Imagine yourself in a spot at sea level on a clear, cloudless day. If you look up you can observe the sky with its characteristic blue color. Now imagine yourself at the same place but aboard a balloon of helium gas with the capacity to ascend to 38,500 m altitude.
During this journey you are observing the sky, which is getting darker and darker. The higher you are, the lower is the concentration of the molecules around you, the atmosphere becomes thinner.
The blue color of the sky is the result of the interaction of light with the molecules present in our atmosphere, this variation in concentration has a direct effect on the visible light spectrum. Thus blue becomes darker as this concentration of molecules decreases.
When you finally reach 38,500 m you look up again. Now the sky, once blue, is totally black.
Between 0 m and 38,500 m there are infinite possibilities for the existing blue tones.
The recording of these tones is the starting point of this study.