<p>I take your question as the question about <em>first</em> 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.</p>
<p>Let's look at a simulation I did using my <a href="https://github.com/10110111/CalcMySky" rel="nofollow noreferrer">CalcMySky</a> software. Here's how the first scattering order luminance looks in equirectangular projection for an observer at <span class="math-container">$1\,\mathrm m$</span> altitude:</p>
<p><a href="https://i.sstatic.net/rLIIY.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/rLIIY.png" alt="" /></a></p>
<p>Now the same for <span class="math-container">$5500\,\mathrm m$</span> altitude:</p>
<p><a href="https://i.sstatic.net/b7VNr.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/b7VNr.png" alt="" /></a></p>
<p>For completeness, let's also see the same from <span class="math-container">$25\,500\,\mathrm m$</span>:</p>
<p><a href="https://i.sstatic.net/3rbcy.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/3rbcy.png" alt="" /></a></p>
<p>So, you're basically right: <span class="math-container">$5500\,\mathrm m$</span> is roughly the altitude where first scattering contribution is the same from below and from above the observer.</p>

CC BY-SA 4.0
2021-08-01T23:26:48.573Z
Oh this is very cool, thanks! Additional links to Eric Bruneton, Fabrice Neyret
 (2008) *Precomputed Atmospheric Scattering*  [1](https://hal.inria.fr/inria-00288758/en), [2](https://hal.archives-ouvertes.fr/inria-00288758), [3](https://onlinelibrary.wiley.com/doi/10.1111/j.1467-8659.2008.01245.x) also potentially helpful [A Qualitative and Quantitative Evaluation of 8 Clear Sky Models](https://arxiv.org/abs/1612.04336) I was going to mention an Astronomy SE question but realized that [you'd already answered it](https://astronomy.stackexchange.com/a/44921/7982)! :-)
uhoh
CC BY-SA 4.0
2021-08-01T23:28:42.877Z
Space SE's [Why does Unity look transparent?](https://space.stackexchange.com/q/54083/12102) was down voted because of title but I think you can use results here to explain what's happening there as well.
uhoh
CC BY-SA 4.0
2021-08-01T23:35:00.687Z
Then was going to mention another Astronomy SE question but I see [you'd already answered it also](https://astronomy.stackexchange.com/a/43814/7982)!
uhoh
CC BY-SA 4.0
2021-08-01T23:10:57.040Z
2021-08-01T23:10:57.040Z
Ruslan
<p><a href="https://en.wikipedia.org/wiki/Rayleigh_scattering" rel="nofollow noreferrer">Rayleigh scattering</a> (mostly) results in a blue sky (<a href="https://en.wikipedia.org/wiki/Diffuse_sky_radiation" rel="nofollow noreferrer">Diffuse_sky_radiation</a>) as seen from Earth's surface. Go up in a plane to cruise altitude and the sky gets noticeably blue-er, and then darker.</p>

<p>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.</p>

<p><strong>Question:</strong> 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?</p>

<p>Origin of this question is from comments below <a href="https://space.stackexchange.com/a/35306/12102">@Ingolifs' answer</a> to <em>Puzzler: help understanding these amazing Curiosity eclipse GIFs</em></p>

<atmosphere><atmospheric-optics>
Where does the sky's blueness come from; at what altitudes is it being produced?
uhoh
# Where does the sky's blueness come from; at what altitudes is it being produced?

(Asked by: username_0 on 2019-04-05)

[Rayleigh scattering](https://en.wikipedia.org/wiki/Rayleigh_scattering) (mostly) results in a blue sky ([Diffuse\_sky\_radiation](https://en.wikipedia.org/wiki/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](https://space.stackexchange.com/a/35306/12102) to *Puzzler: help understanding these amazing Curiosity eclipse GIFs*

**Tags:** atmosphere, atmospheric-optics


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## 1 answers

### Answer by username_1 on 2021-08-01. Score: 4 | Accepted answer

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](https://github.com/10110111/CalcMySky) software. Here's how the first scattering order luminance looks in equirectangular projection for an observer at $1\,\mathrm m$ altitude:

[![](https://i.sstatic.net/rLIIY.png)](https://i.sstatic.net/rLIIY.png)

Now the same for $5500\,\mathrm m$ altitude:

[![](https://i.sstatic.net/b7VNr.png)](https://i.sstatic.net/b7VNr.png)

For completeness, let's also see the same from $25\,500\,\mathrm m$:

[![](https://i.sstatic.net/3rbcy.png)](https://i.sstatic.net/3rbcy.png)

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.

> **Comments:**
> **username_0**: Oh this is very cool, thanks! Additional links to Eric Bruneton, Fabrice Neyret  (2008) *Precomputed Atmospheric Scattering*  [1](https://hal.inria.fr/inria-00288758/en), [2](https://hal.archives-ouvertes.fr/inria-00288758), [3](https://onlinelibrary.wiley.com/doi/10.1111/j.1467-8659.2008.01245.x) also potentially helpful [A Qualitative and Quantitative Evaluation of 8 Clear Sky Models](https://arxiv.org/abs/1612.04336) I was going to mention an Astronomy SE question but realized that [you'd already answered it](https://astronomy.stackexchange.com/a/44921/7982)! :-)  
> **username_0**: Space SE's [Why does Unity look transparent?](https://space.stackexchange.com/q/54083/12102) was down voted because of title but I think you can use results here to explain what's happening there as well.  
> **username_0**: Then was going to mention another Astronomy SE question but I see [you'd already answered it also](https://astronomy.stackexchange.com/a/43814/7982)!  

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