Eli5: If water is transparent, why are clouds white?
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Clouds are white because they are made up of tiny droplets of water or ice crystals that scatter light in all directions. This is known as the Tyndall effect, and it causes the clouds to appear white to our eyes.
Tyndall effect is a phenomenon that occurs when light passes through a mixture of substances, such as water and air. The light is scattered by the tiny particles within the mixture, causing the light to appear different.
In the case of clouds, the Tyndall effect causes the light to be scattered by the tiny droplets of water or ice crystals that make up the clouds. This scattering causes the clouds to appear white to our eyes. The same effect can be observed when a beam of light passes through a glass of water, the beam of light appears to be scattered and the water looks cloudy.
It’s also important to note that the Tyndall effect is not limited to clouds, but can be observed in other natural phenomena such as the blue color of the sky and the brown color of the smog.
Additionally, the sun is reflecting off the clouds, making them appear white.
It’s also important to note that clouds can also appear gray, depending on their density and the amount of light they are reflecting.
Because transparent things in tiny bits look white. Whenever light enters or exits a droplet, it changes direction. When it has to go through billions of droplets, it changes directions so many times that it’s essentially being randomly scattered, which is the same thing that white objects do to incoming light. It’s the same reason snow is white while ice is clear.
Saran wrap is transparent.
Now crumple a bunch of it up into a ball.
It will now look white.
A saran wrap ball, much like a cloud, has a lot of surfaces in it.
It’s not just a solid ball: it’s got tiny (sometimes microscopic) pockets of air.
Light goes through it, comes back out, has to go through it again, come back out again.
That makes it much less transparent than if it were just a solid mass.
Similarly, with clouds, they’re not just solid water.
They’re tiny little droplets of water with air gaps between them, so light has many surfaces to pass through.
When light hits the object, it can either be absorbed, reflected, or pass through (in which case it is refracted, i.e. changes direction).
Depending on type of medium, all 3 can happen at varying degrees. When light hits water, a little bit gets absorbed because water is not 100% transparent, some of it gets reflected, and some of it passes through and gets refracted.
Since water in the clouds is not a straight wall of water, but rather tiny droplets scattered all over the place, light reflects and refracts in random directions, so you completely lose the picture of whatever is on the other side of that cloud.
Because it’s not a cloud of water, it’s water vapor, tiny droplets. When light hits a mist of tiny droplets, the light scatters.
When’s the last time you saw transparent steam?
Light can act in one of three ways when it encounters an interface between two substances (passes from air into a liquid or solid, say): It can reflect, it can transmit (pass through), or it can be absorbed. Which will happen depends on three main things: the wavelength of the light (its energy level, color), the range of energy absorption in the substance(s) – everything has energy to its bonds and electrons and will absorb energy in those ranges, although it might later also re-emit that light, possibly at a different energy level (different color), and the angle that the light is hitting the interface from (at some angle, the light cannot cross the interface and must either absorb or reflect).
It is this last option that makes clouds seem white to us. The surfaces of the water droplets and ice crystals in the clouds provide a lot of surface to the light which is too much of an angle to allow passage, so the light reflects. Scattering of the light usually appears to us as white or gray.
Basically, just like the water in foamy waves seems white to us even though the water is blue in color (lots of light scatter because the angles that the light is hitting are random and in all possible ranges).
Just as an aside, water is not fully transparent, it absorbs color down on the red end of the spectrum more than on the blue end, although the absorbance is weak so the water seems mostly transparent to us except when light passes through a lot of water, which is why there is a depth in water that goes dark, light never gets down that far. This is also why water is a greenhouse gas, a fairly good one actually, because it absorbs infrared energy really well (re-emits it later).
This thing with light scatter is also why sometimes quartz crystals seem clear and sometimes looks white (milky quartz). It is also why glass that is scratched or cracked gets hard to see through, and why mirrors need to be a really smooth surface to work well.
Water isn’t transparent. Ever looked down into a deep river?
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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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