I have noticed clouds appearing black during rain. But I don't know what makes clouds to acquire that colour.
This phenomenon doesn't appear every rainfall. There has to be distinction to white rain clouds.
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$\begingroup$ Cross-link to related question: What causes clouds to appear blue? $\endgroup$– WBTCommented Aug 4, 2015 at 14:27
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2 Answers
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Heavy clouds have condensed to the point of large droplet formation, failing the Rayleigh criterion for visible light and so no longer scatter them. It is a case of absorption being higher than reflection/scattering that causes clouds to look dark.
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$\begingroup$ What absorption is occurring? Why is there no color signature for light absorption in this case? Are the clouds dirty, having some content other than air and water droplets? $\endgroup$– Whit3rdCommented Aug 28, 2016 at 6:41
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$\begingroup$ Worth reading this paper which explains the calculation of photon path length through clouds. At each drop some light is scattered and some transmitted (/refracted) through the drop. You need to show how (and how much) each fate affects photon intensity. And whether it depends on droplet size or just total water stored in the cloud $\endgroup$– FlorisCommented Aug 28, 2016 at 12:33
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Clouds scatter light by a process called Mie Scattering, meaning all wavelengths are scattered equally, so clouds are always white unless the light source itself is missing some wavelengths (e.g. at sunset, when only the red/yellow end of the spectrum hits). However, depending on where you are relative to the cloud and how thick with water droplets the cloud is, it can block sunlight, so rather than a change of colour it is in fact a lack of brightness that you are seeing.
It's worth checking out the summary section of this page for further information. You may also notice that sometimes there are dark clouds and light clouds in the same sky, but if you look at the direction of sunlight you'll usually see that the dark clouds are merely in the shadow of other clouds.
