Blue Sky

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Steven E. Landsburg - 16 Aug 2007 19:59 GMT

I hope this question isn't too elementary for this group.

I (more or less) understand why the sky is blue (Rayleigh scattering
because incoming sunlight turns air molecules into oscillating
dipoles, etc.). But I am vague on why the air all around us isn't
blue for exactly the same reason.

In particular, I'd like to know which of the following statements
are (more or less) true and which are completely off base:

1) Air molecules scatter blue light down here just as they do
up there, but my field of vision takes in a far vaster swath of
the upper atmosphere than of my immediate surroundings, so my
eyes get hit with a lot more blue photons from up there than
down here.

2) By the time sunlight reaches the surface of the earth, it's
had so much blue scattered out of it that we get very little
scattering down here.

3) An air molecule in the upper atmosphere tends to get hit with
sunlight from just one direction, which is what starts it oscillating.
By contrast, an air molecule near the surface is hit with reflected
light from many directions, so we shouldn't expect it to oscillate
(or to radiate) in anything like the same way.

4) Some other explanation I haven't thought of.

Can anyone explain which of the above are right, which are wrong,
and in the case of 4), what the right explanation is?

Steven E. Landsburg
www.landsburg.com/about2.html

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Steven E. Landsburg
http://www.landsburg.com/about2.html

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Uncle Al - 16 Aug 2007 23:33 GMT

> I hope this question isn't too elementary for this group.
>
> I (more or less) understand why the sky is blue (Rayleigh scattering
> because incoming sunlight turns air molecules into oscillating
> dipoles, etc.). But I am vague on why the air all around us isn't
> blue for exactly the same reason.

[snip]

You are looking up through 50 miles of atmosphere against a black
background. You are looking across through a few feet or miles of
atmosphere with a bright polychromatic background.

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Timo Nieminen - 16 Aug 2007 23:33 GMT

> I (more or less) understand why the sky is blue (Rayleigh scattering
> because incoming sunlight turns air molecules into oscillating
> dipoles, etc.). But I am vague on why the air all around us isn't
> blue for exactly the same reason.

It is blue. Look at distant objects and see (well known to landscape
painters).

Alas, your 1-3 are wrong (for 2, yes, some blue has been scattered out of
the direct sunlight, but we get the blue light from the blue sky, and that
will scatter from air down here). For 4, try:

If you were to compress the atmosphere so that it was of uniform density,
it would (iirc) be about 7km thick. How much bluing do you see if you look
at an object that far away? Next, consider that you're also seeing the
light coming directly from the object, which reduces the visibility of the
blueness. The blue sky is competing with the blackness of space (and
with the stars), so it's very easy to see.

Clean air is quite transparent, so you need a lot of (ie in thickness) to
get a noticeable effect.

It's also complicated by the air down here having a lot of particles in it
by comparison with the upper atmosphere. Some of these will scatter
colours other than blue quite effectively, adding to the haze, but
reducing the blueness.

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Timo Nieminen - Home page: http://www.physics.uq.edu.au/people/nieminen/
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Andy Resnick - 17 Aug 2007 15:59 GMT

> I hope this question isn't too elementary for this group.

<snip>

There's a lot of misunderstanding here.

0) the scattering cross section varies inversely with the 6th power of
wavelength in the Rayleigh limit. This means that when you are looking
at the sun (sunset/sunrise), you see mostly the non-scattered long
wavelength light (yellow-red). When you look away from the sun, you see
the scattered light (short wavelengths- blue). Question for you- what
happened to the green?

1) Generally, the atmosphere is weakly scattering- this is why we can
see things long distances away. The main component to air that
increases scattering (in the visible, anyway) is water vapor- humid days
have decreased visibility due to increased scattering. Clear dry
skies have a very deep blue color.

2) The sky is weakly polarized (Haidinger's brush), and the spot of
highest polarization is in a fixed orientation with respect to the sun.

There's a few classic books out there, typical favorite being:

"Light a color in the open air" by Minnaert

There's also a great website:

http://www.atoptics.co.uk/

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Andrew Resnick, Ph.D.
Department of Physiology and Biophysics
Case Western Reserve University

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J. J. Lodder - 18 Aug 2007 14:56 GMT

> I hope this question isn't too elementary for this group.
>
[quoted text clipped - 26 lines]
> Can anyone explain which of the above are right, which are wrong,
> and in the case of 4), what the right explanation is?

In addition to what others have (correctly) said about the physics
there is also a physiological part of the explanation.
Human eyes have 'colour constancy'.
That means that the neural processing in retina and brain
tries to compensate for bias in lighting.
A red apple will appear to us as almost the same red,
both at sunset and under the blue light of north facing window.
In modern camera terms you can say that the human eye
has automatic white balance.

The extra blue in the distance will be partly compensated away.
This is the reason that the distance blueing
often appears to be more pronounced in photographs
than it was to the eye.
You remembered seeing white snow-capped mountaintops in the distance,
but the photo you took is much more blueish and vague.
When looking at the sky otoh there is no reference,
and you just see the blue as it is.

Best,

Jan

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Natural Science Forum / Physics / Research / August 2007

Blue Sky