Why and what makes clouds opaque?

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I know that they are just water droplets in the sky, but why is it opaque?

In: Chemistry

So there’s a fun feature in physics called Mie Scattering, and it only works in spherical drops. If the drops and the wavelength of light are about the same size, then all colors (wavelengths) of light scatter about equally.

In even simpler terms, the length of the waves of light in a cloud are about the same size as the water droplets, so all the colors bounce out into other directions about the same amount. when all these colors reach your eyes, you register that the cloud is white.

The actual process is so complicated that my grad school course in electrodynamics didn’t bother cover it in depth, but hopefully that was clear enough.