Why Do Clouds & Rapids Look White?

  • Thread starter cragar
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In summary: Mie scattering is the only scattering mechanism that can produce white 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... why is it that Mie scattering is the only scattering mechanism that can produce white clouds?I'm not sure what you're looking for. In summary, clouds are white because they scatter all wavelengths approximately equally. Mie scattering is the main scattering mechanism that produces white clouds.
  • #1
cragar
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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.
 
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  • #2
Water is transparent to visible light, so clouds are white because they scatter all wavelengths approximately equally.
 
  • #3
thanks for your answer, what causes something to scatter light?
 
  • #4
A proper surface and a refractive index. In this case, the ice in the clouds are refracting light.
 
  • #5
When the EM wave travels through the ice, does it create surface currents?
 
  • #6
What?
 
  • #7
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
Bloodthunder said:
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.)
 
  • #9
DaleSpam said:
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 [tex] D=\epsilon_0E+P
[/tex]
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?
 
  • #10
Yes. That is essentially the reason why the E field is different from the D field.
 
  • #11
ok thanks for your answer
 
  • #12
DaveC426913 said:
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.
 
  • #13
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).

DaleSpam said:
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.

DaleSpam said:
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.
 
  • #14
ZealScience said:
DaleSpam said:
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.
 
  • #15
D H said:
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.
 
  • #16
I gave the answer, two times now. Google the term "Mie scattering".
 
  • #17
ZealScience said:
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.
 
  • #18
DaleSpam said:
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.
 
  • #19
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.
 
  • #20
ZealScience said:
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.
 
  • #21
DaleSpam said:
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.
 
  • #22
I don't know anything about the polarization of scattered light. Sorry. Perhaps the link on Mie scattering has something.
 
  • #23
DaleSpam said:
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.
 
  • #24
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.
 

1. Why do clouds and rapids look white?

The main reason for the white appearance of clouds and rapids is due to the scattering of sunlight. Clouds and rapids are made up of tiny water droplets or particles that are suspended in the air or water. When sunlight hits these particles, it gets scattered in all directions, making the clouds and rapids appear white to our eyes.

2. Do all clouds and rapids look white?

No, not all clouds and rapids look white. The color of clouds and rapids can vary depending on several factors such as the size and shape of the particles, the angle of the sun, and the amount of water vapor or sediment present. For example, storm clouds may appear dark due to their thickness and the presence of debris, while white rapids may appear more blue or green due to the reflection of the sky.

3. Can clouds and rapids also appear different colors?

Yes, in certain conditions, clouds and rapids can appear different colors. For instance, at sunrise or sunset, the sunlight has to pass through more of the Earth's atmosphere, causing longer wavelengths of light to be scattered. This can give clouds and rapids a pink, orange, or red hue. Additionally, if there are pollutants or smoke particles present, clouds and rapids may appear gray or brownish in color.

4. Why do some clouds and rapids have a silver lining?

The silver lining that is sometimes observed on clouds and rapids is another effect of sunlight scattering. When the sun is at a certain angle, the tops of the clouds and the highest points of the rapids may be illuminated more than the rest of the cloud or water. This creates a bright, silver appearance on the edges, giving the clouds and rapids their characteristic silver lining.

5. Is there a connection between the color of clouds and rapids and the weather?

Yes, there can be a connection between the color of clouds and rapids and the weather. For example, dark, thick clouds often indicate an impending storm, while white, puffy clouds may indicate fair weather. Similarly, white rapids may suggest a strong current, while darker rapids may indicate a calmer flow. However, it is important to note that the color alone cannot accurately predict the weather, as other factors such as temperature and wind patterns also play a significant role.

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