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Clouds can be a variation of grays, from white to some dark gray, almost black. They are clearly grayscale, so it means they reflect all colours uniformly, but some clouds are whiter than others, so the reflectance varies. What makes a cloud to be darker than an another? Typically rain clouds are darker, is the tone a measure of how much "water" they are carrying?

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    $\begingroup$ You're seeing differences in lighting, not differences in the clouds themselves. Large clouds block a lot of sunlight. $\endgroup$ – BowlOfRed May 20 '16 at 7:09
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    $\begingroup$ A more interesting question might be what causes supercells/huge thunderstorms to appear green? I don't really know the answer to that though :p $\endgroup$ – John T. May 20 '16 at 18:10
  • $\begingroup$ "Typically rain clouds are darker..." Also typically, a white house looks darker while overshadowed by a cloud... and in general white objects look darker when there is less light falling on them. $\endgroup$ – Asher May 21 '16 at 22:09
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You are just seeing differently illuminated clouds, as should be clear from this picture of an aircraft flying over a thunderstorm:

enter image description here

Clouds are made of water droplets or ice crystal, which are transparent. When light is incident upon a cloud, it is scattered at the same wavelength (a process known as Mie scattering). This means that it remains of the same color.

Look at this picture of clouds at sunset: they are red because they are scattering red sunlight. The upper part appears more gray-ish because it receives less light. It is clear from the illumination of the clouds that light is coming from below (the upper part is darker) because the Sun's position is low.

enter image description here

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  • $\begingroup$ That is not what I was asking. My question is about the amount of light reflected, when they are white and when they are dark. $\endgroup$ – user May 21 '16 at 15:21
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    $\begingroup$ Uhm...I'll try to be clearer. Clouds are made of water droplets, which are transparent. They scatter light and so appear to be of the color of the incident light. There are no dark or white clouds: dark clouds are poorly illuminated clouds and white clouds are clouds illuminated with white light. $\endgroup$ – valerio May 21 '16 at 21:42

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