What is the secret behind the sky

How do sky blue and sunset arise?

The cloudless sky appears blue during the day, but orange to red in the morning or evening. The secret behind this color duo lies in the way in which the sunlight is scattered in the atmosphere.

Sky blue

The sky palette has a lot to offer: Blue, white and gray, yellow, red and orange alternate and if you add the rainbows, the joy of colors seems limitless. The cloudless sky is dominated by a blue during the day, although behind it there is the blackness of space. On beautiful evenings, the sky turns a breathtaking red, although the sun shines in the same color as during the day. The secret behind this variety of colors lies in the way in which the sunlight is scattered in the atmosphere.

Scattered light

The light of the sun appears yellowish-white to us, but it is made up of all the colors of the rainbow - from violet to blue, green, yellow, orange to red. Each of these colors corresponds to electromagnetic radiation of a certain wavelength. This wavelength is shortest for blue and longest for red.

The different wavelengths now play an important role when the light collides with gas molecules on its way through the atmosphere and changes its direction in the process. Physicists say: the light is scattered. Whenever we are not looking directly into the sun, we only see scattered light that reached our eyes via a few detours from the sun. Hence it is precisely the scattered light that determines the color of the sky.


The smaller its wavelength, the more light is scattered in the atmosphere. Blue light is therefore more strongly scattered than red.

When the sun is high, the path of sunlight through the atmosphere is quite short, mainly blue is scattered, so that the sky appears blue to us during the day. When the sun is low, the path of light through the atmosphere is much longer. The scattering reduces the blue component so much that the red gains the upper hand. The blue was scattered away. Therefore, the cloud-free sky is blue during the day and red at sunrise (dawn) and sunset (sunset).

Rayleigh scattering

In the case of light scattering in the atmosphere, one speaks of Rayleigh scattering, named after the British physicist Lord Rayleigh (1842-1919). Rayleigh scattering occurs whenever radiation is scattered by particles that are much smaller than the wavelength of the radiation. This is the case with the gas molecules in the atmosphere. The water droplets or ice crystals in clouds, on the other hand, are far larger than the wavelength of light. There is no Rayleigh scattering here, rather all wavelengths of sunlight are reflected, so that clouds appear white to us.

Incidentally, the Rayleigh scattering must also be taken into account when evaluating satellite measurement data if, for example, ocean waves or cloud thicknesses are measured with them.