Sky Gradient

[SkepticJ]

So, a little while ago I looked at the bright blue sky we're having here today, and wondered what's up with the gradient in the sky. Why does it have one? The closer to the horizon line one looks, the lighter, whiter-looking the blue gets.

Is it the light reflecting off the ground, back up into the air, and reflecting off of airborne dust?

[grant hutchison]

Even if you removed all the dust, the sky would still show a similar gradient from blue at the zenith to white at the horizon. When you look towards the horizon, you're looking through a long horizontal depth of air, all of it illuminated by the sun. Any bit of air within that column is very likely to scatter blue light towards your eyes, because air is good at scattering blue light. However, that blue light is also very likely to be scattered again before it reaches your eye, so that you never see it. Most of the blue skylight we see therefore comes from close by - within a couple of kilometres of our eyes. In contrast, any bit of air is unlikely to scatter red light towards your eyes, but if red light is scattered towards you, it's unlikely to be rescattered before it gets to you. We therefore see red light coming faintly from the whole depth of the column. Intermediate wavelengths behave in intermediate ways.
If you integrate out all these effects of scattering and rescattering, you find that any sufficiently long column of air will look the same colour as the illuminant: in this case, sun-coloured. The blue signal from nearby adds to the green signal from greater distances, which adds to the red signal from the whole column, and rebuilds the original spectrum. The sky nearer the zenith is not white, because it lacks the visual depth of air to produce the whole effect: the nearby blue gets steadily more dominant as our eyes look through shorter and shorter air columns.
You can actually see the effect of the distant reds and greens isolated for you if you look towards a clear horizon on an otherwise overcast day. The clouds shadow the nearby air which is primarily responsible for the blue shades. The distant sky which is directly illuminated sends you some red, more green, and lots of blue, but the blues are scattered out on their way to your eyes. The result is that the sky looks lemon-yellow or orange under these conditions, even when the sun is high in the sky. A similar effect accounts for the orange-red sky you can see around the horizon during a total solar eclipse.

That said, if you also have fine particulates in the air (water, dust) they'll scatter light in a more uniform manner than air molecules do, and will therefore add white light around the horizon (unless the particulates are themselves very strongly coloured).

Grant Hutchison

[eburacum45]

Very nice explanation, Grant.

Here's a nice article about the colours of the sky, which includes a useful graph showing the way the sky becomes paler and brighter towards the horizon.
http://homepages.wmich.edu/~korista/...ren_Fraser.pdf

If you went to a planet with a thicker atmosphere than the Earth, the pale gradient would reach further up the sky.

[George]

Is it the light reflecting off the ground, back up into the air, and reflecting off of airborne dust?

Though Grant has given the answer, reflected light may add to the intensity, somewhat.

[Nereid]

Another gradient to ponder: the Belt of Venus^WP; on the surface of planet with an atmosphere (dust or no dust), rotating wrt its Sun, will there always be a Belt of Venus?

[George]

Another gradient to ponder: the Belt of Venus^WP; on the surface of planet with an atmosphere (dust or no dust), rotating wrt its Sun, will there always be a Belt of Venus?

[Is ozone present? *wink*]

As long as the shadow contrasts the antiosolar region, why not?

[Ara Pacis]

Not to mention that space is black, so the thinner the atmospheric air column, the darker it will seem. This is why high flying aircraft use black paint as camoflage, because space is black and they want to blend in with that background.

[HenrikOlsen]

... This is why high flying aircraft use black paint as camoflage,...

Very dark blue actually.