Clouds. Why on a clear beautiful day are some pure white and others are black?

Question

You know, I never thought about this until just this afternoon when I was running some errands. It's absolutely beautiful out, blue skies, clear as a bell, lots of clouds..... But some of them are black and some are white and I never thought to wonder why that is? I want to say it's the weight of moisture in some clouds and not others but seriously?

Why is this?

Answer 1

It is the moisture content of the clouds.  Higher content, more light is refracted and thus a darker or less translucent the appearance.  

Answer 2

the darker they are the more built up moisture or whatever they have in them.

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Answer 3

The amount of water and dust and now pollution.

Answer 4

The taller and thicker clouds absorb and reflect more solar radiation because of their higher moisture content.

Bigger Clouds = More Moisture

Answer 5

Clouds are white where sunlight is shining on them.  The tops in sunshine will be white and the bottoms will be black or dark because the upper part of the cloud isn't allowing sunlight to penetrate.