Special case low sun position – The combined scattering of light brings the strongest colors

You are here: Nature Science Photography – Natural light – Scattering phenomena in the atmosphere

Basically, the color change of the particularly low sun from white to yellow to orange and red is also caused by light scattering. Due to the sun’s low position near or just below the horizon, the light must travel a longer distance and pass through a greater number of air masses to reach the observer, compared to a position at the zenith. However, a long path always involves a greater risk than a short one. In this case, a large part of the short-wavelength blue spectrum runs the risk of being scattered by the air molecules. Added to this is the high absorption capacity of ozone (the energy conversion of certain wavelength ranges into thermal radiation), which is also effective in this wavelength range.

In addition to the molecules, the next largest components of the atmosphere, the particles, also play a significant role in this process. Wind and rising hot air bring dust, soil, sand, ash and sea salt into the higher layers of the air, which are also joined by a portion of water vapor. With a diameter of less than 100 nanometers (nm), the particles lie in the size relations between the molecules and the substances responsible for the Mie scattering, exert the greatest influence on the scattering of the short-wave blue-green spectrum, and shift the color of the sun a bit further towards yellow and red.

The fact that not every sunset turns out to be red is due to the fact that the coolness of the evening and the wind that often arises at that time clean the atmosphere a little and that during the daytime hours not always an equal number of haze particles make their way up the thermal elevator. On the other hand, the total amount of particles present, namely aerosols, also plays a major role. As previously mentioned, the artificial emissions from power plants and the material released by volcanic eruptions determine this in turn. The eruptions of Krakatau in 1883 and Pinatubo in 1991, for example, carried millions of tons of ash and other material into the atmosphere, which initially influenced the light scattering of the shorter wavelengths in the morning and evening in the direction of stronger, clearer reds and oranges. After a few months, however, the larger particles responsible for this had settled to earth, and the sulfuric acid molecules remaining in the atmosphere combined with water droplets to form aerosols. These powerful stratospheric layers now in turn favored Mie scattering, or uniform whiteness, and leveled the previously seen rich colors of sunrise and sunset for years to come and almost worldwide. Instead of shining in direct light, landscapes were now under diffuse, because of multiply scattered, illumination. – No more incentive to get up for the early morning alpenglow and sunrise. Under such conditions, it is more rewarding to concentrate on motifs that benefit from this even, flat light, such as macro studies of flowers and blossoms, meadows or the muted tones of a brook landscape, for example.

The horizon is the circle around any observer whose plane is perpendicular to the respective location and which seems to delimit the visible earth from the sky. The following viewing distances may serve as a guide: 27 km at 50 m altitude, 38 km at 100 m altitude and 120 km at 1000 m altitude.

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