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Dorfl
1st July 2009, 11:05 AM
Water is, as far as I know, a very pale blue. So why do clouds, consisting of fine water drops, appear white?

Is it that water only filters out the blue fraction of light passing through, but affects reflected light differently, or are clouds actually blue, but the much deeper blue of the surrounding sky makes them seem white by comparison, or is it something else?

ben m
1st July 2009, 11:48 AM
The clouds are basically giant diffusers. All of those water droplets have reflective surfaces, and those surfaces scattering sunlight in all directions. The outgoing light is therefore the same color as the incoming sunlight, i.e. approximately white.

The total amount of water (which is what would matter for an absorption-based color change) isn't so large, but the amount of reflective surface is huge.

Starthinker
1st July 2009, 11:52 AM
Clouds around here are dark grayish blue.

Marquis de Carabas
1st July 2009, 11:54 AM
Racism.

Monketey Ghost
1st July 2009, 11:55 AM
Once again, the Marquis has distilled the essence of the crux of the biscuit.

maddog
1st July 2009, 11:56 AM
:lfault

Dilb
1st July 2009, 12:03 PM
Tiny particles tend to be white because light will hit a lot of surfaces. Light can be reflected at a surface if it's at the right angle. Because there are so many particles the light can keep reflecting, and the end result is that light gets reflected in all directions. If the particles don't absorb any colour, then all the light will come out, and it will still be white.

A simple example of how light reflects off spheres is the spot of light on a shiny apple: it's at the right angle for light to be reflected, and so it looks white.

Water itself is pretty close to colourless, so clouds don't absorb enough light to change the colour. This is also the reason granulated sugar is white, but rock sugar isn't.

It's also why white t-shirts become see through when they get wet: there's fewer places for light to be reflected, so light goes through instead. Fake edit- Dang it ben m, I was answering this before I got distracted by thinking about wet t-shirts.

Dorfl
1st July 2009, 12:04 PM
The clouds are basically giant diffusers. All of those water droplets have reflective surfaces, and those surfaces scattering sunlight in all directions. The outgoing light is therefore the same color as the incoming sunlight, i.e. approximately white.

The total amount of water (which is what would matter for an absorption-based color change) isn't so large, but the amount of reflective surface is huge.

Thanks :) But shouldn't they still reflect blue light slightly more than other wavelenghts? Is it that they reflect light fairly uniformly, but light actually passing through water gets filtered blue?

Dorfl
1st July 2009, 12:33 PM
Tiny particles tend to be white because light will hit a lot of surfaces. Light can be reflected at a surface if it's at the right angle. Because there are so many particles the light can keep reflecting, and the end result is that light gets reflected in all directions. If the particles don't absorb any colour, then all the light will come out, and it will still be white.

A simple example of how light reflects off spheres is the spot of light on a shiny apple: it's at the right angle for light to be reflected, and so it looks white.

Water itself is pretty close to colourless, so clouds don't absorb enough light to change the colour. This is also the reason granulated sugar is white, but rock sugar isn't.

It's also why white t-shirts become see through when they get wet: there's fewer places for light to be reflected, so light goes through instead. Fake edit- Dang it ben m, I was answering this before I got distracted by thinking about wet t-shirts.

Hmm... Does this mean that if I had a really, really thick cloud layer with a really bright sun above, the clouds would appear blue?

Buckaroo
1st July 2009, 12:37 PM
From my favorite meteorology professor at Penn State, Craig Bohren:

http://homepages.wmich.edu/~korista/atmospheric_optics.pdf

(He was also the advisor of our Penn State Skeptic's Club)

Ziggurat
1st July 2009, 12:39 PM
Thanks :) But shouldn't they still reflect blue light slightly more than other wavelenghts? Is it that they reflect light fairly uniformly, but light actually passing through water gets filtered blue?

Water attenuates light fairly slowly. The light scattered from clouds may travel through a lot of different dropplets, but each droplet is very small, so the distance traveled by light in water before leaving the cloud is typically much shorter than the attenuation length. The frequency differences in attenuation length are therefore basically irrelevant.

