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Why are clouds white?

patchbunny said:
Wouldn't they be grey, like storm clouds?
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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.
 
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.
 
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Buckaroo said:
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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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.
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________

Dorfl said:
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.
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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.
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JoeTheJuggler said:
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.
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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?
 
Dorfl said:
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?
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Same answer: it's only white if the path lengths are short enough that no significant attenuation occurs.
 
Ziggurat said:
If it's thick enough, yes. So does glass. And air.
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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 :)
 
Dorfl said:
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?
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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.
 
Dorfl said:
Thanks. I think that explains everything I wondered about. :)
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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.
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It then proceeds to explain the equation.
 
Ziggurat said:
If it's thick enough, yes. So does glass. And air.
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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 :)
 
Dictator Cheney said:
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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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
 
Dorfl said:
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.
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Sounds like we have something to look forward to when the sun enters its red giant phase. :)
 
Dorfl said:
It's not?!

Is there any short explanation for why the sky is still blue, or should I start a second thread about that?
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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!)
 
Skeptical Greg said:
I'm still trying to figure out why Dorfl thinks that ice is blue ....:confused:
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12-8-gletschereis.jpg
 
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