The summer goes, the autumn comes – this sometimes earlier, sometimes later. For many, Indian summer or golden October are among the favorite seasons. The changing landscape and an often special light mood contribute to this. But what makes autumn "golden"?
Autumn – Time of change!
At first it is still late summer, but the days quickly become shorter. The deciduous trees and larches gradually begin to change color, and in late autumn they finally stand bare. In between, assuming the appropriate high-pressure weather conditions, there is a phase with various names: Indian summer, in North America Indian summer, golden October and later in November sometimes Martini summer. These names are associated with special moods and special light.
The sun does it
And indeed, the light also changes. The position of the sun sinks, it is lower and lower in the sky even at midday. As a result, the sun's rays travel a longer distance through the atmosphere. The light is thereby scattered by air particles – the so-called Rayleigh sc attering (scattering by particles that are small compared to the wavelength). Although sunlight appears basically yellowish-white, it actually consists of a whole spectrum of colors – that becomes visible in a rainbow. The different colors of light have different wavelengths. At the short end of the visible spectrum is the color blue, and at the long wavelength is the color red. Now, Rayleigh scattering has a much stronger effect on the short-wave blue. On its way through the air layer, this part of the light is increasingly scattered out. At the observer, the longer-wavelength yellow and red components then predominate. This is the same effect as during a sunrise or sunset. The longer the path through the atmosphere, the more the light color shifts to the reddish – it becomes "more golden".
Fig. 1: Autumn sunset; Source: pixabay
Longer twilight, longer shadows
In addition to the lower position of the sun, its path also becomes flatter, thus lengthening the twilight phases. In June, for example, the sun rapidly rises higher at a steep angle, and dawn is comparatively short (reversed in the evening). In winter, this part of the day lasts much longer. So, in nice autumn weather, times actually have more of the twilight and thus longer periods of flat light shifted to reddish-yellow. In addition, compared to summer, the shadows also become longer during the day, the contrast between brightly lit and dark areas increases. And this effect is then reinforced by another development in autumn – namely the foliage coloration!
The green of the leaves absorbs a relatively large amount of light, which is crucial for photosynthesis. A green forest appears visually darker. With the yellow and red tones, the trees begin to glow more and then contrast with the shaded areas.
Fig. 2: Sycamore maple in autumn; Source: pixabay
With autumn high pressure conditions, the air is often relatively dry and mild at high elevations, but at lower elevations, fog becomes more of an issue. As the angle of insolation flattens, the energy input also declines, and thermal activity decreases. In autumn, therefore, fewer source clouds form in sunny weather, often only high cloud fields appear in an otherwise blue sky. These do not shade the sun, but only change the mood of the light. All these factors have more or less share and give a beautiful autumn day its own magic!
The content of this article has been at least partially computer translated from another language. Therefore, grammatical errors or inaccuracies are possible. Please note that the original language version of the article should be considered authoritative.