[thaumasiotes]

The article has a section on "why are clouds white?", but it doesn't really address the reason I thought that question should be covered.

It just says that clouds act like a collection of randomly-oriented prisms, such that whenever light of any wavelength comes into the cloud, it is dispersed from the cloud evenly in all directions.

This would explain why a cloud was white if even white light was coming into the cloud. But the rest of the article establishes that the light coming into the cloud is predominantly blue and purple. Why isn't that also true of the light leaving the cloud?

[kazinator]

Well, no; what's coming into the cloud is sunlight, filtered through the atmosphere. It scatters that light and so it has that color. The same like a heap of powdered salt in the palm of your hand, held up to the sun.

Clouds illuminated by the setting sun aren't white.

[thaumasiotes]

> Well, no; what's coming into the cloud is sunlight, filtered through the atmosphere. It scatters that light and so it has that color.

The model the article describes is:

1. The sun puts out a bunch of light.

2. Light that is lower-energy than blue light fails to be effectively scattered by the atmosphere, and follows a path from the sun to you.

3. Light at the levels of blue and up is effectively scattered by the atmosphere, and comes to you from a random direction.

So if you look toward the sun, you receive light that has been depressed of its blue-and-up wavelengths, and the sun appears to be yellow. If you look away from the sun, you receive light that has been enriched in blue-and-up wavelengths, and the sky appears to be blue. Crucially, the sky looks blue because it is sending you more blue light than the background level.

A cloud that isn't between you and the sun is getting its light from the sky background, which is blue. Why is the cloud not blue? It can disperse all the light it receives evenly, but that light is enriched for blue-and-up wavelengths.

[kazinator]

> s sending you more blue light than the background level.

The background level is black! No air, no scattering. d > A cloud that isn't between you and the sun is getting its light from the sky background, which is blue. Why is the cloud not blue?

The cloud is bathed in intense, direct sunlight which is slightly yellow, and it is exposed to a small amount of scattered blue light. It could be that this mixture whitens the color, essentially reconstituting the light.

Why does white paint look pure white? Or snow on the ground? Same reason. It's reflecting the sky blue in a small amount, plus the yellowish sunlight.

If you are in a dark enclosure like a cave or barn, and sunlight is streaming in through a small aperture, if you hold some white object up to that light, it doesn't look the same as if you do that outside because it's not illuminated by the blue sky, only by direct sunlight.

Moreover, the shadowed parts white object sitting outdors, exposed to sunlight. tend to be tinged with blue.

[thaumasiotes]

> The background level is black!

By "the background level", what I mean is the emission spectrum of the sun.

(And by "more blue", I mean in a relative sense, not an absolute sense.)

[thaumasiotes]

Here's a different presentation:

1. Imagine a conceptual sunbeam originating from the sun and passing through the sky high over your head.

2. This sunbeams low-frequency components ("reds") continue on in their straight path, making them invisible to you.

3. The high-frequency components ("blues") are scattered by the atmosphere, going in random directions. When you look at the sunbeam, you can see these scattered blues, making the sky blue.

4. A cloud floating up in the atmosphere is illuminated by some direct sunbeams, which are "red".

5. It's also illuminated by the scatter from the atmosphere, which is "blue".

6. Do those two sources balance out exactly such that the light exiting the cloud has the same profile as light exiting the sun?

[listeria]

If you look at the solar radiation spectrum chart in the section "Why isn’t the sky violet?", you can see that sunlight is not evenly distributed along the visible spectrum--it emit more blue than red, and at sea level it's closer to evenly distributed. So the light that reaches the clouds is still mostly white light.

[joveian]

I think it may also relate to chromatic adaptation. To be white it doesn't need to be any exact absolute color just the color our brain sets our white point to.

Not answering this question but I found an interesting short paper about how at sunset and sunrise the color gamut of shadows doesn't fully overlap with the direct illumination color gamut due to the differences in the paths the light takes:

Hubel. 2000. The Perception of Color at Dawn and Dusk.

https://library.imaging.org/admin/apis/public/api/ist/websit...

[thewakalix]

The cloud is getting light from both the sky background and direct sunlight.