# Why are Clouds white?

1. ### cragar

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.

### Staff: Mentor

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

4. ### Bloodthunder

174
A proper surface and a refractive index. In this case, the ice in the clouds are refracting light.

5. ### cragar

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

174
What?

### Staff: Mentor

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.

8. ### DaveC426913

16,439
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.)

9. ### cragar

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?

### Staff: Mentor

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

### Staff: Mentor

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.

13. ### ZealScience

381
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).

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.

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

Anyways, just some guesses.

### Staff: Mentor

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.

15. ### ZealScience

381
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.

### Staff: Mentor

I gave the answer, two times now. Google the term "Mie scattering".

### Staff: Mentor

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.

18. ### ZealScience

381
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.

19. ### DaveC426913

16,439
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.

### Staff: Mentor

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.