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Why are clouds white?

Dorfl

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Jun 19, 2005
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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?
 
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.
 
Once again, the Marquis has distilled the essence of the crux of the biscuit.
 
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.
 
ben m said:
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.
Click to expand...

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?
 
Dilb said:
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.
Click to expand...

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?
 
Dorfl said:
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?
Click to expand...

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.
 
Dorfl said:
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?
Click to expand...

Aren't many clouds composed of ice crystals rather than water droplets?
 
Ziggurat said:
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.
Click to expand...

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 said:
Just to clarify: When the light is reflected against a droplet, it is not attenuated any?
Click to expand...

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.
 
Section 9 of the above-linked chapter answers pretty much all these questions.

Taking my atmospheric radiative transfer class from Craig was a treat.
 
Dorfl said:
You're right. But ice is bluish too, isn't it?
Click to expand...
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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