# Where does the sky's blueness come from; at what altitudes is it being produced?

Rayleigh scattering (mostly) results in a blue sky (Diffuse_sky_radiation) as seen from Earth's surface. Go up in a plane to cruise altitude and the sky gets noticeably blue-er, and then darker.

The scale height of the Earth's atmosphere is roughly 8 kilometers, which means that half of Earth's atmosphere's mass is below about 5.5 km, again roughly.

Question: Does half of a blue sky's blue light come from this lower 5.5 kilometers as well, or does the effect vary with density such that a disproportionate amount is coming from higher altitudes?

Origin of this question is from comments below @Ingolifs' answer to Puzzler: help understanding these amazing Curiosity eclipse GIFs

Let's look at a simulation I did using my CalcMySky software. Here's how the first scattering order luminance looks in equirectangular projection for an observer at $$1\,\mathrm m$$ altitude:
Now the same for $$5500\,\mathrm m$$ altitude:
For completeness, let's also see the same from $$25\,500\,\mathrm m$$:
So, you're basically right: $$5500\,\mathrm m$$ is roughly the altitude where first scattering contribution is the same from below and from above the observer.