I understand that the blue colour of the sky is because of the scattering of blue light by molecules in earth's atmosphere. The scattering appears to be happening from molecules that are far above in the earth's atmosphere. What about the scattering that happens because of molecules near the surface of the earth? Why can I not see the blue light scattered by molecules closer to the earth?
If I understand you right, you're referring to the phenomenon seen in this picture (from the first Google hit), that near the horison the color of the sky is more light-blue (not exactly white):
The scattering in the atmosphere is for a large part Rayleigh scattering off of nitrogen and oxygen molecules, which are much smaller than the wavelength $\lambda$ of the light, and which has a preference for forward and backward scattering. If the Sun is more or less above you, the horison is roughly at 90º, which is the direction on which scattering is least likely.
Rayleigh scattering has an huge wavelength dependence of $\sim 1/\lambda^4$, meaning that blue light is scattered much more efficiently than red light. This is the reason for the blue sky in the first place, and the reason for the red sun at sunset.Mie scattering
However, close to the ground — which is what you look at when you look at the horison — you have more water droplets which have a size of the order of the wavelength. In this case, the scattering is not Rayleigh scattering, but the more general solution of Mie scattering, which is not strongly wavelength dependent. Thus, all the colors are scattered more or less equally, producing a whitish glare.
(My first answer was not good, and did not take into account the Mie scattering. I'm sorry.)
It's because you're not looking far enough. From personal experience, it takes at least 10 km of atmosphere to build up a really obvious blue (see, for example, this picture), and if you're not in hilly country, the horizon is only 5km away. In contrast, most of the sky has distances to space on the order of hundreds of kilometers.
The scattering separates red/orange and blue in different directions. At sunset you'll see the red parts that are missing from the blue skies by day. This isn't noticeable for objects close by, because those objects surround you. The blue from some objects mixes with the red from others.
Secondly, there is a lot of air between you and the sun. The atmosphere is kilometers thick. In comparison, there's only a few meters of air near you. As each air molecule has a small chance to scatter the light, you need a lot of molecules before you notice significant scattering.
If you actually look at (atmospherically) scattered light, you will see that even at ground-level, there is a distinct blue tint. How do you see that, you ask? Any old shadow will do - find some building that throws a nice big shadow, make a photo with a decent camera, and analyze it thoroughly - you'll find that there's indeed a blue tint, as expected.
Why is this so hard to see, and why doesn't everything look distinctly blue? There's a lot of things at play here:
- The sky is against a black backdrop. Since the almost all of the light that comes from the sky is blue-scattered, the sky is distinctly blue - there's no other colour to it at all. Clouds are yet another way of light scattering, and they end up rather white - that's just because there's no preferential scattering of different visible light wavelengths, unlike with Rayleigh scattering. This is what causes the apparent yellowness of the Sun - while we call Sun a yellow star, that only really matters when comparing with other stars; for Earthly life, it's very much by definition white - as you can see when the light is rejoined around ground level again; at noon, there is hardly any tint to things.
- There are other forms of scattering involved at ground level. For one, there is usually a lot of humidity - and water droplets cause Mie scattering, similar to the clouds. Second, there's the ground itself - it causes a diffuse lighting of everything with the colour of the incident light (which includes the direct (yellowish) sunlight, as well as most of the scattered blue sky-light). When you put a white box on the ground at noon in direct light, it will be white - that's basically what light means, that's where we got through natural evolution of sight.
- The human perception is very skewed. This is a good thing most of the time - for example, it's responsible for brightness correction (so that you can see just as well at noon as in the afternoon, and in direct light as well as in shadow). The more important thing right here is white-balance correction - basically, human sight automatically adapts to the varied colour of ambient light, and corrects the inputs to preserve "whiteness". The easiest way to see this is if you have a camera with configurable white balance - just turn it off completely. Now, compare photos from the outside with photos from indoor-lighted areas - you'll see that everything is tinted somewhat; outside direct light at noon is very much white, while shadows are blueish, and indoor lighting tends to be yellowish or reddish (with incadescent light) or blueish (with old-school fluorescent lighting).
If you want to know more about how human perception gives trouble to photography, have a look at the photography SE. Cameras try to emulate human perception, rather than simply giving you raw light data - when they fail at this, you get a lot of fun effects (remember that gold/blue dress photo from not so long ago? That was also an issue with white balancing).
protected by Qmechanic♦ Nov 17 '15 at 21:11
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