Uneven Scattering of Light to (almost) a Single Color – The Rayleigh Scattering and the Sky-blue

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Looking into the sky, we see more scattered parts of the short blue spectrum than of the long wavelength red spectrum, and therefore the sphere above our head appears blue.

Without this second form of light scattering, the sky above us would be pale and wan, no matter how brilliant the day. The Briton John William Strutt (later the 3rd Lord Rayleigh and Nobel Prize winner for his studies of atmospheric gases) discovered where the blue comes from during his research on light scattering by particles in the late 19th century. His fundamental work on the mathematics of these processes provided proof of our preceding thesis that it takes particles whose diameter is smaller than the wavelength of light to scatter it symmetrically. In his search for such particles, he found them in the air molecules, from whose size he could deduce a direct relation to the spectrum.

From his calculations, it follows that the possibility of scattering a photon by an air molecule is inversely proportional to the fourth power of the wavelength. But only mathematicians understand this. Translated for normal people, it means that light of shorter wavelengths is scattered more strongly than light of longer wavelengths; the blue part of the spectrum with a wavelength of 450 nm about 3.2 times more strongly than the red part with 600 nm. This is what we call Rayleigh scattering for short.

The second, closer look, however, reveals a sky color that is by no means uniform in hue, saturation, and brightness. On the contrary, the number of shades is almost infinite and changes constantly during the day. The explanation for this takes some getting used to, because the color of the sky does not depend on distance but on the number of air molecules in the line of sight: the more molecules there are, the brighter the sky, because more light is scattered by more molecules.

With the air mass (LM), science also provides us with a measure of the number of molecules. 1 LM represents the volume of air in a vertical viewing direction above an observer located at sea level. Practically expressed 1 kg of air per cm². With larger angles, the air mass increases; at sea level, for example, the light passes through about 38 air masses up to the horizon. The large quantity of air molecules in this thick layer not only scatters the short wavelengths of the incident light once or twice, but gradually spreads all ranges of the spectrum several times. And since very little is lost through absorption, the horizon is of the same color as the midday sun – white. Figure 19 shows this typical gradient from white over different shades of blue to the dark area around the zenith. However, we do not always perceive the white because the reflections of the landscape below can overlay it. The water level of the ocean, for example, darkens it, and the pastoral green of wide agricultural areas gives it just as much of a tint.

But the reverse is also true, because at an altitude of 3000, 4000 or 5000 m the sky is a much darker blue than in the plain because the air layer becomes thinner and thinner and the number of molecules suitable for scattering becomes smaller and smaller. Photographs captured from space even depict a nearly black sky, as there are no more air molecules available for scattering. Filters cannot correct this lack of scattering, which makes the dark blue high mountain sky unmanageable from a photographic perspective. At extreme altitudes, the interaction between the relatively dark sky and the heavily sunlit surroundings can potentially cause issues with the exposure range. Ultimately, the exposure is determined by the same amount of sunlight as it is in the lowlands. In these situations, it’s necessary to apply a graduated gray filter on the recording side to somewhat suppress the foreground. But post-exposure and dodging, or their digital equivalents, can also help soften the contrast during image processing at home. Notwithstanding all this, it is of course always important to eliminate the high UV component in the high mountains.

So the increasing scattering with the air mass thickness explains that the day sky is much brighter at the horizon than in the vertical line above our head. However, for us photographers, the sky blue holds significance in a different context. Sky blue, also referred to as airlight, significantly complicates our work, particularly when the sun is at its highest point. Since the air molecules responsible for scattering the sunlight are not only above our heads but also between us and diagonally or horizontally distant objects, they naturally do their work there as well. How effective they are at it depends again on the thickness of the air layer in relation to the object distance. And just as the sky is blue, real airlight is also bluish, almost ethereal blue.

In relation to a mountain range located at a medium distance, this explains its pronounced blue coloration and above-average brightness, especially at midday – the further away, the more light is scattered and the brighter the object. If the mountain lies at a very great distance, the atmosphere can become so cloudy and opaque due to the multiple scattering of light that the light reflected from it is scattered and replaced by airlight – the mountain becomes invisible due to the resulting very low contrast. Thus, there are physical limits to distance visibility. However, with the shadow areas tending to blue in a forest, for example, the airlight also makes itself felt disturbingly at close range.

Airlight is particularly visible between a number of contour lines behind each other. During twilight, it appears in their non-overlapping areas as a thin, bright veil. The color change from blue to almost white is simulated by our visual system because the airlight has the greatest brightness in the case described, which is why it is assigned the value white.

Our photographic antidotes, which are effective in moderation, include an effective UV filter and a warm tone filter such as the KR-6 to compensate for the excess blue. But it is much better to wait for the lower position of the sun in the morning or afternoon. It reduces the airlight permanently because the sun then lands a smaller part of the short-wave blue spectrum suitable for scattering, as the following section explains. Therefore, the higher contrast resulting from the lower scattering leads us to perceive sharper vision during these moments.

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