Why Does the Sky Appear Blue During the Day?

[DaveC426913]

No, it is specifically Rayleigh scattering, and how our human eye has evolved, that are the significant factors. Zz.

What does evolution of the human the human eye have to with anything?

Is there some suggestion that
- animals (those with colour vision) do not see a blue sky?
- cameras do not see a blue sky?

If there were no humans (or animals, or cameras) on the planet, the blue wavelengths of light would still preferentially reach the ground.

 

[Farsight]

Funny thing colour. It's only in our heads. What's real is the frequency. Maybe. Anyhow, I think it's to do with our Red/Green/Blue receptors:

http://www.exploratorium.edu/snacks/colored_shadows.html

http://en.wikipedia.org/wiki/Eyeball

If I shine a blue spotlamp on to a screen and overlap it with a purple spotlamp, I wonder what colour I see? I must try this out.

 

[Claude Bile]

What does evolution of the human the human eye have to with anything?

Is there some suggestion that
- animals (those with colour vision) do not see a blue sky?
- cameras do not see a blue sky?

If there were no humans (or animals, or cameras) on the planet, the blue wavelengths of light would still preferentially reach the ground.

Not true, violet and UV would preferentially reach the ground. In order to explain why we observe the sky to be blue, you need to take the colour responses of the human eye into account.

Cameras see a blue sky, because their RBG algorithms are designed to mimic the response of the human eye. A much more objective measurement would be with a spectrometer, such a measurement clearly shows significant amounts of violet and UV radiation scattered toward the surface in addition to blue.

I think this paper may be of interest to some (also contains the spectrum of the daytime sky I referred to) -

"Human colour vision and the unsaturated blue colour of the daytime sky" Glenn S. Smith, American Journal of Physics, 73 (7), 590-597 (2005).

Claude.

 

[vinter]

hey hey! can someone answer this please - why is the sun red in the evening? isn't THAT also because of scattering? why does scattering sometimes prefers red and sometimes blue?

 

[Farsight]

The scattering means the shorter bluer wavelengths get scattered off sideway more than the longer redder wavelengths.

So when you look up at the midday sky, you're seeing blue wavelengths scattered to one side. When you look straight at the sun in the evening through a lot of air you see more of the longer redder wavelengths because the blue stuff get scattered off to one side before it got to you.