Difference between revisions of "Colors of the world around us"

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and also why the sky is fairly bright in the first place. {{comment|(In contrast, the atmosphereless moon has a pitch black sky, shadows are always crisp, and any light that don't hit anything fly by undisturbed)}}
 
and also why the sky is fairly bright in the first place. {{comment|(In contrast, the atmosphereless moon has a pitch black sky, shadows are always crisp, and any light that don't hit anything fly by undisturbed)}}
  
 
 
 
 
TO READ:
 
https://en.wikipedia.org/wiki/Diffuse_sky_radiation
 
 
https://web.archive.org/web/20161109075152/http://www.osti.gov/accomplishments/nuggets/einstein/daytimea.html
 
 
https://www.researchgate.net/post/Does_Rayleigh_Scattering_really_explain_blueness_of_sky
 
 
http://www.webexhibits.org/causesofcolor/14D.html
 
 
http://www.mike-willis.com/Tutorial/rainscatter.htm
 
  
  
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-->
 
-->
  
==On sunsets==
+
===On sunsets===
  
 
<!--
 
<!--
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Temperature affecting how and how long pollution hangs around also explains why winter gives somewhat nicer sunsets.
 
Temperature affecting how and how long pollution hangs around also explains why winter gives somewhat nicer sunsets.
  
 +
 +
-->
 +
===See also===
 +
<!--
 +
* http://en.wikipedia.org/wiki/Mie_scattering
 +
* http://en.wikipedia.org/wiki/Rayleigh_scattering
 +
 +
* http://hyperphysics.phy-astr.gsu.edu/hbase/atmos/blusky.html
 +
* http://science.howstuffworks.com/nature/climate-weather/atmospheric/sky.htm
 +
* http://optics.kulgun.net/Blue-Sky/
 +
* http://www.sciencemadesimple.com/sky_blue.html
 +
* http://mb-soft.com/public/bluesky.html
 +
 +
* http://www.spc.noaa.gov/publications/corfidi/sunset/
 +
-->
 +
 +
<!--
 +
TO READ:
 +
https://en.wikipedia.org/wiki/Diffuse_sky_radiation
 +
 +
https://web.archive.org/web/20161109075152/http://www.osti.gov/accomplishments/nuggets/einstein/daytimea.html
 +
 +
https://www.researchgate.net/post/Does_Rayleigh_Scattering_really_explain_blueness_of_sky
 +
 +
http://www.webexhibits.org/causesofcolor/14D.html
 +
 +
http://www.mike-willis.com/Tutorial/rainscatter.htm
  
  
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 +
-->
 +
===Might other planet's skies be other colors?===
 +
<!--
  
 +
Yes, but there's a lot of assumptions in there that end up meaning "not by much in a lot of practice".
  
: '''So is the sea blue for the same reason?'''
 
  
Well, partly it's just that water reflects, and the sky is blue.  
+
The simplest change would be to have a different star[https://en.wikipedia.org/wiki/Stellar_classification].
And most of that reflection is from near the horizon so often a lighter blue (because clouds).
+
Even with everything else being equal, a slightly different-colored light source, means different colors.
This is also why polarization filters help look into water and brighten any color of the water itself.
+
Question is whether we could survive.
  
Because yes, the water is also blue by itself. Which is how it can appear a deeper blue than just reflected sky could explain.
+
But let's assume the sun and our eyes as constants.
  
And indeed, this is basically Rayleigh scattering too. The mechanics are slightly different, e.g. that being a lot more matter,
 
it takes out all the light in much less distance, which is why being underwater looks blue-tinted darkness. Underwater it's also more apparent that the absorption is gradual/preferential: red starts going at a few meters, whereas the last one to go, blue, is much deeper.
 
  
  
When not underwater, it matters that waves reflect more sky (and probably the air mixing in also doesn't help see in{{verify}}).
+
Different gases might lead to other colors,  
 +
but there are relatively few gases which would have a significant effect.
  
 +
And if we want to not die from the atmosphere's contents, there's much tighter constraints.
  
Also, pools of water and seas near the shore are usually be the color of what's suspended in there[http://en.wikipedia.org/wiki/Colored_dissolved_organic_matter].  
+
Pressure also has an infuence, but again, not so much in the range where human bodies want to live.
  
Very clear and at least moderately deep water will be dominated by, well, the absence of that.
 
Less light makes it out, and that which does will be more heavily biased to blue.
 
So deeper blues are deeper (clear) water.
 
