The Physics of Sunsets

[Thorsten]

As you all might know from the fact that I wrote ALS, I've long been interested in light passing through the atmosphere, particularly in sunsets.

Doing some research on the topic, I've quickly discovered that many multiple scattering processes are simply too complex to include in a fast rendering framework, yet they lead to quite beautiful results in nature, so some of what I know ended up in ALS, some did in simplified form, some never made it.

Over the years, I've taken (literally) hundreds of sunset photographs - usually from the same location - and in the last years I've also started to film sunsets on video, and I've used that reference material to teach myself all the intricacies of what mechanisms actually drive what we see. Of course the basics are simple - Rayleigh scattering colors the light, Mie scattering creates a halo - but then? There's actually way more to it in the way indirect light plays out for instance, or in the way the view ray (as opposed to the illuminating ray) is affected by scattering processes.

In various conversations I've learned that many science people are simply not aware of some phenomena - for instance that Mie scattering works out in both directions - people recognize the silver lining phenomenon, but are oblivious to the fact that there is dark fringing if one looks the other way (there were a couple of 'bug reports' when that phenomenon was implemented in ALS). Or that Rayleigh scattering is rather generic - the blue sky of Earth is not a particular property of Earth's atmosphere, but driven by the fact that air molecules are small-sized scattering centers, and any colorless gas atmosphere would also appear blue once optically thin (a fact that the otherwise great solar system visualization Celestia got wrong again and again for other planets).

So I've now stared to organize the material that I have into a series of explanations of what we see in the hope that it helps more people to appreciate also the intricacies of what happens in the sky.

Here's a gallery of the vast differences in appearance sunsets can have - this is what needs to be explained:



Please enjoy the first parts of The Physics of Sunsets on my webpage!

[TheEagle]

Nice pictures ! I also have hundreds of photos of sunsets - is it worth uploading them somewhere or do you already have enough ?

[Thorsten]

Do you have a green sunset? Or noctilucent clouds? That I still lack, everything else I think I have covered.

[TheEagle]

Green sunset … no, I'd like to see that, too ! (IRL)
As for noctilucent clouds, I first have to g… what that is exactly … EDIT: I think I've seen those already here at home, but I didn't take any photos, unfortunately !

[www2]

I found some good video for green in the sunset (credits is not by me)

source: https://www.youtube.com/watch?v=lwus2nqU0SY

[Kugelfang]

The science is over my head but the pictures are beautiful. Thank you for sharing them.

--jeff

[GinGin]

Very interesting sharing, thanks Thorsten.
Watching Sun set/rise while being able to understand all the subtilities behind those breathtaking sceneries is really nice.

[Thorsten]

I've now finished the article with a discussion of a few exotic phenomena - among these the mentioned green sunsets and a few of the Halo effects in cold weather

[erik]

About green skies:
I wonder if an event could occur where sunlight is scattered like a prism and where the viewer could be right in the green-section of the light refraction.

Update:
The fact that both yellow skies and green skies can manifest themselves when thunderstorms occur could support the theory:
https://www.greenmatters.com/p/why-is-the-sky-yellow
https://indianexpress.com/article/trend ... m-8014678/

Erik

[wkitty42]

i've seen green clouds before... back in 1982 when i was in Illinois... bright sunny cloudless day... then on the western horizon we could see some line that stretched all the way across the horizon... as we watched, it got larger and larger... then we realized it was a cloud bank (aka weather front) rolling in and it was literally rolling like a barrel on the road...

never before had we seen clouds rolling on their side like that... and green... not like grass, emerald, or lime... more of a grey-green... as this front rolled over us the temperature dropped from a lovely shorts and tank tops 80F (26.6C) to like 50F (10C) in a matter of a couple of minutes and some were scrambling for jackets and long pants...

i don't remember what the solid cloud cover behind the initial rolling edge looked like but the green tint went away and it was all more grey and bubbly, i think, like mammatus clouds... the cooler temperatures stayed with us for several days... it was like we rolled from mid-summer into late-fall in the blink of an eye...

[Thorsten]

I wonder if an event could occur where sunlight is scattered like a prism and where the viewer could be right in the green-section of the light refraction.

It begs the question why you should see so large parts of the sky in green - having the viewer in the green section is comparatively easy, but all else that he sees? The rest of the sky would show prismatic colors in this case.

The striking F-14 picture for instance shows colors consistent with a dirty yellow moving into green - but clearly not a prism-like color spectrum.

So no, I don't really buy that explanation, it seems easier to start with bluish light and remove all the blue to end up green, at least for that I have found plenty of examples - albeit not very striking ones - but the base mechanism seems to be there.

[erik]

You seem to think of a prism close by. I was more thinking of a prism (ice-crystals/hail) far away (at the horizon) which means the light will be spread over a much larger area. Which means you won't see much difference at ground level versus higher up .

Erik

[Thorsten]

which means the light will be spread over a much larger area.

In this case it'd be impossible to see any blue sky (the light in the green part of the spectrum really only has green wavelengths, so it'd be impossible to get any other color from it by Rayleigh scattering). That is however not the case in the pictures I'm trying to understand - the F-14 one for instance shows a high blue sky and a yellow Sun (and a yellow reflection on the plane).

[erik]

I used the color picker tool on the image and actually what you are seeing is more blue to the top gradually changing to more yellow and red to the bottom. And that's pretty consistent for the whole sky. Which tends to lean towards prism-effect.

Erik

[Thorsten]

I used the color picker tool on the image

Read this appendix for why this isn't really a meaningful way to settle this question

And that's pretty consistent for the whole sky. Which tends to lean towards prism-effect

Prism colors are very pure. The colors in question are anything but. So no, I don't think the fact that the high sky is blue and low clouds are more red supports any particular theory, that is the normal state of affairs in the sky, Rayleigh scattering does that for you.

Also note that in a rainbow, red is actually up and blue is down - you somehow need to opposite and quickly gloss over that annoying fact.