Nib asked for
Why Clouds Are White
Cloud droplets are far bigger than air molecules, so they scatter every color about equally, and sunlight comes back out still mixed, which looks white.
4 steps, drawn by Nib. Read it here, or watch him draw it. Updated .
1size matters
Everything so far depended on air molecules being tiny, much smaller than the wavelength of light. That tininess is what makes blue scatter far more than red. A cloud is different: it is packed with water droplets, each many times larger than a molecule and comparable to or bigger than a wavelength of visible light.
Clouds are made of droplets, not molecules, and droplets are much bigger.
a common mix-upClouds are just thicker blue sky, so they should look blue.
Which scatters light more strongly by color: a tiny molecule or a large droplet?
Show the answer
The tiny molecule, because the strong favoring of blue only works when the scatterer is much smaller than the wavelength.
2no favorites
When the scatterer is this large, the strong preference for short wavelengths disappears. A droplet redirects red, green, and blue by about the same amount. No color is favored, so no color gets removed from the mix more than any other.
Large droplets scatter all colors about equally.
3balance survives
Sunlight is white because it contains all colors in balance. If a droplet scatters every color equally, that balance is preserved in the light it sends out. Equal parts of every color is exactly what white is, so the scattered light is still white.
Equal scattering keeps the mix balanced, and a balanced mix is white.
If a droplet scattered red twice as much as blue, what color would the cloud look?
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Reddish, since the balance would tip toward red; clouds are white only because the scattering is even.
4the cloud
A cloud holds billions of droplets, so light bounces from one to the next, changing direction again and again until much of it exits back toward your eyes. Thanks to all that scattering, sunlit clouds look bright white. Thick, dense clouds let less light through, so their undersides look gray.
Countless droplets bounce white light every which way, so the cloud glows white.
a common mix-upGray clouds are a different material; really they are the same droplets with less light getting through.
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.