Randall Monroe, a credible source in my opinion, says that the sky is blue because the air is:

Normal light interacts with the atmosphere through Rayleigh scattering. You may have heard of Rayleigh scattering as the answer to "why is the sky blue." This is sort of true, but honestly, a better answer to this question might be "because air is blue." Sure, it appears blue for a bunch of physics reasons, but everything appears the color it is for a bunch of physics reasons.

But is the air really blue? Color is created by matter absorbing some wavelengths, while reflecting or transmitting others (as explained, for example, here). Air transmits the entire spectrum equally, but in different directions, which would make it colorless (white?), not blue.

So, is Randall wrong?

To clarify about possible duplicates - I'm asking whether the air, not the sky, is blue. I don't ask about the physical phenomena that make them so - this one was asked and answered.


marked as duplicate by ACuriousMind, John Rennie, Sebastian Riese, Martin, Norbert Schuch Jan 14 '16 at 22:31

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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    $\begingroup$ Ever seen a red sunset? If air actually were blue, what new thing would be there to let you perceive the sunset as red? Or a different question could be: what color is the air in your apartment? Or at night? $\endgroup$ – Clever Jan 14 '16 at 9:40
  • $\begingroup$ @Clever that sounds like it should be an answer $\endgroup$ – David Z Jan 14 '16 at 9:43
  • $\begingroup$ @Clever, Sunset's a good point. The air in my apartment would be very very slightly blue, but the sky's just more air. At night, the sky is still blue, just darker (see long-exposure night photos). $\endgroup$ – ugoren Jan 14 '16 at 9:50
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    $\begingroup$ If air were blue then it would absorb all other colors except blue and it would be blue regardless of the angle you look at the sky; looking right above your head it is blue but moving toward the horizon it becomes pale blue. $\endgroup$ – Amin R. Jan 14 '16 at 11:25
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Air is blue.

White light is incident on it, and the scattered light that reaches your eye is blue.

A leaf is green. White light is incident on it, and the scattered light that reaches your eye is green.

Smoke is gray.

Chlorine gas is yellow-green (I think. I really don't know what color chlorine is.)

Air is blue, but only very faintly so. So faint that you can see it only when you have a great deal of it, the atmosphere, for example.

Edit after comment

Air is red.

The word "scattered" does not appear in that article. But your point is taken. A piece of red glass is red because the blue is scattered out. So in that sense, air is red. So I see no single answer to the question. The answer depends on the what you are observing and how you observe it. The impetus of my original answer was: what does your brain perceive? In the case of air, it can be red or blue.

I think Randall (like me) didn't think this one through. Also, one can argue that this is not a physics question. No one is uncertain about the mechanisms involved. We are arguing over the definition of the concept "color of an object", which by its nature is a psychophysical question.

  • $\begingroup$ You're saying color is a matter of what's scattered compared to what's absorbed/transmitted? The explanation I quoted says it's what's scattered/transmitted compared to what's absorbed. $\endgroup$ – ugoren Jan 14 '16 at 14:06
  • $\begingroup$ I edited my answer to address this. $\endgroup$ – garyp Jan 14 '16 at 14:22
  • $\begingroup$ F.Y.I., The Cleveland Museum of Natural History used to have a display that was supposed to illustrate solid, liquid, and gaseous phases of matter (solid, liquid, gas). The display featured large sealed flasks, illuminated from behind, and containing; Chlorine (pale, nasty-yellowish gas), Bromine (orange vapor above a dark red liquid), and iodine (purple vapor above dark crystals). It's been decades since the last time I saw it. I don't know whether it's still there. $\endgroup$ – Solomon Slow Jan 14 '16 at 15:39

is Randall wrong?

No, Randal is simplifying.

His point is that the colour we perceive from various objects and materials is produced by a tremendous variety of physical phenomenon. Generally the lay public are not interested in the deeper causes. He gives the example of the green appearance of the statue of liberty. People who ask why it is green almost certainly don't want an explanation in terms of fundamental physics, they want to know it is because the copper surface has tarnished and the result is a mixture of compounds that is coloured green.

Of course, you can reasonably argue that all simplifications are incorrect. But we can be reasonably certain that Randal Monroe has a reasonable understanding of Rayleigh scattering, Mie scattering and other processes. Therefore he is not wrong in the sense of making an error through ignorance.

Because it is a simplification, it doesn't adequately explain why, for example, sunsets are red. So, many of us would regard it as an oversimplification. Nevertheless Randall's point remains - there are many levels at which you can explain something and it isn't always necessary or useful to explain it from fundamental principles.

It is also worth remembering that a key element of XKCD is humour.

  • $\begingroup$ But is the simplification "the sky is blue because the air is" different from "the statue of liberty is green because the oxidized copper is"? Everybody's happy with the latter, but not with the former. $\endgroup$ – ugoren Jan 14 '16 at 16:37
  • $\begingroup$ @ugoran: good point, maybe RM should have said something like "the sky is blue because nitrogen gas is". $\endgroup$ – RedGrittyBrick Jan 14 '16 at 16:52
  • $\begingroup$ I don't see how it matters. If air is blue, it doesn't matter which of its components causes it, or via what physical process. $\endgroup$ – ugoren Jan 14 '16 at 17:03
  • $\begingroup$ "Why is the Statue of Liberty green?" "Because it's made of copper." "Why is copper green?" "Because blah blah physics." ... "Why is the sky blue?" "Because it's made of air." "Why is air blue?" ... $\endgroup$ – endolith Jan 14 '16 at 18:38

Sunlight reaches Earth's atmosphere and is scattered in all directions by all the gases and particles in the air. Blue light is scattered in all directions by the tiny molecules of air in Earth's atmosphere. Blue is scattered more than other colors because it travels as shorter, smaller waves. This is why we see a blue sky most of the time. Closer to the horizon, the sky fades to a lighter blue or white. The sunlight reaching us from low in the sky has passed through even more air than the sunlight reaching us from overhead. As the sunlight has passed through all this air, the air molecules scatters blue light more than other colors Also, the surface of Earth has reflected and scattered the light. All this scattering mixes the colors together again so we see more white and less blue. As the sunlight has passed all this air, the air molecules have scattered and rescattered the blue light many times in many directions. As the sun gets lower in the sky, its light is passing through more of the atmosphere to reach you. Even more of the blue light is scattered, allowing the reds and yellows to pass straight through to your eyes.

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    $\begingroup$ Correct, but it doesn't answer the question. $\endgroup$ – garyp Jan 14 '16 at 11:56

From a quite different perspective - consider that air is comprised mainly of nitrogen and oxygen. If you liquefy the air and separate the two gases each into separate, clear dewers the nitrogen will appear clear (transparent), but the oxygen will appear a light blue color.


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