JoeTheJuggler
1st July 2009, 12:51 PM
Thanks :) But shouldn't they still reflect blue light slightly more than other wavelenghts? Is it that they reflect light fairly uniformly, but light actually passing through water gets filtered blue?

Aren't many clouds composed of ice crystals rather than water droplets?

Paulhoff
1st July 2009, 12:57 PM
:lfault

:lfault2

You should use the one I changed............

Paul

:) :) :)

Dorfl
1st July 2009, 12:59 PM
Water attenuates light fairly slowly. The light scattered from clouds may travel through a lot of different dropplets, but each droplet is very small, so the distance traveled by light in water before leaving the cloud is typically much shorter than the attenuation length. The frequency differences in attenuation length are therefore basically irrelevant.

Just to clarify: When the light is reflected against a droplet, it is not attenuated any? If so, then I think I understand it :)

Dorfl
1st July 2009, 01:02 PM
Aren't many clouds composed of ice crystals rather than water droplets?

You're right. But ice is bluish too, isn't it?

Ziggurat
1st July 2009, 01:11 PM
Just to clarify: When the light is reflected against a droplet, it is not attenuated any?

If it's reflected from the outer surface (some light enters the droplet and reflects off the inner surface too). But my point was that even the light that passes through the droplet is hardly attenuated at all, because each droplet is so small.

Buckaroo
1st July 2009, 01:16 PM
Section 9 of the above-linked chapter answers pretty much all these questions.

Taking my atmospheric radiative transfer class from Craig was a treat.

patchbunny
1st July 2009, 01:21 PM
Hmm... Does this mean that if I had a really, really thick cloud layer with a really bright sun above, the clouds would appear blue?

Wouldn't they be grey, like storm clouds?

JoeTheJuggler
1st July 2009, 01:23 PM
You're right. But ice is bluish too, isn't it?
Not in my experience. It's mostly white (think mountain glaciers, ice cubes, snow, polar ice, etc.)

Still. . it sounds like that's not the answer to why clouds are often white.

Dorfl
1st July 2009, 01:23 PM
From my favorite meteorology professor at Penn State, Craig Bohren:

http://homepages.wmich.edu/~korista/atmospheric_optics.pdf

(He was also the advisor of our Penn State Skeptic's Club)

Cool. I'll read it tomorrow, since it's getting kind of late here, but it looks interesting.

Dorfl
1st July 2009, 01:28 PM
Wouldn't they be grey, like storm clouds?

I don't think so. If the reason that the light is unattenuated is that it travels through so little water, it should in principle be possible to make it bluer just by increasing the amount of water—in other words, make a thicker cloud layer. The problem is that you would have to make the sun brighter to compensate for the darkening effect, which might not be very realistic in practise.

The reason I asked is just to see if I more or less understood the idea.

shadron
1st July 2009, 01:32 PM
It's glare - you are seeing billions of very small reflections of the sun in a cloud facing both you and the sun. Cloud in the shadow turns gray, which is simply relatively dimmer reflections of dimmer light - dark clouds are dar k for the same reason night is - no light. I don't think any deeper science is involved than that. You would also see a rainbow in there if it is at the right angle, and the white reflections aren't totally glaring it out. Like any convex mirror surface, you will get a solar reflection whenever there is clear line of sight from the sun to the droplets to you.

And clouds are usually water droplets, not ice, at least until the altitude gets very high or the air itself is very cold.

JoeTheJuggler
1st July 2009, 01:37 PM
Section 9 of the above-linked chapter answers pretty much all these questions.

Taking my atmospheric radiative transfer class from Craig was a treat.

The penultimate paragraph of that section:

The scattering coefficient of cloud droplets, in contrast with that of air molecules, is more or less independent of wavelength. This is often invoked as the cause of the colorlessness of clouds. Yet wavelength independence of scattering by a single particle is only sufficient, not necessary, for wavelength independence of scattering by a cloud of particles (see Sec. 2.4). Any cloud that is optically thick and composed of particles for which absorption is negligible is white upon illumination by white light. Although absorption by water (liquid and solid) is not identically zero at visible wavelengths, and selective absorption by water can lead to observable consequences (e.g., colors of the sea and glaciers), the appearance of all but the thickest clouds is not determined by this selective absorption.

