Is the sky blue or red

Why is the sky blue?

If the sky is in its most beautiful blue when the sun is shining, it is because the sun's light is deflected by the smallest particles in our atmosphere. To put it simply, sunlight consists of many individual rays of light that travel like a wave. When we see all of these rays of light at once, the light appears white to us. As soon as the light hits a prism or raindrops at a certain angle, we can see the individual rays of light, the so-called spectral colors: for example in a rainbow made up of the colors red, orange, yellow, green, blue and Violet consists. The light of each of these spectral colors has a different wavelength, i.e. a different distance between two successive wave crests. Red light, for example, has a long wavelength, while blue light has a very short one.

The blue sky during the day

When the sun is high in the sky, the path that light has to travel through the earth's atmosphere is relatively short. The earth's atmosphere consists largely of the molecules nitrogen and oxygen. The rays of light hit these small particles in the air and are deflected by them, or more precisely: they are scattered. The short-wave blue light is scattered more strongly by the air molecules than the long-wave red light. Because it is mainly blue light that is reflected by the smallest air particles, the cloudless, clear sky appears blue to us.

Rayleigh scattering gives the sky its color

This physical principle was discovered in the 19th century by the Englishman John William Strutt, third Baron Rayleigh. In his honor, the phenomenon that gives us the blue sky is called Rayleigh scattering. It not only describes in concrete terms what happens when sunlight is scattered by air molecules, but also applies in general to all types of electromagnetic radiation and to all particles that are smaller than the wavelength of this radiation. The power with which these particles emit the absorbed radiation is inversely proportional to the fourth power of the wavelength of this radiation: the smaller the wavelength of the radiation, the more it is scattered.

Red at dusk, gray in bad weather

In the evening or in the morning, however, the conditions are completely different: At dawn or sunset, the sun is low in the sky, and the path that the light has to cover before it reaches the viewer is much longer. The short-wave blue light is then intercepted by the molecules after a short distance, and we only get the long-wave red part.

But as we know, the sky does not always show us its most beautiful colors, but can also look gray and cloudy. This is always a sign of dusty or humid air - not to mention continuous cloud cover. The sunlight is not even split into its spectral colors on the relatively large dust and water particles, but is reflected directly, like on a mirror - and makes the sky appear white-gray.

The skies of other planets

But what about other planets? Is the sky blue or red too? That depends entirely on the atmosphere of the heavenly body - more precisely on its density and its composition. On the moon, for example, there is no atmosphere at all - the sky is always black there and the light from the sun hits the surface unhindered, so it appears bright white. Mars, on the other hand, has a very thin atmosphere, which consists mainly of carbon dioxide and small rust particles (iron (III) oxide). The NASA probes that explore Mars therefore always look into a yellow-red sky. The atmosphere on Venus also consists mainly of carbon dioxide, but it is very dense. In addition, it is surrounded by a 20-kilometer-thick layer of cloud, most of which consists of sulfuric acid. It is difficult for sunlight to penetrate these thick layers. However, color images from Soviet Venus probes suggest that the sky looks yellow-orange when viewed from the surface of Venus.