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redwithpurpleflashes
2003-Jul-09, 10:06 PM
A co-worker said that a radio station had a trivia contest this morning (don't know which station). The question was, "What color is the sky?". The correct answer according to them was black. They also gave the wrong explanation as to why it appears blue. They said it is the reflection of the oceans (I gave her this web site for the correct reason).

Is the answer, "Black" correct? I say that on earth under normal/clear daylight conditions it's blue (let's ignore night, cloudy days, and sunset/sunrise). My thinking is that if it "appears" blue because of the scattering of light then you could safely say that it is blue. With their line of thinking, the oceans wouldn't be blue either.

I know there are examples that may throw a wrench in my theory, where colors are not as they appear(i.e. : http://www.stud.idb.hist.no/~espeegot/images/synvilla.jpg) but I don't think we are dealing with an optical illusion when talking about the color of the sky

I apologize if this was discussed already, I did a search and didn't find anything.

Musashi
2003-Jul-09, 10:09 PM
Well, one could possibly assume that its true color is clear. This could be seen as black (at night, when there is little light), or blue because of the refraction of light (or whatever reason it appears blue, dust?). That's just my take.

daver
2003-Jul-09, 11:14 PM
I think asking "what color is X" is pretty much a grade school exercise, and you should expect grade school answers. So, green should be a valid answer to what color leaves are, or what color grass is, or what color mommy is after a boat ride. And blue should be perfectly acceptable as what color the sky is.

eburacum45
2003-Jul-09, 11:59 PM
absolutely.
You could look at the blue or green coloration of a parrot or macaw, and say that the colour was caused by diffraction instead of pigmentation, and you would be wrong, because it looks blue and it is blue.
The Blue sky is caused by the distance between air molecules in the upper atmosphere being the right scale to cause Rayleigh scattering.
We live and breathe in the thick lower atmosphere where the separation is too small for Rayleigh scattering, so the air down here really does have no colour.

Musashi
2003-Jul-10, 12:27 AM
Actually, here in souther California, our air is usually some shade of brown.

Archer17
2003-Jul-10, 12:32 AM
Actually, here in souther California, our air is usually some shade of brown.better than Pittsburgh, although it's been sunny lately, our usual shade is slate grey. This is not a preferred place for astronomers to go to! Our winter skies tend to be clearer but then you have frost-bite to worry about! :(

The Supreme Canuck
2003-Jul-10, 12:42 AM
Ah, Canadian skies, blue and clear. Don't you just hate it? :P

Musashi
2003-Jul-10, 12:48 AM
Just wait! One day you Canadians will destroy your skies too!

Humphrey
2003-Jul-10, 12:50 AM
Ahhh...But we Floridians have the best skies of all. :-)

The Supreme Canuck
2003-Jul-10, 12:58 AM
Except for the hurricanes and tornados. :o :(

Humphrey
2003-Jul-10, 01:14 AM
True, but they do make great waves! And where else does it rain every day at 4:00 p.m. during the summer?

Weeeee.....

The Supreme Canuck
2003-Jul-10, 01:17 AM
It rains at 4:00 pm everyday in England. Or so I'm told. It's why they go in for tea! :D

*Runs from angry Brits*

freddo
2003-Jul-10, 01:20 AM
It's that old frame of reference issue again isn't it - only with colour and not motion.

Isn't it enough to say that the color of the sky is blue because thats how it appears to everone (colorblind excluded)??!?!?

The more fun part of the trivia, and decidedly less contentious, is to ask why the sky appears that way...

But black? Sheesh, even objectively that's just wrong.

Humphrey
2003-Jul-10, 01:21 AM
It rains at 4:00 pm everyday in England. Or so I'm told. It's why they go in for tea! :D

*Runs from angry Brits*

But it rains every day in England. I am talking about just the summer. Try again. :-)

Musashi
2003-Jul-10, 01:22 AM
I still stand by the clear theory!

The Supreme Canuck
2003-Jul-10, 01:28 AM
Humphrey:

You asked: "And where else does it rain every day at 4:00 p.m. during the summer?"

You also said that it rains every day in England. This means that it rains every day in summer in England, as well as the rest of the year. Gotcha now!

Humphrey
2003-Jul-10, 01:29 AM
Arrrggg...The English language you will be the death of me!!!!

The Supreme Canuck
2003-Jul-10, 01:32 AM
MWAHAHAHAHA!!! Such a powerful tool...

