Why does the Sun appear white through clouds? It seems there should not be any absorption of, say, the reddish component, as this would not produce a white colour. So what is going on? Am I right that clouds are white due to direct yellow light from the Sun and scattered blue light arriving from all directions?
[Physics] Why does the Sun appear white through clouds
atmospheric sciencespectroscopysunvisible-light
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The sky does not skip over the green range of frequencies. The sky is green. Remove the scattered light from the Sun and the Moon and even the starlight, if you so wish, and you'll be left with something called airglow (check out the link, it's awesome, great pics, and nice explanation).
Because the link does such a good job explaining airglow, I'll skip the nitty gritty.
So you might be thinking, "Jim, you half-insane ceiling fan, everybody knows that the night sky is black!" Well, you're only half right. The night sky isn't black. The link above explains the science of it, but if that's not good enough, try to remember back to a time when you might have been out in the countryside. No bright city lights, just the night sky and trees. Now when you look at the horizon, can you see the trees? Yes, they're black silhouettes against the night sky. But how could you see black against black? The night sky isn't black. It's green thanks to airglow (or, if you're near a city, orange thanks to light pollution).
Stop, it's picture time. Here's an above the atmosphere view of the night sky from Wikipedia:
And one from the link I posted, just in case you didn't check it out:
See, don't be worried about green. The sky gets around to being green all the time.
I get that the sun is producing white light which is scattered threw our atmosphere so that the light of the sun reaching our eyes is yellow.
Not very much. When the sun is high in the sky, most would describe the light as "white", not "yellow". That would be more true for a sun low to the horizon.
So how come if I look to a piece of white paper under sunlight or a cloud in the sky, I see it white ?
The human visual system does not have a fixed mapping from input wavelengths to color perception. There are many types of "filters" in place between the two. In particular, we have an automatic "white balance" system. When you walk from a room with a single incandescent light to outside, the light hitting your clothes is very different, but you will still identify (mostly) the same colors in both places.
This system isn't foolproof, but works very well for most lighting systems you find.
Best Answer
It's kind of a funny misconception that the sun is yellow. I mean, astronomically speaking it is indeed a yellow star, more precisely G-type main sequence / yellow dwarf... but don't be fooled by the terminology: astronomically speaking, you'll also find that the Earth consists completely of metal!
Actually you should consider the sun as white.
The main reason, strangely enough, why we think the sun is yellow is that we never look at it. That is, directly enough to judge its colour. When the sun is high in a cloudless sky, it's just too bright to see its colour (and evolution has trained us to not even try, because it would damage the eyes). Only near sunrise or sunset do we actually get to look at the sun, but then it's not so much the colour of the sun but the colour of the atmosphere we're noticing – and the atmosphere is, again counter to perception, yellow-orange-red in colour. Well, not quite – the point is that the atmosphere lets red / yellow light through in a straight line whereas bluer frequencies are more Rayleigh scattered. That's the reason why the sky is blue, and also adds to the perception of the sun being yellow: it's yellow-ish in comparison with the surrounding sky colour.
When you see the sun through clouds, you get to see its actual colour more faithfully than usual, both because (as Mark Bell wrote) Mie scattering doesn't have the colour-separating effect that Rayleigh scattering does, and because you then see it against a grey / white backdrop instead of against the blue sky.