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So the article doesn't actually provide the answer to the question, which is Rayleigh scattering [1]. Higher energy (shorter wavelength - blue) photons are more

So the article doesn't actually provide the answer to the question, which is Rayleigh scattering [1]. Higher energy (shorter wavelength - blue) photons are more likely to be scattered (reflected in a random direction) by molecules in the air than lower energy ones (longer wavelength - red), so the further it travels through the atmosphere, the more blue is removed from direct sunlight. The blue component is scattered in all directions, which means the sun's disc appears yellow, and the rest of the sky appears blue. The methane/orange sky theory is news to me, does anyone have any ideas what might cause this? Rayleigh scattering's high-energy bias is independent of the type of gas. Methane itself is colourless, so I suspect sunlight must be causing either ionisation or a chemical reaction which splits the molecule, with the reverse reaction emitting orange light? Or am I way off? [1] http://en.wikipedia.org/wiki/Rayleigh_scattering http://en.wikipedia.org/wiki/Rayleigh_scattering

drostie 15y ago
It's worth telling the folks roughly why Rayleigh scattering is independent of the type of gas. ^_^ I'm only a physics grad student; but let me try. Okay, so basically, atoms are really small. They're so small that they're smaller than light -- light has a size over which it likes to interact and interfere called its wavelength, and thousands of atoms can fit side-by-side across the visible wavelengths. (Smaller light also has a little bit more kick to it per photon, which is why we physicists tend to think that cell phones cannot cause cancer.) The smaller light is closer to the size of the atoms that it's scattering over and that's principally why it scatters more strongly. It's what we call elastic scattering; the light doesn't lose any energy and therefore keeps the same color, rather than becoming redder. So the light bounces off of the air without kicking it. One curious fact that you might think about, which bugs me a little bit, is that the color of the sky is not isotropic. It really depends what direction you're looking. Imagine that you're facing the setting sun, and you look up, and the sky's still actually pretty blue, but over "next to the Sun" the sky is pretty orange. But now teleport yourself to the distant fields that this sky is right above, and you will be jumping closer to the Sun -- so the Sun will actually be higher, there will be less red in your sky, you look up and the sky also seems quite blue to you. Step back and look at the globe rotating from space, half black, half in light -- as sunset comes, you're rotating away from the Sun, into the black. The only isotropic way to get a red sky around the Sun is to have the horizon be a sort of red band of sky, but that would have to "wash over" the sky as we crossed over it. I mean, I'm sure that if you looked from space the objects would seem redder, because the Sun gets all coppery as it sets, but the color of a given box of air in our atmosphere is actually as far as I can tell different in different directions, and nobody ever seems to want to calculate this. Point being, it's not like a blue wall, where the wall is glowing blue in all directions. The same sky glows in different colors in different directions. As for your specific question, if you're right that methane is colorless (i.e. that it has no absorption frequencies in the visible spectrum) then that's certainly strange, yes. Are you sure it's not colorless in the sense that air is colorless -- that you need to look through some kilometers of air before you see some tiny-in-probability scattering event? Some searching on DuckDuckGo reveals that there are a couple of different effects; one is that methane might form smoggy cloudy effects as on Titan and Venus which are not colorless; but that without those effects methane does indeed weakly absorb red. They claim that this might cause the sky to become blue-green under our sort of illumination: http://boards.straightdope.com/sdmb/showthread.php?t=534194 http://boards.straightdope.com/sdmb/showthread.php?t=534194
- klodolph 15y ago
  And if you need more proof that the color is not isotropic, try looking to other wavelengths. If you look through a near IR filter, you'll see a bright sky near the sun and a very dark sky far away from the sun. (Okay, so technically you'll see black everywhere because the human eye is so insensitive to IR.) This is a major reason why IR photographs are so dramatic looking -- the isotropic sky allows photographers to create a black sky with white clouds in full daylight, and foliage is also fairly bright to IR.
  - pmjordan 15y ago
    That gels pretty well with the general observation that the sky becomes "bluer" at angles further away from the sun. IR scatters even less than visible red, so you'd expect its angular distribution to be more strongly skewed as well. I'm having a hard time trying to qualitatively estimate the angular distribution of perceived spectrum at a particular location, as opposed to the distribution of the angles of scattering events. I may have to try simulating this when I have some time.
- pmjordan 15y ago
  I agree that the function of spectral intensity depending on position and direction would be interesting. It's clearly non-trivial to calculate: it's a 5-dimensional field for any given wavelength, so the whole thing is a 6-dimensional function. I wonder if it could be represented and simulated in terms of a finite element system. Otherwise there's always the myriad ways of solving the rendering equation. :-) I'd love to know what physical effect causes this red haze. As you say, if methane weakly absorbs red light, we should be seeing green/blue, not red. A cursory search of mine also revealed the Titan example, but I couldn't find anything terribly specific linking it to methane in particular. (Titan appears orange, methane is present in its atmosphere; it's not clear to me if and how the two are connected)
lutusp 15y ago
> The methane/orange sky theory is news to me, does anyone have any ideas what might cause this? The article could have mentioned that Saturn's moon Titan has (a) an atmosphere containing some methane, and (b) an orange appearance. But (b) is thought to be the result of organics in the atmosphere, not so much the methane (http://en.wikipedia.org/wiki/Titan_(moon) http://en.wikipedia.org/wiki/Titan_(moon)): "The atmosphere of Titan is largely composed of nitrogen; minor components lead to the formation of methane and ethane clouds and nitrogen-rich organic smog." So it seems the orange color results from ordinary preferential reflection of certain wavelengths from smog particles (as in many U.S. cities), not Rayleigh scattering or another similar effect as in earth's atmosphere. As to the early earth's atmospheric color, I think that's a stretch, depending on things we just don't know. Also, the Wikipedia article's use of "organic" should be understood to mean "containing carbon compounds", not necessarily organic in a broader sense.