Humphrey
2003-Jul-10, 01:34 AM
MWAHAHAHAHA!!! Such a powerful tool...

You misspelled "Muuaaahahahahahaha!!!" :-)

The Supreme Canuck
2003-Jul-10, 01:37 AM
Regional dialect.

sarongsong
2003-Jul-10, 05:30 AM
And where else does it rain every day at 4:00 p.m.
during the summer?
Hawaii---where a light rain is considered a good omen.

kilopi
2003-Jul-10, 07:49 AM
Is the answer, "Black" correct? I say that on earth under normal/clear daylight conditions it's blue (let's ignore night, cloudy days, and sunset/sunrise).
Certainly, they were going for a trick answer (nevermind the nonsense about the reflection of the oceans), but I gotta agree with them that nighttime is a normal condition. Around here, the skies are black more often than they are blue or white or gray or red.


We live and breathe in the thick lower atmosphere where the separation is too small for Rayleigh scattering, so the air down here really does have no colour.
Are you saying that less dense air causes more Rayleigh scattering than more dense air?

Musashi
2003-Jul-10, 08:04 AM
Well, there is approx. 76 centimeters of air between my eyes and my monitor and that air does not seem to be affecting the color of the things i see on the monitor, so...

clear, clear, clear, clear, clear!!!! :D

Mainframes
2003-Jul-10, 09:03 AM
It rains at 4:00 pm everyday in England. Or so I'm told. It's why they go in for tea! :D

*Runs from angry Brits*

And at 5pm, 6pm, 7pm, 8pm....... stupid wet country. Of course that may be just the localised weather phenomenon that occurs around Bath. :(

Glom
2003-Jul-10, 11:19 AM
I, of course, wish our rain was that regular. If I knew it was going to be everyday at 4 then I'd know that I'd be okay for my flying lesson a 9:30 on Monday.

BlueAnodizeAl
2003-Jul-10, 01:06 PM
It rains at 4:00 pm everyday in England. Or so I'm told. It's why they go in for tea! :D

*Runs from angry Brits*

And at 5pm, 6pm, 7pm, 8pm....... stupid wet country. Of course that may be just the localised weather phenomenon that occurs around Bath. :(

Hence the name.

eburacum45
2003-Jul-10, 01:46 PM
Are you saying that less dense air causes more Rayleigh scattering than more dense air?
Well, yes and no.
From here-
http://www.sundog.clara.co.uk/atoptics/sunsets.htm#ray
Scattering by the air
Air molecules in the low density upper atmosphere behave as Rayleigh scatterers. However, closer to the ground the air is too dense for Rayleigh scattering to be significant because waves from the closely adjacent scattering molecules overlap and destructively interfere. In the middle atmosphere random molecular motion and collisions produce momentary density fluctuations that scatter the sunlight. The effect, first described by Einstein and Smoluchowski, is similar to, but not the same as Rayleigh scattering.
So it does happen, but it is cancelled out by the thickness of the air.
I am intrigued by the Einstein/Smouluchowski scattering, though- I wonder if this is similar to scintillation effects.

Mainframes
2003-Jul-10, 02:06 PM
It rains at 4:00 pm everyday in England. Or so I'm told. It's why they go in for tea! :D

*Runs from angry Brits*

And at 5pm, 6pm, 7pm, 8pm....... stupid wet country. Of course that may be just the localised weather phenomenon that occurs around Bath. :(

Hence the name.

Surely the name would be Shower.....

RichField
2003-Jul-10, 02:21 PM
Well, there is approx. 76 centimeters of air between my eyes and my monitor and that air does not seem to be affecting the color of the things i see on the monitor, so...

clear, clear, clear, clear, clear!!!! :D
But that's air, how much of the sky is between your eyes and the monitor. :)
And how many times do you view the monitor through a vacumn to check the difference? *ducks*

It's semantics really, but it's along the same lines of saying water is (generally) clear but the ocean can be blue, grey, green, etc.

redwithpurpleflashes
2003-Jul-10, 04:14 PM
Sky - 1 : the upper atmosphere or expanse of space that constitutes an apparent great vault or arch over the earth
http://www.m-w.com/home.htm.
So according to Merriam-Webster, the sky is the upper atmosphere so it wouldn't be clear. The question isn't, "What color is air?". Clear would be a good answer for night though (black would not make sense because if the sky was black, we would not see the moon, stars, etc.). I say if it looks blue, then it's blue (or whatever color it may be at that time of day).



edited to fix quote code and url

SeanF
2003-Jul-10, 04:22 PM
Sky - 1 : the upper atmosphere or expanse of space that constitutes an apparent great vault or arch over the earth
http://www.m-w.com/home.htm.
So according to Merriam-Webster, the sky is the upper atmosphere so it wouldn't be clear. The question isn't, "What color is air?". Clear would be a good answer for night though (black would not make sense because if the sky was black, we would not see the moon, stars, etc.). I say if it looks blue, then it's blue (or whatever color it may be at that time of day).

