prominent.


Fourth picture of mirage The fourth image, taken only 12 seconds after the third, is also looking away from the Sun at a distant island. Here the effect of increasing object distance is very strong: the sides of the image are almost vertical. This means that the object is close to the point conjugate to the observer, if we regard the atmosphere as acting like a cylindrical lens. As the island is farther away, only a little of it is visible above the duct; probably very little of it would normally be seen at this range. The result is “looming” of land that would normally lie below the horizon.

It's worth pointing out that this display took place under very nearly ideal circumstances. The time of the year (late Spring) is exactly the season when superior mirages are most common: the water still has the chill of winter, but the air is warming up. And, indeed, the observer noted that it had been a warm day. The air was calm: no wind to mix the cold air next to the ocean with the warm air overlying it. The latitude was high, just above the Arctic Circle; thermal inversions are common at high latitudes. You could hardly ask for a better recipe for a superior mirage!

Fifth picture of mirage This final wide-angle shot was taken almost exactly one minute from the first image of the mirage. It shows the setting of the “keyhole” feature seen in the second of the mirage frames.

Once again, note that these pictures are copyrighted by the photographer, and cannot be used without his permission. Thanks to him for bringing these pictures to my attention! (And further thanks to Olaf Squarra for getting us together!)

            — A.T.Y.

Images © 2002 Wim van Bochoven, the Netherlands

Text © 2003, 2006 – 2008, 2012 Andrew T. Young

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