  
You don't see these deeper blues much because water still enough to let things sink down, and be wave-less so you can find a spot with less reflection (or a polarization filter has a uniform effect), is rare.
 
  
  
This effect is stronger from straight up, which is why deep clear water can appear nearly black on satellite images, though there's always some white reflection, particularly on regular photos from space.
+
If we're talking Rayleigh scattering,  
 +
we might get more or less of it of it,
 +
but this is mostly going to give us brighter or darker blues skies, not
  
  
https://earthobservatory.nasa.gov/features/ColorImage/page2.php
 
  
 +
'''Mars''' ''seems'' like a really interesting case,
 +
with photos on mars showing a red sky, and a sun with a blue halo (visible mostly around sunrise/sunset) seemingly the inverse of earth.
  
  
: '''Might skies be other colors?'''
+
Mars looks red, but not because of its atmosphere - the martian atmosphere is so thin that the Rayleigh components is tiny (TODO: figure out rayleigh on CO2), so actually, a clear ''clear'' martian sky looks almost black.
  
Yes, but there's a lot of assumptions in there.
+
But a clear sky is rare.  
  
 +
There's a lot of dust, which is large enough to not involve much of the above scattering.
 +
Some of it is still small enough for Mie scattering, but a lot of it is larger.
  
For example, do you want to not die from this atmosphere, there's much tighter constraints.
+
Large enough to mainly have color the classical way, in this case because it has has rust mixed in.
  
The simplest change would be to have a different star[https://en.wikipedia.org/wiki/Stellar_classification].
+
So light reflecting off this dust looks red, but more than a little of it mostly just blocks all light, which is why visibility is never great on mars.
Even with everything else being equal, a slightly different-colored light source,
+
means different colors.
+
Question is whether we could survive.
+
  
But let's assume the sun and our eyes as constants.
 
  
 +
That relatively bluish (to human eyes) halo around the sun is only really visible only around sunsurse/set basically because of the nature of Mie scattering, which unlike Rayleigh is somewhat directional - blue light scatters forwards a little more than others, so (only) when you look relatively straight at the light source, the net effect is that it takes out red more than blue.
  
 +
Note that the same can happen on earth, but it requires fine dust. Volcanos have done this,
 +
and this seems to be the main reason for [https://en.wikipedia.org/wiki/Blue_moon blue moons].
  
  
Mars ''seems'' like a really interseting case, a red sky with with a blue halo (visible mostly around sunrise/sunset) seemingly the inverse of earth.
 
  
However, martian atmosphere is so thin that it's basically black - there is no Rayleigh component here
 
  
Mars is red, but not because of its atmosphere - it barely has any (TODO: figure out rayleigh on CO2).
+
Some atmospheric gas might have a color, not by merit of Rayleigh scattering, but by the more usual way of things having colors: absorbing specific wavelengths.
Without all the dust, its sky would actually be close to black.
+
And the dust, being large, doesn't involve any of the above scattering.
+
It actually mostly blocks all colors equally, which is why visibility is never great.
+
  
The dust that's there both:
+
Nitrogen and oxygen don't.
: is still small enough for Mie scattering
+
In fact, most gases have little or no color,
: has color the classical way, in this case because it has has rust mixed in.
+
and most of the more exciting-colored gases are things we really don't want to breathe.
So light reflecting off it looks red.
+
 
 +
For example, Ozone is a sligtly blue gas, and while its density puts it higher up,
 +
there isn't enough of it to contribute much.
 +
While ozone is good for us in that it takes out UV-C,
 +
ozone as a gas is bad for us at more-than-tiny concentrations
 +
(e.g. why you don't want certain types of [[air ionizers]]; see also [[ozone]]),
 +
so we're quite happy with it staying 10+ kilometers up.
 +
 
 +
 
 +
 
 +
As to Rayleigh scattering, it depends a lot on the gas.
 +
 
 +
The gas has to be polarizabile for Rayleigh scattering to apply, which narrows things down.{{verify}}
 +
 
 +
If the atmosphere primarily contained ''slightly'' larger sized molecules, you might get the most scattering elsewhere in the visible spectrum, probably red - basically the same thing we have here, with red and blue reversed.
 +
 
 +
Much larger and you wouldn't get this effect in the visible range.  
 +
If anything it's Mie again, going for white and, if there's a lot of it, gray or no color (and quite possibly no life, because little to no heat, which sits to the side of human-visible).
 +
 
 +
For example, the third on the list of our atmosphere, argon, would not contribute in visible light even if it were a large enough constituent[https://physics.stackexchange.com/questions/243975/what-colour-would-the-sky-of-a-high-oxygen-high-argon-high-water-vapour-atmosp])
 +
 
 +
 
 +
CH4, CO2, Freon
  
  
There is an actually relatively bluish (to human eyes) halo around the sun,
 
only really visible only around sunsurse/set basically because of the nature of Mie scattering, which unlike Rayleigh is somewhat directional - blue light scatters forwards a little more than others, so (only) when you look relatively straight at the light source, the net effect is that it takes out red more than blue.
 