I agree with most of this, except the part I highlighted. Some stars, and definitely the moon, are visible during the day, against the "blue sky," ain't they?

The Supreme Canuck
2003-Jul-11, 04:04 AM
At least in England you can see a storm coming. In my corner of Canada, it could be a wonderful sunny day, without a cloud in the sky. Before the trouble starts. You see a dark band of cloud on the horizon. The sun fades. The sky becomes overcast. You hear thunder. The torrent begins. And all in a matter of minutes. You can actually see the edge of the rain moving along the ground. In fact, I've had it rain on one side of my backyard but not the other for an hour! We have weird weather...

Musashi
2003-Jul-11, 04:25 AM
But that's air, how much of the sky is between your eyes and the monitor.

I was mostly kidding, but I stand by my assertion of clear.


if the sky was black, we would not see the moon, stars, etc.

I agree with most of this, except the part I highlighted. Some stars, and definitely the moon, are visible during the day, against the "blue sky," ain't they?

Clear sky! :D

captain swoop
2003-Jul-11, 11:12 AM
It rains at 4:00 pm everyday in England. Or so I'm told. It's why they go in for tea! :D

*Runs from angry Brits*

And at 5pm, 6pm, 7pm, 8pm....... stupid wet country. Of course that may be just the localised weather phenomenon that occurs around Bath. :(

Apart from a shower yesterday and a shower on tuesday we haven't had any rain for about a month in our corner of North Yorkshire, at the moment it's a clear, blue sky and hot sun.

Kaptain K
2003-Jul-11, 11:35 AM
We must have gotten all your rain. Yesterday was the first day in July that it didn't rain. Definitely unusual for central Texas. I've seen years in which it did not rain in July! :o

Prester John
2003-Jul-11, 02:58 PM
Hmm Rains most of the time in Cardiff, Wales too.

I guess the question might have been phrased, what colour is the sky usually? for which the correct answer could easily be Black i guess.

PJ

man on the moon
2003-Jul-11, 03:06 PM
It rains at 4:00 pm everyday in England. Or so I'm told. It's why they go in for tea! :D

*Runs from angry Brits*

And at 5pm, 6pm, 7pm, 8pm....... stupid wet country. Of course that may be just the localised weather phenomenon that occurs around Bath. :(

Hence the name.

Surely the name would be Shower.....

ha ha! true, but maybe they didn't have pressured water when they named the town, no showers. only baths in big wooden rain tubs! :lol: :lol:

oh, and as for the rain in only one side of your yard SC? thats what you get for living in canada :lol:

*ducks. swim!*

Humphrey
2003-Jul-11, 05:32 PM
When i lived in Toronto i had that happen too Canuck.

The Supreme Canuck
2003-Jul-13, 05:39 PM
I blame the government. (The easy way out)

kilopi
2003-Jul-13, 06:56 PM
why do mountains in the distance appear blue?

Humphrey
2003-Jul-13, 07:10 PM
why do mountains in the distance appear blue?

Lots of paint? :D


I am guessing the same effect as why the sky looks blue.

Musashi
2003-Jul-13, 07:12 PM
perspective of distance, or something like that. I think DaVinci or someone came up with that.

eburacum45
2003-Jul-13, 07:50 PM
I don't know...
but looking around on Google, this seems to be due to Rayleigh scattering too,
but due to small dust and water droplet particles rather than the air.

One indicator of this might be that the blue color is variable according to weather conditions.

skyglow1
2003-Jul-18, 09:11 PM
I thought black isn't a colour because there is no light reflected to make colours.

skyglow1

milli360
2004-Mar-03, 01:56 PM
We live and breathe in the thick lower atmosphere where the separation is too small for Rayleigh scattering, so the air down here really does have no colour.
Are you saying that less dense air causes more Rayleigh scattering than more dense air?
I know redwithpurpleflashes doesn't post often, but once fairly recently. Perhaps we can rejoin this discussion.