________

I don't think so. If the reason that the light is unattenuated is that it travels through so little water, it should in principle be possible to make it bluer just by increasing the amount of water—in other words, make a thicker cloud layer. The problem is that you would have to make the sun brighter to compensate for the darkening effect, which might not be very realistic in practise.
From earlier in the section Buckaroo mentioned:

Despite their sometimes solid appearance, clouds are so flimsy as to be almost nonexistent – except optically. The fraction of the total cloud volume occupied by water substance (liquid or solid) is about 10−6 or less. Yet although the mass density of clouds is that of air to within a small fraction of a percent, their optical thickness (per unit physical thickness) is much greater.

Dorfl
1st July 2009, 01:37 PM
Not in my experience. It's mostly white (think mountain glaciers, ice cubes, snow, polar ice, etc.)

Still. . it sounds like that's not the answer to why clouds are often white.

Yeah... All your examples are of ice containing large amounts of air bubbles. I think that pure ice is still blue. Which raises the opposite question: since air is blue, why is a large number of small air bubbles inside ice white?

Ziggurat
1st July 2009, 01:44 PM
Yeah... All your examples are of ice containing large amounts of air bubbles. I think that pure ice is still blue. Which raises the opposite question: since air is blue, why is a large number of small air bubbles inside ice white?

Same answer: it's only white if the path lengths are short enough that no significant attenuation occurs.

DC
1st July 2009, 01:45 PM
Water has a color? :confused:

Dorfl
1st July 2009, 01:47 PM
The penultimate paragraph of that section:

Thanks. I think that explains everything I wondered about. :)

Ziggurat
1st July 2009, 01:55 PM
Water has a color? :confused:

If it's thick enough, yes. So does glass. And air.

DC
1st July 2009, 02:06 PM
If it's thick enough, yes. So does glass. And air.

i know i better would research first, but i claim, no, thats not the water that gives the color, its plankton for example, pure water is always transparent.

pls correct me if im wrong, a while ago i went true that in school :)

JoeTheJuggler
1st July 2009, 02:27 PM
Yeah... All your examples are of ice containing large amounts of air bubbles. I think that pure ice is still blue. Which raises the opposite question: since air is blue, why is a large number of small air bubbles inside ice white?
Air also isn't blue. Clouds do have a lot of "air" in them--in fact, they're almost entirely air.

At any rate, scattering is the short answer to why clouds are white.

JoeTheJuggler
1st July 2009, 02:31 PM
Thanks. I think that explains everything I wondered about. :)

Thank Buckaroo. He linked that pdf. I just pulled out the least technical bit of it. The equation in that section gives a more complete description. As the article says,

Equation (42) is the key to the vastly different optical characteristics of clouds and of the rain for which they are the progenitors. For a fixed amount of water (as specified by the quantity fh), optical thickness is inversely proportional to mean diameter. Rain drops are about 100 times larger on average than cloud droplets, and hence optical thicknesses of rain shafts are correspondingly smaller. We often can see through many kilometers of intense rain whereas a small patch of fog on a well-traveled highway can result in carnage.

It then proceeds to explain the equation.

DC
1st July 2009, 02:35 PM
If it's thick enough, yes. So does glass. And air.

oops i was wrong. its like you say, it is slightly blocking the red spectrum and thus it is slightly blue and can be seen when its thick enough :)

Ziggurat
1st July 2009, 02:35 PM
i know i better would research first, but i claim, no, thats not the water that gives the color, its plankton for example, pure water is always transparent.

pls correct me if im wrong, a while ago i went true that in school :)

Various impurities including plankton can contribute to coloring water, but even pure water still has an intrinsic color:
http://www.dartmouth.edu/~etrnsfer/water.htm#blue

DC
1st July 2009, 02:37 PM
:)

patchbunny
1st July 2009, 10:11 PM
I don't think so. If the reason that the light is unattenuated is that it travels through so little water, it should in principle be possible to make it bluer just by increasing the amount of water—in other words, make a thicker cloud layer. The problem is that you would have to make the sun brighter to compensate for the darkening effect, which might not be very realistic in practise.