drostie 15y ago

It's worth telling the folks roughly why Rayleigh scattering is independent of the type of gas. ^_^ I'm only a physics grad student; but let me try. Okay, so basically, atoms are really small. They're so small that they're smaller than light -- light has a size over which it likes to interact and interfere called its wavelength, and thousands of atoms can fit side-by-side across the visible wavelengths. (Smaller light also has a little bit more kick to it per photon, which is why we physicists tend to think that cell phones cannot cause cancer.) The smaller light is closer to the size of the atoms that it's scattering over and that's principally why it scatters more strongly. It's what we call elastic scattering; the light doesn't lose any energy and therefore keeps the same color, rather than becoming redder. So the light bounces off of the air without kicking it. One curious fact that you might think about, which bugs me a little bit, is that the color of the sky is not isotropic. It really depends what direction you're looking. Imagine that you're facing the setting sun, and you look up, and the sky's still actually pretty blue, but over "next to the Sun" the sky is pretty orange. But now teleport yourself to the distant fields that this sky is right above, and you will be jumping closer to the Sun -- so the Sun will actually be higher, there will be less red in your sky, you look up and the sky also seems quite blue to you. Step back and look at the globe rotating from space, half black, half in light -- as sunset comes, you're rotating away from the Sun, into the black. The only isotropic way to get a red sky around the Sun is to have the horizon be a sort of red band of sky, but that would have to "wash over" the sky as we crossed over it. I mean, I'm sure that if you looked from space the objects would seem redder, because the Sun gets all coppery as it sets, but the color of a given box of air in our atmosphere is actually as far as I can tell different in different directions, and nobody ever seems to want to calculate this. Point being, it's not like a blue wall, where the wall is glowing blue in all directions. The same sky glows in different colors in different directions. As for your specific question, if you're right that methane is colorless (i.e. that it has no absorption frequencies in the visible spectrum) then that's certainly strange, yes. Are you sure it's not colorless in the sense that air is colorless -- that you need to look through some kilometers of air before you see some tiny-in-probability scattering event? Some searching on DuckDuckGo reveals that there are a couple of different effects; one is that methane might form smoggy cloudy effects as on Titan and Venus which are not colorless; but that without those effects methane does indeed weakly absorb red. They claim that this might cause the sky to become blue-green under our sort of illumination: http://boards.straightdope.com/sdmb/showthread.php?t=534194 http://boards.straightdope.com/sdmb/showthread.php?t=534194

klodolph 15y ago
And if you need more proof that the color is not isotropic, try looking to other wavelengths. If you look through a near IR filter, you'll see a bright sky near the sun and a very dark sky far away from the sun. (Okay, so technically you'll see black everywhere because the human eye is so insensitive to IR.) This is a major reason why IR photographs are so dramatic looking -- the isotropic sky allows photographers to create a black sky with white clouds in full daylight, and foliage is also fairly bright to IR.
- pmjordan 15y ago
  That gels pretty well with the general observation that the sky becomes "bluer" at angles further away from the sun. IR scatters even less than visible red, so you'd expect its angular distribution to be more strongly skewed as well. I'm having a hard time trying to qualitatively estimate the angular distribution of perceived spectrum at a particular location, as opposed to the distribution of the angles of scattering events. I may have to try simulating this when I have some time.
pmjordan 15y ago
I agree that the function of spectral intensity depending on position and direction would be interesting. It's clearly non-trivial to calculate: it's a 5-dimensional field for any given wavelength, so the whole thing is a 6-dimensional function. I wonder if it could be represented and simulated in terms of a finite element system. Otherwise there's always the myriad ways of solving the rendering equation. :-) I'd love to know what physical effect causes this red haze. As you say, if methane weakly absorbs red light, we should be seeing green/blue, not red. A cursory search of mine also revealed the Titan example, but I couldn't find anything terribly specific linking it to methane in particular. (Titan appears orange, methane is present in its atmosphere; it's not clear to me if and how the two are connected)

lutusp 15y ago

> The methane/orange sky theory is news to me, does anyone have any ideas what might cause this? The article could have mentioned that Saturn's moon Titan has (a) an atmosphere containing some methane, and (b) an orange appearance. But (b) is thought to be the result of organics in the atmosphere, not so much the methane (http://en.wikipedia.org/wiki/Titan_(moon) http://en.wikipedia.org/wiki/Titan_(moon)): "The atmosphere of Titan is largely composed of nitrogen; minor components lead to the formation of methane and ethane clouds and nitrogen-rich organic smog." So it seems the orange color results from ordinary preferential reflection of certain wavelengths from smog particles (as in many U.S. cities), not Rayleigh scattering or another similar effect as in earth's atmosphere. As to the early earth's atmospheric color, I think that's a stretch, depending on things we just don't know. Also, the Wikipedia article's use of "organic" should be understood to mean "containing carbon compounds", not necessarily organic in a broader sense.