Why is the sky a different shade of blue at different places?

Question

We all know that the sky is blue, but I have seen the color of the sky in the day (because during sunrise and sunset it is obviously different) it is not the same shade of blue across places. Why is that? There isn’t any fixed shade of blue which we can say is the color of the sky.

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Answer 1

It depends on where you are, where the sun is, and what’s in the sky.

The sky will scatter the light from the sun, that’s why you see the sky at all and not blackness with the other stars. It actually scatters starlight and moonlight too, but since they’re so much dimmer we don’t notice much. If you’re careful, you could notice a difference between the brightness of the sky in full moon vs new moon, but it’s tricky to do that comparison when you have to wait two weeks to do it.

The sky scatters different colors of light differently, with blue being much much more strongly scattered than red. This is really noticeable at sunrise and sunset, as you say, because the light goes through a lot of air. Looking at or near to the sun, you see the red light coming through straight and the blue is missing as it’s been scattered away before it got to you.

But this effect doesn’t just stop at other times of day. At midmorning, the sun might be up and to your left. If you look near it, the sky will be more whitish blue than the blue of the rest of the sky, and for the same reason. The direct path to you scatters more blue than red, so it looks a little less blue.

Now look up and to your right, so that the angle between the sun, you, and your new gaze is at 90 degrees to the sun. Now, *only* the light that scatters directly at 90 degrees hits you, and that will be almost all blue, so the most intense, bluest sky will be there, because it’s also a short path after it scatters once.

Polarized sunglasses will enhance this effect, as the scattering will polarize the light when it bounces.

This all ignores things like water vapor and clouds, which cause a different kind of scattering that is uniform in color and looks white. A thin wispy white in part of the sky will change its apparent color, even if you don’t identify it as being an obvious cloud.