Arctic reindeer and The Blue Hour - a photo on Flickriver

This post gives a description of the sky during 'The Blue Hour' some 26 minutes before winter sunrise in Scotland. This is representative of periods during the Arctic winter to which the reindeer have adapted their vision to optimise the ability to feed and survive predation during the long winter twilight north of 60° latitude.

The image shows the reflective mirror (tapetum lucidum) revealed during the dissection of an eye from an animal during the regular cull of animals in northern Norway by the Sámi reindeer herders. The tapetum changes from a golden-turquoise colour in summer to a rich blue in winter.

The photograph of the sky https://www.flickr.com/photos/190452030@N06/52638473207/ was taken by Tom Fosbury in January this year (2023) and illustrates the physical mechanisms responsible for the intense blue environmental light that triggers this seasonal adaptation.

A Commentary on the Colour of the Pre-dawn Twilight Sky

For a direct link to the photo, see the comment below this text.

Looking towards the southwest from Cairngarroch before the winter sunrise, Tom's photograph of light snow-cover in the pre-dawn twilight provides a masterclass in the study of the formation of the colours of the sky.

With the yet-to-appear Sun over your left shoulder, the sky above the shallow peak called Millfore (656m) is beginning to show the familiar pale blue of the clear daytime sky above the pale yellow horizon that precedes sunrise. This is the pure blue produced by the Rayleigh scattering of sunlight by air molecules, mostly of them nitrogen. Unlike many natural pigment colours, this Rayleigh blue carries no hint of purple and demands that the artist chooses a colour that provides this hue.

As we traverse to the top right of the picture, the blue deepens to a fundamentally different hue that perfectly justifies the use of ultramarine pigment made from ground and purified lapis lazuli. This much prized and very costly material was used by Titian to such effect in his Bacchus & Ariadne (1520-3) with the starry sky in the top left of his painting giving the hint that scene is lit by twilight. Titian learned, from his pigment advisor Cennino Cennini, that: "Its hue is that which marks the transition from dusk to night, with a purple tint to enhance its majesty."

The remarkable property of this photograph is that it so beautifully illustrates the transition between these two blues that are entirely different in their physical origin. The "Blue Hour" ("l'heure bleue"), so favoured by painters around the turn of the 19th – 20th centuries, is now known (Hulburt, 1953) to be the result of the absorption of horizon-grazing sunlight by ozone in a layer between about 12 and 40 km altitude. With its long tangential passage through the atmosphere, much of the yellow-orange region of the solar spectrum is removed by the 'Chappuis' absorption band that results in the continuous destruction of the ozone gas by causing this rather unstable molecule to furiously vibrate and self-destruct.

Fortunately for us, the sunlight also stimulates the formation of new ozone at almost exactly the same rate. If it did not, life on the surface of the Earth would shrivel under a rain of deadly ultraviolet photons. We live under a fragile shield of ozone gas that we nearly destroyed last century with leaking refigerators and air-conditioning systems.

It is not widely appreciated that that atmosphere provides us with the delight of two entirely different blues. One of the reasons that we are not so aware of this is that our vision changes dramatically during the passage of twilight. As the sky dims at sunset (or brightens at sunrise), we switch from our 'cone' photoreceptors, whose (usually) three types give us daytime colour vision, to our more sensitive 'rods' that are tuned to blue/green light but give us no colour perception. The remarkable blue of deep twilight when the Sun is below the horizon is well captured by modern digital cameras that are both sensitive and can be made to control their colour balance (white balance) to avoid automatic colour-correction that would unwittingly compensate for the real ozone-blue.

One of the most remarkable animal visual adaptations to the extreme blue of twilight is seen in Arctic reindeer (caribou) that live and feed for about 10 hours a day in the twilight conditions of winter. These animals seasonally and reversibly change the reflected colour from the mirror behind their eyes (the tapetum lucidum) to match the spectrum of the winter sky (Fosbury and Jeffery, 2022).

The Chappuis band of ozone is the strongest indicator of oxygen that we may see in the visible spectrum of transiting exo-planets as an indicator of the presence of oxygen and the possibility of life.

This activity on the sky is reflected in the snow-dusted landscape with the lighter patches reflecting the brightening eastern horizon while the apparently shaded regions reflect the deep, metallic ozone blue of the western sky and the Earth shadow. The fresh snow has a very high albedo due to the efficient Lambertian scattering of light from myriads of tiny ice crystals.

This whole photograph is suffused with a mixture of colours derived from the two dominant physical mechanisms that paint the sky: scattering and molecular absorption.

References

Hulburt EO. 1953 Explanation of the brightness and color of the sky, particularly the twilight sky. J. Opt. Soc. Am. 43, 113–118. (doi:10.1364/JOSA.43. 000113) opg.optica.org/josa/viewmedia.cfm?uri=josa-43-2-113&s... (unfortunately not open access, but see the book: "Why the Sky is Blue" by Götz Hoeppe, Princeton University Press, English translation 2007, for an excellent explanation of the nature of ozone absorption in the atmosphere.)

Fosbury Robert A. E. and Jeffery Glen 2022 Reindeer eyes seasonally adapt to ozone-blue Arctic twilight by tuning a photonic tapetum lucidum Proc. R. Soc. B.2892022100220221002
doi.org/10.1098/rspb.2022.1002

Appendix: Image and location geometry

The image was exposed at 2023-01-20 07:57 UT at a location in Dumfries & Galloway called Cairngarroch.

Lat: 55° 04' 12" N, Long: 4° 21' 54" W at an altitude of 548 m

The broad, shallow peak visible at the left side of the image at a bearing (cw from N) of around 210°, consists of a high point (Millfore, 656m) at 2.6km with a 2 km ridge dropping slowly to the southwest for a further 2 km. The photo covers an azimuth angle of 74° from approximately S to WSW.

Using the spreadsheets available from gml.noaa.gov/grad/solcalc/calcdetails.html, we can calculate the solar position at the time of the photograph. The solar elevation (at sea level) was then -4.5° and the azimuth was 119° which results in an irradiance on a horizontal surface about 1/15 of that at sunrise.

At the camera altitude above sea level of 550m, the zenith distance of the Sun at sunrise can be calculated using the the approximate formula given in the Explanatory Supplement to the Astronomical Ephemeris, HM Stationery Office (1961), p401:

zd_Sun = 90° 50' + 2.08 * √h(m)

= 91° 39'

where h is the altitude of the camera in metres. Using the NOAA tables above, this indicates the time of sunrise with a clear horizon of 08:23 UT, ie. 26 min after the photo was taken, in the direction of Castle Douglas about 31 km away.

Taken on: August 18, 2017

Tags:

sky

twilight

Rayleigh

scattering

ozone

absorption

reindeer

vision

tapetum lucidum

adaptation

photonic-crystal

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winter

snow

reflection

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Camera: Sony DSC-RX100M4, f/8.0, 1/80 sec, 24.5mm, ISO 125 (more info) (hide info)

Camera:	Sony DSC-RX100M4
Lens:	24-70mm F1.8-2.8
Aperture:	f/8.0
Exposure Time:	0.013 sec (1/80)
Focal Length:	24.5mm
ISO:	125
Exposure Bias:	-2 EV
Flash:	Off, Did not fire
Exposure Program:	Aperture-priority AE
Exposure Mode:	Auto
Metering Mode:	Multi-segment
White Balance:	Auto
Color Space:	sRGB
Software:	Adobe DNG Converter 9.9 (Macintosh)
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