I still dont understand why we see red at sunset.

The sun emits light that we see as white becuase it emits at least all the visible wavelengths which when combined and reach our eyes, we see as white

We see blue during the day becuase the shorter wavelengths get reflected off the particles in the atmosphere, and are subjected to lots of scatter interactions which results in shorter wavelenghts traveling in all sorts of directions and locations other than eminating from the sun, and inevitably reach our eyes. The longer wavelengths dont get reflected as much an keep going without being reflected which is why we dont see them as much.

Yet at sunrise/sunset, we see the longer wavelengths being scattered just as much as the shorter wavelengths were during the day.



Why is it that the longer wavelengths dont scatter like this during the day? Why dont we see any blue? where have the shorter wavelengths gone?

At sunrise/sunset, the sun light has further to travel to reach our eyes, it also appears lower in the atmosphere form our point of view, and as the denisty is thicker there, more scattering occurs as there are more particles. How does this contribute to us not seeing the shorter wavelengths but the longer ones.

Your understanding is correct. When the sun is high in the sky, light rays of short wavelength get scattered all around the sky and hence the sky appears blue.

When the sun is very low on the horizon, then the light rays have to travel a much longer distance through Earth's atmosphere before they reach you. Most of the light rays of short wavelength get scattered before they can reach you. That is, they get scattered onto the Earth, back into space, or simply get absorbed by the atmosphere after too many reflections. However, the longer wavelength light rays get scattered a little but not nearly as much. So all that you see are the remaining light rays which have been filtered out of their short wavelength contents.

Why is it that the longer wavelengths dont scatter like this during the day?

They do a little bit, but we just can't see it because a lot more of short wavelength rays get scattered and sent to our eyes. You have to think of this phenomenon as two distinct effects. There is scattering and absorption. Scattering causes all wavelengths to "spread" when passing through the atmosphere (of course, the short wavelengths spread a little more than the longer wavelengths). Absorption causes the luminosity of all wavelengths to decrease (again, short wavelengths have their luminosity decrease faster than longer wavelengths). You have to combine those two effects with the fact that absorption is small when the sun is high in the sky and absorption for short wavelengths is total when the sun is low.

So would it be correct to say that during the day, when the light has less distance to travel to reach our eyes, and as such, scatters less, we see the result of the wavelengths that scatter the most, which is the shorter wavelengths. The longer wavelengths still scatter, but much much less, meaning that not enough of them reach our eyes for us to perceive red/yellow in the sky.

During the morning/evening, the light has to travel a much greater distance, which means the amount scattering in both shorter and longer wavelengths is greatly increased. This increase causes the shorter wavelengths to scatter so much that the absorbtion between all the interactions causes them to diminish in power.The increase in the scattering now also means that the longer wavelengths now actually scatter enough for them to reach our eyes, and we now see red/yellow.

Why does the sky appear whiter at the bottom?



I thought this was due to Mie scattering, which affects all wavelengths equally, causing a kind of desaturation look. But if blue is shorter wavelengths, and whitish is all wavelengths, then wouldnt that mean that theres is mostly shorter wavelenghts higher up in the image, and a greater range of wavelenghts lower in the image?

Wikipedia seems to explain it quite well: http://en.wikipedia.org/wiki/Sunset#Colors

It doesnt answer my last question

It is white at a distance because as the distance increases the length through the atmosphere is so long that you get enough scattering of all wavelengths to make it uniform.

If you reduce this to the simplest approximation, where blue is the only color that scatters and the other wavelengths do not, than all of the sky except for the sun would be the same shade of perfect blue. However in reality the other wavelengths do scatter a little bit, but when looking directly overhead, the distance through the atmosphere is so thin that the blue scattering dominates all of the other scattering.

However as the length through the atmosphere approaches infinity, the sky will become white, because the small amount of scattering of the other wavelengths start to accumulate until eventually everything is completely randomly scattered.

It's the same reason why fog makes everything look white, because it's totally jumbling up all the light wavelengths together because the scattering is very strong.

In short:

1) thin atmosphere doesn't scatter a significant amount of non-blue wavelengths, so these aren't distributed like blue
2) thick atmosphere scatters everything so the color appears uniform.

Do note that the atmosphere is not uniform, it's consistency changes with altitude. Lower layers contain are thicker and contain more dust and water particles than higher layers, which combine to act like a perpetual 'fog' - hence the difference in color.

When you talk about thick atmosphere having this effect, are you talking about the larger particles, ie Mie scattering? Or does Rayleigh scattering have the same effect?

Say you were just modeling the effects of Rayleigh scattering, would you still get the whitening towards the bottom? From what I have read, Rayleigh scattering models the interactions of the smaller particles, so the fact that there are more of the them(in the high density parts), wouldnt make them scatter the longer wavelengths equally.

Is it more a case of just modeling Ralyeigh Scattering would give you an almost even blue sky, which changes in density affecting how blue. And that when adding Mie to this, then you would get the whitening?

It's important to note that in places where there are more particulates in the air, the redness at sunset is even more dramatic. This is why sunsets in hawaii and other volcanic islands are so ridiculously awesome.