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?

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    $\begingroup$ You can check the actual color of the Sun if you use a solar filter to look at the solar disk when it's high in the sky. (spoiler: it's white) $\endgroup$
    – Ruslan
    Apr 5 at 13:10
  • $\begingroup$ physics.stackexchange.com/questions/147639/… $\endgroup$
    – ProfRob
    Apr 5 at 18:35
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    $\begingroup$ Sun is white. What else would it be? What else would white even mean? :D Don't forget white is not a physical colour - it's a perception. It changes. $\endgroup$
    – Luaan
    Apr 5 at 20:11
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    $\begingroup$ The Sun is white almost by definition. If the Sun was of different temperature or in some other way emitted different spectrum, then we would still call it white. Under this hypothetical Sun the evolution would make us perceive its light as the most neutral and natural, this would be the white. But even if our current Sun instantly changed its color slightly, our brains would adapt. In my room I have two bulbs of slightly different tints. At night I can perceive any one as white, if only they don't work together. "White" is an impression. $\endgroup$ Apr 6 at 23:36

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.

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    $\begingroup$ In fact, the Sun is yellower than the sRGB white point. It's easy to see by comparing the effective temperature of the photosphere, 5777K, with CCT of the sRGB white point, 6500K. $\endgroup$
    – Ruslan
    Apr 5 at 13:04
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    $\begingroup$ @Ruslan you mean “the CIE standard illuminant D65 has a higher colour temperature than sunlight”. That's true, but a) they're actually pretty similar b) D65 can't by any means claim to be “the” standard white – it was originally based on overcast-day light, but the reason it found its way into sRGB is probably because fluorescent lights were dominant in 1990's office space, where most monitors were placed. Arguably, D65 is more of a blueish white than 5777K is yellowish, though of course there is no absolute definition for either of this. $\endgroup$ Apr 5 at 14:22
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    $\begingroup$ ...though indeed this leads us to a question-relevant point: why is daylight blueish in overcast weather? $\endgroup$ Apr 5 at 14:26
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    $\begingroup$ @Ruslan: And if you put your monitor outside in the sunshine, your #ffffff will look blueish. What is white depends on the illumination. $\endgroup$ Apr 5 at 15:15
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    $\begingroup$ @Mark The Earth as a whole (including oceans and atmosphere) is about 0.03% Hydrogen by mass, whilst the amount of helium is negligible. 99.97% is pretty damn near 100%, though the distinction is pretty important given we are each about 10% hydrogen by mass! $\endgroup$
    – stuart10
    Apr 6 at 9:19

Sunlight is "white". Blue sky is due to Rayleigh scattering, where the intensity of scattered light depends on the fourth power of the frequency, then this is why the sky is blue, since blue is in the upper bound of the visible spectrum in frequency. This is because the molecules in atmosphere have a size much smaller than the wavelength of light.

The clouds contain droplets of water that are bigger (1-100 $\mu m$) than the particles mentioned before. Since here the size of droplets is bigger than the wavelength of light, we have to use Mie scattering. And in Mie scattering we have that the wavelengths are equally scattered in all directions and that determines the white color.

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    $\begingroup$ Why the quotes around "white"? $\endgroup$
    – void_ptr
    Apr 5 at 14:49
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    $\begingroup$ Presumably because white light is actually a collection of colors. You have to know this in order to understand Rayleigh scattering. $\endgroup$ Apr 5 at 15:27
  • $\begingroup$ since the "sky" is blue because of Rayleigh scattering shouldn't then the color of clouds depend on their altitude? $\endgroup$
    – hyportnex
    Apr 6 at 21:13
  • $\begingroup$ The color of the sky change with altitude, becoming darker, because the density of the atmosphere and other parameters change. Then even the brightness or the color of the clouds are influenced by the background, mostly if we are talking about thin clouds. Also it depends from the point of view: position of the sun, and position of the observer. $\endgroup$
    – Mark_Bell
    Apr 6 at 22:22
  • $\begingroup$ @hyportnex yes, it does depend on altitude.. $\endgroup$
    – Ruslan
    Apr 6 at 23:00

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