Why are clouds white?

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

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

Originally Posted by ben m

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

Originally Posted by Dilb

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/...ric_optics.pdf

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

Ziggurat · 1st July 2009, 12:39 PM

Originally Posted by Dorfl

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

Originally Posted by Dorfl

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

Originally Posted by maddog

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

Paul

Dorfl · 1st July 2009, 12:59 PM

Originally Posted by Ziggurat

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

Originally Posted by JoeTheJuggler

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

Originally Posted by Dorfl

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

Originally Posted by Dorfl

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

Originally Posted by Dorfl

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

Originally Posted by Buckaroo

From my favorite meteorology professor at Penn State, Craig Bohren:

http://homepages.wmich.edu/~korista/...ric_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

Originally Posted by patchbunny

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

Originally Posted by Buckaroo

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:

Quote:

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.

________

Originally Posted by Dorfl

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:

Quote:

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

Originally Posted by JoeTheJuggler

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

Originally Posted by Dorfl

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?

Dorfl · 1st July 2009, 01:47 PM

Originally Posted by JoeTheJuggler

The penultimate paragraph of that section:

Thanks. I think that explains everything I wondered about.

Ziggurat · 1st July 2009, 01:55 PM

Originally Posted by Dictator Cheney

Water has a color?

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

DC · 1st July 2009, 02:06 PM

Originally Posted by Ziggurat

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

Originally Posted by Dorfl

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

Originally Posted by Dorfl

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,

Quote:

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

Originally Posted by Ziggurat

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

Originally Posted by Dictator Cheney

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

Originally Posted by Dorfl

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

Originally Posted by JoeTheJuggler

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

Originally Posted by Dorfl

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

Originally Posted by patchbunny

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 ....

DC · 2nd July 2009, 06:18 AM

Originally Posted by Skeptical Greg

I'm still trying to figure out why Dorfl thinks that ice is blue ....

1st July 2009, 11:05 AM	#1
Dorfl Muse Join Date: Jun 2005 Location: Sweden Posts: 523	Why are clouds white? 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?

1st July 2009, 11:48 AM	#2
ben m Illuminator Join Date: Jul 2006 Posts: 4,637	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.
ben m Illuminator Join Date: Jul 2006 Posts: 4,637

1st July 2009, 11:52 AM	#3
Starthinker Philosopher Join Date: Oct 2005 Location: Denver, CO Posts: 5,014	Clouds around here are dark grayish blue.
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1st July 2009, 11:54 AM	#4
Marquis de Carabas Penultimate Amazing Join Date: Dec 2002 Location: vuori Posts: 27,106	Racism.
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1st July 2009, 11:55 AM	#5
Monketey Ghost Body of Work Join Date: May 2003 Location: I'm on your screen! Posts: 14,806	Once again, the Marquis has distilled the essence of the crux of the biscuit.
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1st July 2009, 11:56 AM	#6
maddog Intimidating Terrapin Join Date: Apr 2004 Location: The People's Republic of Maryland Posts: 11,819
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1st July 2009, 12:03 PM	#7
Dilb Muse Join Date: Oct 2004 Posts: 738	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.
Dilb Muse Join Date: Oct 2004 Posts: 738

1st July 2009, 12:04 PM	#8
Dorfl Muse Join Date: Jun 2005 Location: Sweden Posts: 523	Originally Posted by ben m 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?

1st July 2009, 12:33 PM	#9
Dorfl Muse Join Date: Jun 2005 Location: Sweden Posts: 523	Originally Posted by Dilb 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?

1st July 2009, 12:37 PM	#10
Buckaroo Graduate Poster Join Date: Feb 2006 Location: Washington, D.C. Posts: 1,715	From my favorite meteorology professor at Penn State, Craig Bohren: http://homepages.wmich.edu/~korista/...ric_optics.pdf (He was also the advisor of our Penn State Skeptic's Club)
	__________________ "You ask me do I love you... does the pope live in the woods? Quad Erat Demonstrandum, baby... " "Oh! You speak French!" -- Airhead, by Thomas Dolby "When you're slapped you'll take it and like it." -- Sam Spade Last edited by Buckaroo; 1st July 2009 at 12:38 PM.

1st July 2009, 12:39 PM	#11
Ziggurat Penultimate Amazing Join Date: Jun 2003 Posts: 26,173	Originally Posted by Dorfl 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.
	__________________ "As long as it is admitted that the law may be diverted from its true purpose -- that it may violate property instead of protecting it -- then everyone will want to participate in making the law, either to protect himself against plunder or to use it for plunder. Political questions will always be prejudicial, dominant, and all-absorbing. There will be fighting at the door of the Legislative Palace, and the struggle within will be no less furious." - Bastiat, The Law

1st July 2009, 12:51 PM	#12
JoeTheJuggler Penultimate Amazing Join Date: Jun 2006 Location: St. Louis Posts: 26,653	Originally Posted by Dorfl 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?
	__________________ "That is a very graphic analogy which aids understanding wonderfully while being, strictly speaking, wrong in every possible way." —Ponder Stibbons

1st July 2009, 12:57 PM	#13
Paulhoff You can't expect perfection. Join Date: May 2005 Location: South Florida Posts: 12,570	Originally Posted by maddog You should use the one I changed............ Paul
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1st July 2009, 12:59 PM	#14
Dorfl Muse Join Date: Jun 2005 Location: Sweden Posts: 523	Originally Posted by Ziggurat 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

1st July 2009, 01:02 PM	#15
Dorfl Muse Join Date: Jun 2005 Location: Sweden Posts: 523	Originally Posted by JoeTheJuggler Aren't many clouds composed of ice crystals rather than water droplets? You're right. But ice is bluish too, isn't it?

