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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?

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    $\begingroup$ I'm not sure you got my question right. Why is the blue coming out of the less dense part of atmosphere? Why isn't the denser atmosphere (present near the surface of the earth) scatter blue light? Sky looks blue. Surroundings look white. Why isn't the scattering happening here? $\endgroup$ – coder Nov 17 '15 at 11:35
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    $\begingroup$ @coder: What do you mean surroundings are white? Nearby objects look like whatever colour they have, whether form very close or from further away, but that is not scattered light. You might conclude that the intermediate air is not scattering, but there is also much less air involved then when you look at the sky. To do a proper experment you should look at a (large) black object about 10km away horizontally, and see what colour it appears to have. This is very hard to realise. Haze, mist, clounds are due to a different form of scattering (off water droplets). $\endgroup$ – Marc van Leeuwen Nov 17 '15 at 16:27
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    $\begingroup$ To clarify, is the question: "If you are standing in a room with a window and sunlight coming in, and the room is full of air, why is there not a blue tint to everything because of the air scattering the light?" $\endgroup$ – pentane Nov 17 '15 at 17:26
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    $\begingroup$ You sure you can't? You may just need to be in the right place (clear dry air + distant backdrop = mountains, typically) to notice it clearly. $\endgroup$ – Ilmari Karonen Nov 17 '15 at 17:34
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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):

blue skies

Rayleigh scattering

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.)

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  • $\begingroup$ Thank you for your answer. In gist, the lower atmosphere has water droplets which are responsible for Mie scattering. Hence no colour predominates in the scattered light. Did I get it right? I didn't understand why the word horizon is used here. I am talking about the whitish glare that we see around us on the surface of the earth. Why isn't that bluish is the question :) I hope my question has been understood :) $\endgroup$ – coder Nov 18 '15 at 1:31
  • $\begingroup$ I've never ever seen horizon spelled the way you do before, by British nor American. $\endgroup$ – Octopus Nov 18 '15 at 4:18
  • $\begingroup$ @coder: You got it right. Maybe I didn't, though. When you look at the sky around you along the surface of the earth, you are looking at the horizon, no? $\endgroup$ – pela Nov 18 '15 at 7:48
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    $\begingroup$ @pela I am talking about the white light around me and I am not talking about the horizon. The light around me ie the illumination of my surroundings is white light. So what I meant to ask is why isn't the light getting scattered in my surroundings. Why isn't the illumination blue? $\endgroup$ – coder Nov 18 '15 at 11:06
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    $\begingroup$ @coder: Oh, then I completely misunderstood. Although I don't really know what "white light" around you are referring to. Do you mean "Why don't thing nearby look bluish"? In that case, Mark answers you question. Light is scattered in the atmosphere close to the ground, but the optical depth of the atmosphere is quite large, i.e. the light has to travel quite far before there's a significant probability of being scattered. Thus things on the surface of the Earth look bluish/whitish only when they're far away. The contrast between colors decrease with their distance from you. $\endgroup$ – pela Nov 18 '15 at 15:10
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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.

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  • $\begingroup$ You're not really looking at the horizon, but just above it. Surely if one looks at a low angle through the atmosphere, one looks through more atmosphere, not less. As per @pela's answer, the phenomenon in your link is fogging due to water vapour. $\endgroup$ – Keith Nov 18 '15 at 4:33
  • $\begingroup$ Are you looking at the same blue hills I am? The white near the horizon is Mie scattering, but the blue of the hills is Rayleigh. $\endgroup$ – Mark Nov 18 '15 at 7:02
  • $\begingroup$ @Keith: Those blue hills are definitely below the horizon $\endgroup$ – slebetman Nov 19 '15 at 4:53
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Two reasons:

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.

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  • $\begingroup$ Firstly, thanks for your answer! I am not sure if I am right here. Please correct me if I am wrong. The atmosphere around me is a lot pressurized than the atmosphere high above. Which means air molecules are at the highest in the lower atmosphere. Yet I don't see a wavelength being scattered in the lower atmosphere (surrounding me) .But I see blue being scattered in upper atmospheres. (High above) $\endgroup$ – coder Nov 18 '15 at 1:39
  • $\begingroup$ @coder: The pressure gradient isn't that steep. That's why the atmosphere extends about 100 kilometer. There are more air atoms above one kilometer than below. Secondly, you don't see where the blue is being scattered. Like the scattering of rainbows, the blue light scattering changes the direction of the light which makes it impossible to see where it's actually coming from (i.e. the sun). $\endgroup$ – MSalters Nov 18 '15 at 7:54
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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).

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protected by Qmechanic Nov 17 '15 at 21:11

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