# Why are Clouds white?

Why are clouds white and rapids white, same kind of thing. I was reading about it and it said something about the EM wave inducing a dipole in the water. If white light shines through a large body of water it doesn't look white. So does it have something to do with them being in droplet form. I am looking for an answer on the atomic scale and how Maxwells equations describe the interaction with the water. Any input will be much appreciated. I guess i just don't understand the mechanism behind scattering.

Dale
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Water is transparent to visible light, so clouds are white because they scatter all wavelengths approximately equally.

A proper surface and a refractive index. In this case, the ice in the clouds are refracting light.

When the EM wave travels through the ice, does it create surface currents?

What?

Dale
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Probably not much free currents, but polarization currents certainly. However, any time the speed of a wave is different in one medium vs another you will get refraction, and if you refract and reflect off of a bunch of surfaces in different directions then you get scattering.

DaveC426913
Gold Member
A proper surface and a refractive index. In this case, the ice in the clouds are refracting light.

For the record, almost all clouds do not have ice, they have water droplets. (For ice, you're looking at cirrus clouds - the high altitude wispy mare's tails that portend a cold front.)

Probably not much free currents, but polarization currents certainly. However, any time the speed of a wave is different in one medium vs another you will get refraction, and if you refract and reflect off of a bunch of surfaces in different directions then you get scattering.

By polarization currents you mean $$D=\epsilon_0E+P$$
When the EM wave is in the medium it is constantly inducing electric dipoles, And do these dipoles affect the E and B components of the EM wave?

Dale
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Yes. That is essentially the reason why the E field is different from the D field.

D H
Staff Emeritus
For the record, almost all clouds do not have ice, they have water droplets.
Those water droplets are very, very small. Average size is about 10 microns, but some are micron-sized or less -- about the same size as typical visible light wavelength. This means that Mie scattering is significant in clouds, and it is Mie scattering that explains why clouds are white.

I don't konw either. BUt my guess is that there are so many of the small droplets. The light dispersed by droplets would simply mix together.

Color of sky is caused by polarization of reflection, where as polarization of refraction in the water droplets are mixed (exactly the seven colors of rainbow which combine into white).

Water is transparent to visible light, so clouds are white because they scatter all wavelengths approximately equally.

I'm not quite with this explanation, because this contradicts with phenomenon of rainbows. So in my guess, Rainbow is because of a very thin layer or spherical shell of water that light doesn't mix that much.

Yes. That is essentially the reason why the E field is different from the D field.

Here I think it's just the difference between free E field and net E field

Anyways, just some guesses.

D H
Staff Emeritus
Water is transparent to visible light, so clouds are white because they scatter all wavelengths approximately equally.
I'm not quite with this explanation, because this contradicts with phenomenon of rainbows. So in my guess, Rainbow is because of a very thin layer or spherical shell of water that light doesn't mix that much.
Don't just guess! Google is your friend. A simple search would cut down on the guessing.

Rainbows form when light scatters just once (primary rainbow) or twice (double rainbow). Clouds are white because light passing through a cloud scatters many, many times, and does so in a complex manner due to the tiny size of the droplets. Those tiny droplets are about the same size as a typical visible light wavelength, so Mie scattering is a key feature of what is going on inside of clouds.

Don't just guess! Google is your friend. A simple search would cut down on the guessing.

Rainbows form when light scatters just once (primary rainbow) or twice (double rainbow). Clouds are white because light passing through a cloud scatters many, many times, and does so in a complex manner due to the tiny size of the droplets. Those tiny droplets are about the same size as a typical visible light wavelength, so Mie scattering is a key feature of what is going on inside of clouds.

But I still don't think that explanation from DaleSpam is sufficient for this question. I know about physics behind primary and double rainbow, they are caused by refraction and total internal reflection. So what I'm thinking here is that these monochromatic lights mix into white light, as cloud is much thicker than layers of single droplets.

D H
Staff Emeritus
I gave the answer, two times now. Google the term "Mie scattering".

Dale
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I'm not quite with this explanation, because this contradicts with phenomenon of rainbows. So in my guess, Rainbow is because of a very thin layer or spherical shell of water that light doesn't mix that much.
My answer doesn't contradict rainbows. Those are generally caused by larger droplets of water where there is nice clean refraction and internal reflection rather than scattering. They are different processes that depend on different sizes of the droplets. This is why you see rainbows more around your sprinkler than around your teapot.

My answer doesn't contradict rainbows. Those are generally caused by larger droplets of water where there is nice clean refraction and internal reflection rather than scattering. They are different processes that depend on different sizes of the droplets. This is why you see rainbows more around your sprinkler than around your teapot.

But rainbow is similar to prism also. How can rainbow be created if droplet diffract different frequencies exactly the same? Like what I said total internal reflection, refraction only create primary and secondary rainbows. But the difference in color is dispersed by droplets.

DaveC426913
Gold Member
Zeal: clouds are formed of droplets in the microns size, very close to the wavelength of light where Mie scattering occurs. Rainbows are formed of droplets many times larger than the wavelength of light, where normal refraction occurs.

You see clouds all the time but you only tend to see rainbows when there's actual rain.

I learned about Mie scattering with a 30 second visit to Wiki.

Dale
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2020 Award
How can rainbow be created if droplet diffract different frequencies exactly the same?
Diffraction is not relevant AFAIK. The small water droplets scatter all frequencies approximately the same. The larger droplets refract different wavelengths differently. You are mixing up different phenomena. Refraction is different from scattering is different from diffraction.

Diffraction is not relevant AFAIK. The small water droplets scatter all frequencies approximately the same. The larger droplets refract different wavelengths differently. You are mixing up different phenomena. Refraction is different from scattering is different from diffraction.

Sorry, I mean refract, misprinted. But why doesn't scattering like that won't cause polarization of light? Because light scattered by air molecules is polarized.

Dale
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2020 Award
I don't know anything about the polarization of scattered light. Sorry. Perhaps the link on Mie scattering has something.

I don't know anything about the polarization of scattered light. Sorry. Perhaps the link on Mie scattering has something.

Sorry, I don't know either, I just can't quite understand it, so I'm asking it. Because air molecules polarizes light by reflection. But the reflection of light is similar pattern to the mie scattering that I looked at. Sorry for my deficiency of knowledge in this area. But I just don't quite get it. Probably I can start a thread about that.

Redbelly98
Staff Emeritus
Homework Helper
The number of scattering events is important too. For just one scattering event (rainbows, blue sky), polarization and color-separating effects are easily visible.

For many many scattering events (clouds, whitewater), the polarization gets randomized, and colors get mixed together.