One from my 14-year-old - who refuses to register here - and it is, to me, a poser.
Simply: Is the sky ‘bluer’ at the poles.
She posits that the sky is blue due to airs ability to scatter blue wavelengths of light. Well, she reasons, light coming in at the poles is subject to being in air longer - and therefore scattered more - due to coming to Earth at an oblique angle. Hence more scattering, hence ‘bluer’.
I told her I knew who to ask.
More air means the sky will appear as a lighter shade of blue. If you have ever been at high altitude on a clear day you may have noticed the sky was a bit darker blue.
To simplify, the ecliptic (the path the sun follows in the sky) will be much lower at the poles. Because of this, the sun spends a lot of its time hanging closer to the horizon, and because the sun rays have to cut through so much more atmosphere, you get a lot more golden/red light scattered in your direction than blue.
So I’d say, overall, a bit darker blue during day and a lot more “golden hour” sunrise/sunset colors.
Since blue light scatters more than red light, when the sunlight comes in at an oblique angle, more of the blue light gets scattered before it reaches you. You’re left with more red light.
This is what you see when the Sun is setting: a red sunset, since much of the blue light has been removed, leaving relatively more red.
How blue can it get?
Your daughter’s argument, as I understand it, is that because sunlight is at a lower elevation above the horizon at the poles, that more of its blue light is scattered, and therefore that the light from the sky will be bluer. However, there’s nothing particularly special about the poles in this argument; what really matters is the elevation angle of the Sun above the horizontal, because that’s what determines the length of time that a light ray spends in the atmosphere. So the question should really be: “Is the sky bluer when the Sun is lower in the sky?”
This seems doubtful to me; certainly it’s not an effect I’ve noticed. The reverse effect is definitely true, though: If you look directly at the Sun when it’s at low elevation, then all the blue light is scattered away, and the red light is all that’s left. Thus, the Sun turns red at sunrise & sunset. The actual amount of blue light that reaches you from a given (non-Sun) direction, though, is rather more complicated, and I’m honestly not sure how it would work out. I’ll have to try out some calculations if I have time.
Finally, I’ll point out the following: The sky is often bluer at the poles, but not for the reason you’re thinking. The light from the sky is not only due to scattering by molecules, but also due to aerosols: fine droplets of things like water or pollutants. This scattering, by objects that are much larger than the wavelength of light, doesn’t have the characteristic blue color of scattering from molecules; rather, it’s basically white light. So the light you see from the sky is a combination of these two lights, and the white light from the aerosols will tend to “wash out” the blue from the molecules. Near the poles, though, it tends to be much drier (since it’s cold) and less polluted (since nobody lives there and the winds don’t tend to carry pollution there.) So the blue light is more “pure” and the sky would appear bluer.
Like, really blue man.
Oh wow, you just blew my mind.
Very true. I always notice how much darker blue the sky is at home (I live in the mountains) than say at the beach.
Don’t forget that the earth’s axis is tilted relative to the ecliptic, so it’s not hitting at the poles at an extreme angle. Light at the poles travels through the same length of air that it does at some point during the day at any latitude, and well before sunset. The sky is blue at the poles – have a look at the many photos available.
The physics of the color of the sky can be surprisingly complex. You can get quite far by invoking Rayleigh scattering, but there’s much more involved. Pick up a copy of M. Minnaert’s book The Nature of Light and Color in the Open Air (which was published in paperback by Dover). It’s a trove of weird and fascinating optical effects. But the real source for understanding sky colors is W.J. Humphreys’Physics of the Open Air, which is a lot more difficult to get, but which goes into why sky blue differs from place to place, and what other scattering mechanisms come into play. It’s actually a pretty old book (so is Minnaert’s, for that matter), but I don’t know of any newer ones that have come in to cover the topic for the general student.
Actually, I keep forgetting the advantages of the internet, which can be a crapshoot. But you win this time. Humphreys is available online:
“The answer is: None. None more blue.”
Not blue, blue blue. Toilet bowl blue.