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Why is the sky blue?

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Re: Why is the sky blue?

#261

Post by listeria »

thaumasiotes wrote: post #226
thaumasiotes wrote: post #191

Earlier quoted context omitted.

> 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 a…

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 bl…

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.

Re: Why is the sky blue?

#262

Post by joveian »

listeria wrote: post #261
thaumasiotes wrote: post #226

Earlier quoted context omitted.

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 bl…

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.

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...

Re: Why is the sky blue?

#263

Post by thaumasiotes »

kazinator wrote: post #256
thaumasiotes wrote: post #191

Earlier quoted context omitted.

> 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 a…

> 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 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.)

Re: Why is the sky blue?

#264

Post by mncharity »

yawpitch wrote: post #183
mncharity wrote: post #140

Earlier quoted context omitted.

Perhaps "transparent with a blue tint"?

I’ll allow “transparent with an occasional blue tint under the current conditions of its nearest star”.

Does that confuse sales staff when shopping for clothes? ... :) The general observation being that sometimes educational descriptions of things get hedges which wouldn't usually be applied in everyday life. Yes, the nice red shirt will look black under some lighting, like some meters underwater... but it usually isn't mentioned. Yet for example, colors of unfamiliar objects in education content can get an "appears" hedge -"it appears white", rather than the more usual, simpler concept of "if its light looks white, it's 'white'".

Re: Why is the sky blue?

#265

Post by mncharity »

reactordev wrote: post #153
mncharity wrote: post #123

Earlier quoted context omitted.

I puttered on a color interactive where, to emphasize this distinction between world-spectra vs brain-color, you could swap in color deficiencies, a non-primate mammal ( dichromats), and a monochromat.

this is fascinating because I'm red/green color deficient yet I have no problem seeing most reds or greens. I feel there's a "spectrum" of color that we all see and each of us is slightly different. My shade of green may not be your shade of green. Yet, when I point out my shade of green - it matches your shade of green because of our eyes. Even though we may be perceiving entirely different colors.

Imagine a chromaticity diagram, but on a perceptual color space where long-range euclidean distance at least attempts to describe the magnitude of perceived difference (rather than the usual model artifacts) - so round-ish. Then decreasing perceived difference between red and green shows up as a smush to oval.

Re: Why is the sky blue?

#266

Post by mncharity »

eru wrote: post #210
mncharity wrote: post #125

Earlier quoted context omitted.

I saw a Microsoft talk decades back, that was a dispirited "the people of India could be buying educational materials and... but no, all the money is in ringtones". For some kinds of business perspective, ok I guess. But for others, and for civilizational change, what's going on in the tail can matter a lot. Does China become a US engineering/science peer in early 21st C absent an internet/WWW?

Well, the thing is that the educational materials are largely free. That's why the people of India don't need to buy them. Isn't that a better world than one where the ringtones were free?

Ah, perhaps I should have said something like "educational materials, and apps, and other useful things" (disapproving judgement in the original).

> Well, the thing is that the educational materials are largely free.

A triumph and fruition of these last decades of massive effort. Now we just need to deal with their quality (with commercial as bad as free). AI may help, by reducing barriers to content creation - you might for example, now more easily author an intro astronomy textbook, one that doesn't reinforce top-30 common misconceptions, something the most used (US; commercial) texts still don't manage.

Re: Why is the sky blue?

#267

Post by snowwrestler »

The mental model that helps visualize why the sky is blue—blue photons flying in every direction—is also the mental model that helps understand the greenhouse effect.

Visible light passes through the atmosphere, gets absorbed by the ground, and then re-radiated at infrared (IR) wavelengths. Atmospheric gases like water vapor, methane, and CO2 cause a certain amount of that IR light to shoot in every direction. So a portion of inbound solar energy gets trapped bouncing around inside the atmosphere for a while before it finally makes it out.

Basically the greenhouse effect is the sky glowing in infrared in addition to blue. We can’t see infrared, but we can feel it with our skin. It feels like warmth.

Re: Why is the sky blue?

#268

Post by fivestones »

mncharity wrote: post #266
eru wrote: post #210

Earlier quoted context omitted.

Well, the thing is that the educational materials are largely free. That's why the people of India don't need to buy them. Isn't that a better world than one where the ringtones were free?

Ah, perhaps I should have said something like "educational materials, and apps, and other useful things" (disapproving judgement in the original). > Well, the thing is that the educational materials are largely free. A triumph and fruition of these last decades of massive effort. Now we just need to deal with their quality (with commercial as bad as free). AI may help, by reducing barriers to content creation - you m…

I’m pretty curious. What are those too 30 misconceptions in US commercial astronomy texts? Is there a list somewhere? Or can you name some?
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