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I was taught in school that clouds are white due to the scattering of light. Since all rays are reflected it appears as white. But I am wondering about rain clouds. Why are rain clouds darker?

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Rain clouds are dark because the part of the cloud you see is in the shade.

Clouds are white because they contain tiny water droplets that scatter light of all colors equally in all directions. "Scatters light of all colors equally in all directions" means "white".

But if you put a layer of white stuff over another layer of white stuff, the top layer will scatter light from the Sun, reflecting a lot of it into space. That means there's less left to light up the layer underneath. Compared to the top layer, the bottom layer will look darker.

For a cloud to produce rain, it needs to be fairly tall (thick). That means the upper parts of the cloud reflect away most of the sunlight, leaving the lower parts in the shade. If you're under the cloud, the lower part is all you see -- and it looks dark.

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  • $\begingroup$ Is this the only reason? I'd have imagined that the dust particles that allow the droplets to form in the first place to play some kind of role too. $\endgroup$ – Denis de Bernardy Nov 16 at 17:16
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    $\begingroup$ @DenisDeBernardy I don't think the dust matters. After all, rain clouds look beatifully white when looked at from afar (like the typical vertical cloud towers that turn into thunderstorms). The same cloud can look pitch black from beneath, if it's large and dense enough. $\endgroup$ – cmaster Nov 16 at 17:46
  • $\begingroup$ @cmaster: What prompted me to ask is the amount of dirt I need to wipe off my white plastic chairs after it rains. I don't live in a particularly polluted location (on the contrary), and there's little dust around where I live. The rain is as transparent as in other places. Yet the thin layer of dirt on my garden chairs prompts me to wonder if there might not be a tiny bit more to it than just thickness, given how water drops only form around, you know, other particles. It would still be compatible with the upper parts of clouds being white. $\endgroup$ – Denis de Bernardy Nov 16 at 17:56
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    $\begingroup$ @DenisdeBernardy It's true that microscopic dust particles in the atmosphere provide the nucleation sites on which water vapor can condense into droplets to become clouds. But, the dirt you see on your furniture after rain is more likely dust that has already settled there before it rained; the rain just rearranged it into larger particles that have become more visible, combined with some of the rain water that hasn't yet evaporated. $\endgroup$ – Anthony X Nov 16 at 19:47
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    $\begingroup$ Do you have sources? The first part seems to be clear, this is called Mie scattering. Why do you not think that there is a transition from Mie Scattering to absorption when the droplet size increases? (I dont know if this is the case) $\endgroup$ – lalala Nov 18 at 11:34
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This is due to the scattering of light.As the density increases scattering increase and light rays coming from sun dispersed among them. So the lower part of that clouds appear as darker but above part will be white, you can observe it on your flight, but you have to grab a window seat for a beautiful view

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Rain clouds have larger droplets, which decreases the cloud's albedo (i.e. the fraction of light “reflected” by the cloud).

From https://en.wikipedia.org/wiki/Cloud_albedo:

The smaller the drops and the greater the liquid water content, the greater the cloud albedo, if all other factors are the same.

And a few lines lower:

In macrophysically identical clouds, a cloud with few larger drops will have a lower albedo than a cloud with more smaller drops.

Ironically, according to the same Wikipedia article, aerosols have a tendency to increase the cloud albedo (make the cloud appear brighter) because they decrease the droplet size.

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