Why Does the Sky Appear Blue During the Day?

[Cyrus]

Quite right Russ. I did not consider that effect.

Around the edges of the picture, you can see a layer of the atmosphere in it's blue glow. Are you supporting that this blue glow is due to the scattering of light from the atmosphere, or that it is the color of the air due to 'thickening'?

 

[Gokul43201]

The simplest description I've seen for why the sky appears blue is this:


The sky appears blue because air is blue.

...except at dusk and dawn, when the air is either yellow, pink, or orange.

(So in the specific context of the question - being during times when the sky is blue - the answer provided by Hurkyl is something of a tautology.)

 

[Farsight]

I think that's pretty good Dave.

 

[russ_watters]

Around the edges of the picture, you can see a layer of the atmosphere in it's blue glow. Are you supporting that this blue glow is due to the scattering of light from the atmosphere, or that it is the color of the air due to 'thickening'?

Scattering. And...

...except at dusk and dawn, when the air is either yellow, pink, or orange.

If you mean the atmosphere itself (as opposed to the clouds in the atmosphere), no. No amount of thickness will make it appear red/orange, otherwise the horizon would always appear red/orange. What happens at dawn/dusk is that the sun/moon shine through/reflect off of thin clouds/haze and the atmosphere scatters out the blue light due to the added thickness, leaving the couds/haze red.

As seen in The Swerve's award winning photo, it is the clouds, not the sky behind them that are colored: http://i22.photobucket.com/albums/b326/PaidVacation/NaplesSunsetredgold.jpg

 

[Hurkyl]

Are you supporting that this blue glow is due to the scattering of light from the atmosphere, or that it is the color of the air due to 'thickening'?

Both. The latter is the simple, straightforward explanation, and the former is the low-level underlying physical mechanism that causes the latter to happen!

 

[Gokul43201]

...otherwise the horizon would always appear red/orange.

Not necessarily true (I think the mean free path of a photon in the lower atmosphere is smaller than a meter, but I may be wrong). But I haven't thought about this enough to say anything intelligent...so I recant my previous objection for now.

Edit : The more I think about it (okay that's only 20 minutes so far), the more I think it is wrong to speak of the "color of a gas". The phenomenon that "gives color" to a gas is very different from the thing that gives color to a solid. With a solid (to be precise, an opaque solid), once you go beyond the nanometer size range, color is a material property. The color of a thing does not depend on how much of the thing there is. The phonon dispersion relation is size independent. So you can speak of the color of a solid without having to specifiy how much of the solid there is.

I don't believe the same is true of fluids - where the physics itself is completely different from that of solids. The color of a fluid is determined roughly by a Beer-Lambert relation with the extinction coefficient determined by something like the Rayleigh formula. That makes it seem plausible to me that the "color of a fluid" is not a material property.


Edit2 : Looking for a reliable opinion, I decided to see if Baez has anything to say about this. From his website:http://math.ucr.edu/home/baez/physics/General/BlueSky/blue_sky.html

Sunsets

When the air is clear the sunset will appear yellow, because the light from the sun has passed a long distance through air and some of the blue light has been scattered away. If the air is polluted with small particles, natural or otherwise, the sunset will be more red. Sunsets over the sea may also be orange, due to salt particles in the air, which are effective Tyndall scatterers.

Too handwavy, by Baez's standards but he seems to be saying that it is the sky that is yellow (though the wording is admittedly a little ambiguous to me). Also, his picture shows that it is the sky, not the clouds in front, that is colored.

Also, I think back about the colorimetry experiments I did in college, where you determined the concentration of a solution by matching colors with a standard solution in a column where you could adjust its height. The color of the standard solution is a function of the height of the liquid column, though admittedly, a blue solution never turned red by making it a few inches taller.

Looking back through the thread, I see Zz has linked a couple of AJP articles. Better read those first.

 

[Farsight]

Oh hurkyl. Next you'll be telling me oil is rainbow-coloured!

 

[Hurkyl]

The more I think about it (okay that's only 20 minutes so far), the more I think it is wrong to speak of the "color of a gas". The phenomenon that "gives color" to a gas is very different from the thing that gives color to a solid.

I agree that they are different phenomena, but given the diverse use of the word "color" in everyday speech, I don't think it would be reasonable to restrict the word to mean nothing but diffuse reflection.

Oh hurkyl. Next you'll be telling me oil is rainbow-coloured!

I wasn't planning on it. The scattering we see in the air and diffuse reflection off of a solid have roughly the same bulk effect on light -- they select certain frequencies of light, and diffuse them. These two effects are far more similar than anything else we might call "color", so I think I'm justified in the use of the term.

Thin film interference is not a similar effect.

 

[russ_watters]

Edit : The more I think about it (okay that's only 20 minutes so far), the more I think it is wrong to speak of the "color of a gas". The phenomenon that "gives color" to a gas is very different from the thing that gives color to a solid.

Well, I was going to comment on Hurkyl's tautology before, but I guess now would be a good time: So what? So what if the mechanism by which different things cause a certain wavelength of light to get to your eye is different? Why does that make it wrong to say (for example) that hydrogen is red because of its emission spectrum while a brick is red because it absorbs everything else while reflecting red light. Sure, Hurkyl's tautology doesn't explain the mechanism (and there are several), but it most certainly is correct.