The reason I asked is just to see if I more or less understood the idea.

Sounds like we have something to look forward to when the sun enters its red giant phase. :)

Dorfl
2nd July 2009, 05:45 AM
Air also isn't blue.

It's not?!

Is there any short explanation for why the sky is still blue, or should I start a second thread about that?

realpaladin
2nd July 2009, 05:48 AM
It's not?!

Is there any short explanation for why the sky is still blue, or should I start a second thread about that?

Funny, that is the introduction of 'The cuckoo egg' by Mr. Clifford Stoll.

He states that to explain that, you will go into the frequencies of light and into subatomic reasoning as well... :)

(yeah, I know, there is no such thing as 'subatomic reasoning', but... meh!)

Dorfl
2nd July 2009, 05:57 AM
Sounds like we have something to look forward to when the sun enters its red giant phase. :)

Yay, blue clouds! :) But until then, I will have to play with Terragen instead.

Skeptical Greg
2nd July 2009, 06:05 AM
I'm still trying to figure out why Dorfl thinks that ice is blue ....:confused:

DC
2nd July 2009, 06:18 AM
I'm still trying to figure out why Dorfl thinks that ice is blue ....:confused:

http://www.5reicherts.com/skan03/12-8-gletschereis.jpg

Dorfl
2nd July 2009, 06:23 AM
I'm still trying to figure out why Dorfl thinks that ice is blue ....:confused:
From Ask a Scientist, http://www.newton.dep.anl.gov/askasci/gen01/gen01514.htm:

Large blocks of ice absorb light from the visible spectrum, preferentially absorbing from the red end of the spectrum so thicker volumes of ice will let blue light through. Thus the ice will have the color blue. Also, the purer the water that is used in making the ice, the more pronounced the blue color will be. Man- made artificial ice will not be as bluish in color because of impurities. I hope that this is helpful.


Admittedly, the answers on that page do not quite agree with each other. Some of them claim that the color is due to scattering, and not absorbtion:

Ice is colorless in that it does not absorb any visible light; however, thick slabs of ice like liquid water can scatter light giving it a bluish cast.


But either way, a large slab of ice will look blue.

Paulhoff
2nd July 2009, 03:57 PM
It's not?!

Is there any short explanation for why the sky is still blue, or should I start a second thread about that?
It is mainly because of dust in the air. A lot of the dust is small enough that longer wavelengths of light, at the red end of the spectrum, aren't reflected but go around the dust but the shorter wavelengths like blue are and comes to you eyes.

Paul

:) :) :)

rjh01
2nd July 2009, 07:47 PM
To add to post 42 the red light is not scattered so much so you get a red sunset and sunrise.

CaveDave
3rd July 2009, 01:50 AM
Funny, that is the introduction of 'The cuckoo egg' by Mr. Clifford Stoll.

He states that to explain that, you will go into the frequencies of light and into subatomic reasoning as well... :)

(yeah, I know, there is no such thing as 'subatomic reasoning', but... meh!)

LOVED that book! (must have read it about 20 years ago - before the public had heard of the Internet:D)

Have you seen his current endeavor, Acme Klein Bottle (http://www.kleinbottle.com/)? Hilarious stuff, there. Educational even if you don't want to be.

Check out the "Portraits of Gauss" (http://www.kleinbottle.com/gauss.htm) and Cup of Tantalus (http://www.kleinbottle.com/Tantalus.html). Hell, read it all, even the "boring" FAQ; that guy has a great sense of humor.

And then, here find a
Photo of Cliff with his masterpiece. (http://www.kleinbottle.com/images/giantKleinbotandCliff2.jpg) Bet that babe masses a kg or three!

Cheers,

Dave