1st July 2009, 01:11 PM	#16
Ziggurat Penultimate Amazing Join Date: Jun 2003 Posts: 26,173	Originally Posted by Dorfl 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.
	__________________ "As long as it is admitted that the law may be diverted from its true purpose -- that it may violate property instead of protecting it -- then everyone will want to participate in making the law, either to protect himself against plunder or to use it for plunder. Political questions will always be prejudicial, dominant, and all-absorbing. There will be fighting at the door of the Legislative Palace, and the struggle within will be no less furious." - Bastiat, The Law

1st July 2009, 01:16 PM	#17
Buckaroo Graduate Poster Join Date: Feb 2006 Location: Washington, D.C. Posts: 1,715	Section 9 of the above-linked chapter answers pretty much all these questions. Taking my atmospheric radiative transfer class from Craig was a treat.
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1st July 2009, 01:21 PM	#18
patchbunny Graduate Poster Join Date: May 2004 Location: Right about... here. Posts: 1,551	Originally Posted by Dorfl 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?
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1st July 2009, 01:23 PM	#19
JoeTheJuggler Penultimate Amazing Join Date: Jun 2006 Location: St. Louis Posts: 26,653	Originally Posted by Dorfl 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.
	__________________ "That is a very graphic analogy which aids understanding wonderfully while being, strictly speaking, wrong in every possible way." —Ponder Stibbons

1st July 2009, 01:23 PM	#20
Dorfl Muse Join Date: Jun 2005 Location: Sweden Posts: 523	Originally Posted by Buckaroo From my favorite meteorology professor at Penn State, Craig Bohren: http://homepages.wmich.edu/~korista/...ric_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.

1st July 2009, 01:28 PM	#21
Dorfl Muse Join Date: Jun 2005 Location: Sweden Posts: 523	Originally Posted by patchbunny 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.

1st July 2009, 01:32 PM	#22
shadron Philosopher Join Date: Sep 2005 Location: Colorado Posts: 5,717	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.
	Last edited by shadron; 1st July 2009 at 01:37 PM.

1st July 2009, 01:37 PM	#23
JoeTheJuggler Penultimate Amazing Join Date: Jun 2006 Location: St. Louis Posts: 26,653	Originally Posted by Buckaroo 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: Quote: 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. ________ Originally Posted by Dorfl 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: Quote: 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.
	__________________ "That is a very graphic analogy which aids understanding wonderfully while being, strictly speaking, wrong in every possible way." —Ponder Stibbons

1st July 2009, 01:44 PM	#25
Ziggurat Penultimate Amazing Join Date: Jun 2003 Posts: 26,173	Originally Posted by Dorfl 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.
	__________________ "As long as it is admitted that the law may be diverted from its true purpose -- that it may violate property instead of protecting it -- then everyone will want to participate in making the law, either to protect himself against plunder or to use it for plunder. Political questions will always be prejudicial, dominant, and all-absorbing. There will be fighting at the door of the Legislative Palace, and the struggle within will be no less furious." - Bastiat, The Law

1st July 2009, 01:45 PM	#26
DC dedicated aphilatelist Join Date: Mar 2008 Location: Switzerland Posts: 21,634	Water has a color?
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1st July 2009, 01:47 PM	#27
Dorfl Muse Join Date: Jun 2005 Location: Sweden Posts: 523	Originally Posted by JoeTheJuggler The penultimate paragraph of that section: Thanks. I think that explains everything I wondered about.

1st July 2009, 01:55 PM	#28
Ziggurat Penultimate Amazing Join Date: Jun 2003 Posts: 26,173	Originally Posted by Dictator Cheney Water has a color? If it's thick enough, yes. So does glass. And air.
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1st July 2009, 02:06 PM	#29
DC dedicated aphilatelist Join Date: Mar 2008 Location: Switzerland Posts: 21,634	Originally Posted by Ziggurat 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
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1st July 2009, 02:27 PM	#30
JoeTheJuggler Penultimate Amazing Join Date: Jun 2006 Location: St. Louis Posts: 26,653	Originally Posted by Dorfl 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.
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1st July 2009, 02:31 PM	#31
JoeTheJuggler Penultimate Amazing Join Date: Jun 2006 Location: St. Louis Posts: 26,653	Originally Posted by Dorfl 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, Quote: 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.
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1st July 2009, 02:35 PM	#32
DC dedicated aphilatelist Join Date: Mar 2008 Location: Switzerland Posts: 21,634	Originally Posted by Ziggurat 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
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1st July 2009, 02:37 PM	#34
DC dedicated aphilatelist Join Date: Mar 2008 Location: Switzerland Posts: 21,634
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1st July 2009, 10:11 PM	#35
patchbunny Graduate Poster Join Date: May 2004 Location: Right about... here. Posts: 1,551	Originally Posted by Dorfl 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.
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2nd July 2009, 05:45 AM	#36
Dorfl Muse Join Date: Jun 2005 Location: Sweden Posts: 523	Originally Posted by JoeTheJuggler 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?

2nd July 2009, 05:48 AM	#37
realpaladin Master Poster Join Date: Apr 2007 Location: I live not very often where my home is. Posts: 2,084	Originally Posted by Dorfl 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!)
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