With a solid (to be precise, an opaque solid), once you go beyond the nanometer size range, color is a material property.

Who says that only solids that reflect and absorb are allowed to have a property called "color"?

The color of a thing does not depend on how much of the thing there is. The phonon dispersion relation is size independent. So you can speak of the color of a solid without having to specifiy how much of the solid there is.

That's correct, but so what? I guess you are saying that since it takes more atmosphere to be bluer, it isn't an innate property. But the same applies to solids: slice any solid thin enough and it starts to lose its color as well. So you can still say the same about a gas - not to mention a translucent material like the plastic wrap I make red filters. I fold it several times to make it block enough light to be useful on my laptop. So what if it is only slightly red at 1-ply? It is still red.

I have some filters for my telescope that are extremely high transmittance color filters. Because of their transmittance, you cannot tell what color they are just by looking at them (they completely transmit one color and completely absorb or reflect the rest). So the only way to tell the color is to hold them up against a white piece of paper or white light. But I guess for the sake of quibbling, we could say the red filter is red because it transmits red or blue/green because it reflects blue/green. But either way, the color is an innate property of the material.

I don't believe the same is true of fluids - where the physics itself is completely different from that of solids. The color of a fluid is determined roughly by a Beer-Lambert relation with the extinction coefficient determined by something like the Rayleigh formula. That makes it seem plausible to me that the "color of a fluid" is not a material property.

What determines the Beer-Lambert relation? Is it the same for all fluids. It seems pretty self-evident that if it depends on the properties of the fluids and is unique for each, then it can be said that the resulting color itself is a property.

Edit2 : Looking for a reliable opinion, I decided to see if Baez has anything to say about this. From his website:http://math.ucr.edu/home/baez/physics/General/BlueSky/blue_sky.html

Too handwavy, by Baez's standards but he seems to be saying that it is the sky that is yellow (though the wording is admittedly a little ambiguous to me). Also, his picture shows that it is the sky, not the clouds in front, that is colored.

You're talking about this:

When the air is clear the sunset will appear yellow, because the light from the sun has passed a long distance through air and some of the blue light has been scattered away. If the air is polluted with small particles, natural or otherwise, the sunset will be more red. Sunsets over the sea may also be orange, due to salt particles in the air, which are effective Tyndall scatterers. The sky around the sun is seen reddened, as well as the light coming directly from the sun. This is because all light is scattered relatively well through small angles--but blue light is then more likely to be scattered twice or more over the greater distances, leaving the yellow, red and orange colours.

He's talking about the sun itself except where he specifically mentions the sky around it. "Sunset" = setting sun in that context.

 

[Gokul43201]

What determines the Beer-Lambert relation? Is it the same for all fluids.

The relation is just that - what differs is the extinction coefficients.

It seems pretty self-evident that if it depends on the properties of the fluids and is unique for each, then it can be said that the resulting color itself is a property.

The relation tells you that different colors have different intensities that depend on the type and amount of fluid that the light passes through. This makes the color of air dependent on both the nature of the molecules and the amount of air along the path of light.

 

[DaveC426913]

No, it is specifically Rayleigh scattering, and how our human eye has evolved, that are the significant factors. Zz.

What does evolution of the human the human eye have to with anything?

Is there some suggestion that
- animals (those with colour vision) do not see a blue sky?
- cameras do not see a blue sky?

If there were no humans (or animals, or cameras) on the planet, the blue wavelengths of light would still preferentially reach the ground.

 

[Farsight]

Funny thing colour. It's only in our heads. What's real is the frequency. Maybe. Anyhow, I think it's to do with our Red/Green/Blue receptors:

http://www.exploratorium.edu/snacks/colored_shadows.html

http://en.wikipedia.org/wiki/Eyeball

If I shine a blue spotlamp on to a screen and overlap it with a purple spotlamp, I wonder what colour I see? I must try this out.

 

[Claude Bile]

What does evolution of the human the human eye have to with anything?

Is there some suggestion that
- animals (those with colour vision) do not see a blue sky?
- cameras do not see a blue sky?

If there were no humans (or animals, or cameras) on the planet, the blue wavelengths of light would still preferentially reach the ground.

Not true, violet and UV would preferentially reach the ground. In order to explain why we observe the sky to be blue, you need to take the colour responses of the human eye into account.

Cameras see a blue sky, because their RBG algorithms are designed to mimic the response of the human eye. A much more objective measurement would be with a spectrometer, such a measurement clearly shows significant amounts of violet and UV radiation scattered toward the surface in addition to blue.

I think this paper may be of interest to some (also contains the spectrum of the daytime sky I referred to) -

"Human colour vision and the unsaturated blue colour of the daytime sky" Glenn S. Smith, American Journal of Physics, 73 (7), 590-597 (2005).

Claude.

 

[vinter]

hey hey! can someone answer this please - why is the sun red in the evening? isn't THAT also because of scattering? why does scattering sometimes prefers red and sometimes blue?

 

[Farsight]

The scattering means the shorter bluer wavelengths get scattered off sideway more than the longer redder wavelengths.

So when you look up at the midday sky, you're seeing blue wavelengths scattered to one side. When you look straight at the sun in the evening through a lot of air you see more of the longer redder wavelengths because the blue stuff get scattered off to one side before it got to you.