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Why is the sky blue, and sunsets red? (for a curious adult who has never studied physics). Cover, in this order: What is white light made of?; What does air do to a beam of light?; Why does blue light scatter more than red?; So why isn't the sky violet?; Why are sunsets red?; Why are clouds white? — page 6: Why Clouds Are White
Air molecules are tiny. A cloud is different: it's full of water droplets, vastly bigger than any molecule, bigger even than a wavelength of light.
A droplet is so big that it scatters red, green, and blue about equally, so no colour gets favoured the way small molecules favour blue.
Sunlight looks white because it holds every color mixed in equal amounts. When a droplet scatters all of them evenly, that balance survives, so the scattered light still looks white.
A cloud has billions of droplets. Light ricochets between them until it escapes toward you, still white. Thick clouds let less through, so their bases look gray.
So the sky is blue because tiny molecules favor blue, and clouds are white because big droplets favor nothing. The color you see depends on the size of what scatters the light.