DID YOU KNOW...
WHERE THE COLORS IN THE SKY COME FROM?

Recently, I had a visit from my cousin's family. His daughter is three years old and is currently in the well-known "Why?" phase. During our walk, it didn't take long before one of the absolute classics from this phase came up: "And why is the sky blue?" Good question, I thought, as I hadn’t given it much thought in a long time. I had a rough idea, but I wasn't completely sure. "Why is the car red?" came right after from the little one. So, an answer wasn’t immediately necessary for her. However, I wanted to know because just because we stop marveling at something after all these years doesn’t mean it can’t be exciting to find out the answer.

Let’s start with the physical explanation: When we look up at the sky, we are looking at the air, the Earth’s atmosphere. This atmosphere consists of various gas particles, such as oxygen and nitrogen. As sunlight enters this atmosphere, we see it as a bright whitish light because it is a mix of many different colors – we recognize these colorful hues from the rainbow. The different colors of light vary in their wavelengths.

When a ray of sunlight enters the Earth’s atmosphere, it does not travel through "nothing," as it would in a vacuum. Instead, during its journey, it encounters many different gas particles, causing the light to refract and slightly change direction. However, the angle of this change in direction depends on the wavelength of the refracted light.

of light. The blue part of the sunlight has a very short wavelength and is therefore deflected at a large angle – this is also referred to as being strongly scattered. In contrast, the red part of the light has a relatively long wavelength and is scattered less strongly.

What does this mean for the colors in the sky? When we have a cloudless sky and are under the bright midday sun, the sunlight strikes the Earth at a steep angle. The distance that the light has to travel to reach our eyes is relatively short. As we gaze up into the sky, we notice blue light rays coming from all directions because they are strongly scattered. In contrast, when we look at the sky just before sunset, the sun is close to the horizon, and the light has to travel a much longer distance before it reaches our eyes. Along this long path, the blue light is scattered in all directions, so only a small amount of it still reaches our eyes. However, the less deflected red light manages to reach our eyes. The same phenomenon occurs in the early morning, just from a different direction in the sky.

Mountains in the distance often appear bluish for two reasons: First, there is so much air between the mountain and the observer that the previously mentioned scattering effect comes into play. However, this effect is further enhanced in densely vegetated mountains by the natural UV protectants produced by the plants. To protect themselves from heat and sunlight, many plants emit the hydrocarbon isoprene. This isoprene vapor in the air also appears bluish when viewed over large areas, as impressively seen in the Blue Mountains in eastern Australia.

Additionally, I promised you a psychological answer. It goes something like this: The sky appears blue to us because we have a word for it.

Of course, we also have words for green and beige, yet we don't perceive the sky in those colors. What is meant here is that we typically categorize the sky, with its various shades depending on the weather throughout the day, as "blue" because we have grouped all these nuances under this color term in our language. Here are a few examples to make this clearer:  

The Russian language distinctly separates a word for "light blue" and a word for "dark blue." For native Russian speakers, these are not simply shades of the same color. Even we can differentiate between a dark blue and a light blue in extreme edge cases (otherwise, the RWTH logo would be poorly chosen). However, when it comes to the transitional areas between the two colors, experiments by Jonathan Winawer at MIT show that the Russian brain can assign colors to one of the words more quickly, allowing for a clearer distinction between them. Therefore, native Russian speakers likely perceive the sky not just as blue, but rather as dark blue, for example.  

But we can also look at it from the other direction: The indigenous Bolivian language Tsimané has fewer color words than German, roughly translating to "light," "dark," and "red." Not only do native speakers not refer to the sky as blue, but they also do not necessarily perceive it as distinctly bluish from the horizon. For the Tsimané, when there is a blue sky over a lush green meadow, studies show that this color spectacle appears to them more like a color with varying shades. This may be hard to imagine, but it becomes more understandable when we remember that we too perceive only different shades while native Russian speakers can clearly distinguish between them. So, one might envision this sight from the Tsimané perspective as somewhat akin to viewing a blue sky over a blue sea.

Who would have thought that so much lies behind the seemingly simple question of a three-year-old? After all, we now know the answer. And the next time she asks me why the sky is blue, I’ll just hand her this printout and hope that she can read by then...