Why is the sky blue?

On a clear day, the sky appears to be more blue toward zenith than it does towards the horizon. why?
I thought it was because the atmosphere is denser higher up than it is at the Earth's surface but a friend told me "the temp of the upper atmosphere is higher than its at the Earth's urface and that's why?" is he correct? we did some research on google and it kind of talked about both but he still argues he's right and I am wrong. So which is it?
dx

 

[Hyperreality]

I think your explanation is more correct than your friend's, but I do not think it is the correct one.

I think it has something to do with the rotation of the Earth and the Doppler Effect. During a day, a particular region of the surface of the Earth will get closer and closer to the sun in the morning, and further away from the sun as it approahces afternoon due to its rotation aroundthe Earth. As the distance between the particular region of the surface of the Earth and the Sun changes, the wavelength ofthe sunlight that shines on the surface of the Earth changes. As the sun approaches the particular region of the Earth, the wavelength of the sulight is "compressed", and that means the frequency of the sunlight is increased and therefore the sky will appear to be blue during most of the day. And as the particular region of the surface of the Earth turns away from the Sun, the wavelength of the sunlight is "stretched", therefore the colour of the sky durgin sunrise and dawn appears to be red.

You shold really look up Rayleigh's Blue Sky Law. But I'm pretty confident my explanation is correct.

 

[HallsofIvy]

That response was a joke, right? The "blue sky" remark was to let us know that, right?

Just on the chance that someone didn't realize that: light waves are not "compressed" or "stretched" passing through the atmosphere. The distance from the Earth to the sun is over 93 million miles- the atmosphere directly over head is not signifcantly "closer" to the sun than any other part.

By the way, DX, the upper atmosphere is LESS dense, not denser. However, temperature also has nothing to do with the color of the atmosphere.

 

[arcnets]

More blue? WHOA!
Well, every painter knows (or should know) that a cloudless sky appears brighter towards the horizon, and darker towars the zenith. I think this is because near the horizon, the line of sight crosses more distance inside the atmosphere, i.e. there are more light-scattering particles in the line of sight.