Eli5: If water is transparent, why are clouds white?
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Have a look at the side of a glass full of water. It’s transparent, yes, but distorted. Like a magnifying glass, the curved surface where it switches from air to glass and water makes the light bend, which makes things look all, well, bent. But the glass is big enough that you can still make things out through it.
Now imagine a hundred glasses of water in a big cluster on a table, and imagine looking through them all. The image would be so scrambled, it would be like trying to watch free porn on cable. (Hey, five-year-olds: don’t try to watch porn. Also, stop living 20 years in the past, it’s weird.)
A cloud is made of up a *huge* number of *very* small droplets of water. They’re curved, like the glasses, so each one distorts light. And because there are so many, they’re absolutely impossibly scrambled. We say they “scatter” light, which means they mix it up so much we could never see what the original image was, like squirting colors of paint into a cup and stirring them together. And like the paint, what we get depends on what colors of light went in.
Now, in the sky, you might think that would make a cloud blue, because you’d only be stirring up the blue light from the sky behind it. But it’s not just the blue sky that adds light to a cloud, because the sun shines directly on a cloud too! So it’s all of the colors that sunlight makes, which, mixed together, give you white.
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At sunset, though, the sunlight has more air to go through before it gets to the clouds that you can see. The sun is farther away from *your* clouds now and closer to other people’s clouds—people in a different timezone, where it’s not sunset yet. Redder light has an easier time getting through all that air (for reasons best left to another answer), which means the light that reaches your clouds is redder. When the clouds mix up *those* colors, you get a beautiful sunset!
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