

Why is the Sky Blue? No, Seriously - acqq
http://news.discovery.com/earth/why-is-the-sky-blue-no-seriously.html

======
pmjordan
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>

~~~
drostie
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>

~~~
klodolph
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
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.

------
btilly
If you could go back in a time machine, I confidently predict that, despite
being orange, it would still have looked relatively blue to you.

Why? Our eyes are very, very good at adjusting to ambient light and factoring
it out. Thus things still look to us like they have the same colors when
viewed under natural light, or when viewed under incandescent lighting (which
is far more red). Ditto if you wear colored glasses or goggles for a bit. Even
though the ambient light would have been orange, Rayleigh scattering says that
the sky would still have been blue relative to the ambient light, so would
look blue.

The exception is if the sky actually reflected light from the ground or
blocked it out. Then it would have looked white or dark, and the natural color
would have become evident. (This happens with clouds, and is why a cloudy sky
does not look so blue to us.)

~~~
pmjordan
Interesting way of looking at it (as it were), though in principle if there
was some other scattering effect specific to methane, I don't see why this
couldn't dominate the effects of Rayleigh scattering. (as I've said in the
other comments, I'm not convinced that such an effect actually exists)

------
stanmancan
I love reading articles like this. They've done a great job at simplifying a
bunch of complex information into a short, easy to read post. I know it's
certainly peaked my interest in a couple different subjects I'll be searching
for on Wikipedia later today.

------
gtani
from Clifford Stoll's Cucckoo's Egg (PhD defense)

[http://books.google.com/books?id=9B1RfCAar2cC&pg=PA339&#...</a>

