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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

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I take your question as the question about first scattering event, ignoring all the subsequent ones (because they bounce light rays between the top and bottom halves of atmosphere). Also, I'm assuming daylight conditions, like 45° solar elevation.

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.

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