Well, there is approx. 76 centimeters of air between my eyes and my monitor and that air does not seem to be affecting the color of the things i see on the monitor, so...

clear, clear, clear, clear, clear!!!!
That's more a function of distance though, not density, isn't it?

But if air is clear, and the blue color is refracted sunlight, is it valid to say that the sky is black? Of course some of the time it clearly (eh) is.

What do we mean by "sky" when we are asking the question, what color is the sky? Is it the molecules, or is it the overall "dome" taken as a whole? With or without clouds? Or sunsets?

Spaceman Spiff
2004-Mar-03, 03:25 PM
absolutely.
You could look at the blue or green coloration of a parrot or macaw, and say that the colour was caused by diffraction instead of pigmentation, and you would be wrong, because it looks blue and it is blue.
The Blue sky is caused by the distance between air molecules in the upper atmosphere being the right scale to cause Rayleigh scattering.
We live and breathe in the thick lower atmosphere where the separation is too small for Rayleigh scattering, so the air down here really does have no colour.

The Rayleigh scattering cross section is FAR too small to be the cause of our blue sky IF the only scattering that occurred happened in the low density upper atmosphere (even perhaps the stratosphere). We don't live inside glass (in which scattering is mostly coherent), nor do we live in the laboratory like vacuums of the upper atmosphere (in which scattering is mostly incoherent).
And the world is rarely black or white.
What you are saying is that absent aerosols and such, a light source sitting on a high hilltop near the horizon wouldn't be "obscured" at all, or a dark mountain side seen at a distance wouldn't have lower contrast. This isn't the case even in "clean" air.

Here are some numbers.

The Rayleigh scattering transmission at the zenith for a location near sea level at 5500 Angstroms (550 nm) is about 0.91 (Table 11.25, Allen's Astrophysical Quantities).

The Rayleigh scattering cross section is something like 4.7e-27 cm^2 at that same wavelength (and somewhat smaller in the stratosphere due to its lower index of refraction). In Allen's Astrophysical Quantities is an equation that indicates the scattering opacity due to Rayleigh scattering in Earth's atmosphere:

optical depth = 0.008569 * (lambda/1micron)^-4 * p/p_o,
(with some correction factors of order unity due to wavelength dependence of the index of refraction), where p is the pressure, and p_o is the standard atmospheric pressure. At 0.55 microns (550 nm), this optical depth amounts to 0.094 * p/p_o.

Due to the exponential behavior of the gas density in the atmosphere, one can closely approximate things by taking the density at sea level, and then multiplying it by one atmospheric scale ht, about 8.4 km, to determine the column density of gas molecules overhead. This column density of air is related to the optical depth by:

optical depth = cross section * column density

In other words, from sea level and looking straight up p/p_o is very nearly 1. Thus the optical depth is 0.094. The transmission is equal to e^(-tau) or 0.91.

Now if Rayleigh scattering were to occur only in the stratosphere and above, say starting at 2 scale hts, then the optical depth is e^(-2) times smaller or 0.013. Have you ever flown at 40,000 ft (about 1 scale ht up), looked up and seen how much darker the sky overhead appears? Some of this is due to being above most of the aerosols that have a smaller scale ht., but some of it is due to the 1/e smaller Rayleigh scattering optical depth.

Even in the clearest of skies, the sky turns whiter and brighter near the horizon because the optical depth to Rayleigh scattering is large enough for multiple scattering to dominate the light we see.

Putting it more directly, how does one account for the zenith Rayleigh scattering optical depth of approximately 0.1 at 550 nm at sea level, if it essentially never happens within the first scale ht?

sarongsong
2004-Mar-03, 03:35 PM
"...The sky looks blue, not violet, because our eyes are more sensitive to blue light (the sun also emits more energy as blue light than as violet)..."
http://cimss.ssec.wisc.edu/wxwise/bluesky.html

George
2004-Mar-03, 07:22 PM
The Blue sky is caused by the distance between air molecules in the upper atmosphere being the right scale to cause Rayleigh scattering.
We live and breathe in the thick lower atmosphere where the separation is too small for Rayleigh scattering, so the air down here really does have no colour.

[I might as well add my 2 cents.]

You may have seen Rayleigh scattering demonstrated using an aquarium and a little milk. Wouldn't this molecular density be contradictory?

Also, if Rayleigh scattering was dominant in the upper atmosphere, I would suspect we would see redder clouds (due to less blue).