Note that the same can happen on earth, but it requires fine dust. Volcanos have done this.
 
  
  
Line 241: Line 271:
 
https://www.quora.com/Does-Mars-have-a-blue-sky
 
https://www.quora.com/Does-Mars-have-a-blue-sky
  
 +
* [https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19770012747.pdf
 +
ABSOLUTE RAYLEIGH SCATTERING CROSS SECTIONS OF GASES AND FREONS OF STRATOSPHERIC INTEREST IN THE VISIBLE AND ULTRAVIOLET REGIONS]
  
  
  
 +
-->
  
 +
==Aurora==
 +
<!--
  
 
[https://en.wikipedia.org/wiki/Aurora], by the way, is basically an ionization effect,
 
[https://en.wikipedia.org/wiki/Aurora], by the way, is basically an ionization effect,
 
and the color dependent on the present gases.  
 
and the color dependent on the present gases.  
  
You don't want this to be very intense on your planet, because the same solar radiation
+
You don't want this to be very intense on your planet, because the main reason aurora is visible
is bad for us and our infrastructure if intense enough. In that regard we're lucky
+
is ionizing radiation making it further into the atmosphere.  
it fluctuates from little to some, not from some to more.
+
(It's stronger near the poles because of the shape of the [https://en.wikipedia.org/wiki/Magnetosphere#Structure magnetosphere])
  
 +
Ionizing radiation that reaches us is bad for us, and our electronics (so varied infrastructure), if intense enough.
 +
In that regard we're lucky it fluctuates from little to some, not from some to more.
  
 +
-->
  
  
  
Some atmospheric gas might have a color, not by merit of Rayleigh scattering, but by the more usual way of things having colors: absorbing specific wavelengths.
+
==The color of water==
 +
<!--
  
Nitrogen and oxygen don't, and so don't a lot of other gases.
 
And the more exciting-colored gases are things we really don't want to breathe,
 
or even be in your atmosphere.
 
  
Well, if density means it's high up, then it may be okay. For example, Ozone is sligtly blue,
+
: '''So is the sea blue for the same reason?'''
but there isn't really enough of it to contribute much.
+
While ozone is good for us in that it takes out UV-C, ozone as a gas is bad for us
+
(e.g. why you don't want certain types of [[air ionizers]]), so we want it to stay 10+ kilometers up.
+
  
 +
Well, partly it's just that water reflects, and the sky is blue.
 +
And most of that reflection is from near the horizon so often a lighter blue (because clouds).
 +
This is also why polarization filters help look into water and brighten any color of the water itself.
  
 +
Because yes, the water is also blue by itself. Which is how it can appear a deeper blue than just reflected sky could explain.
  
As to Rayleigh scattering, it depends a lot on the gas.  
+
And indeed, this is basically Rayleigh scattering too. The mechanics are slightly different, e.g. that being a lot more matter,
 +
it takes out all the light in much less distance, which is why being underwater looks blue-tinted darkness. Underwater it's also more apparent that the absorption is gradual/preferential: red starts going at a few meters, whereas the last one to go, blue, is much deeper.
  
The gas has to be polarizabile for Rayleigh scattering to apply, which narrows things down.{{verify}}
 
  
If the atmosphere primarily contained ''slightly'' larger sized molecules, you might get the most scattering elsewhere in the visible spectrum, probably red - basically the same thing we have here, with red and blue reversed.
+
When not underwater, it matters that waves reflect more sky (and probably the air mixing in also doesn't help see in{{verify}}).
  
Much larger and you wouldn't get this effect in the visible range.
 
If anything it's Mie again, going for white and, if there's a lot of it, gray or no color (and quite possibly no life, because little to no heat, which sits to the side of human-visible).
 
  
For example, the third on the list of our atmosphere, argon, would not contribute in visible light even if it were a large enough constituent[https://physics.stackexchange.com/questions/243975/what-colour-would-the-sky-of-a-high-oxygen-high-argon-high-water-vapour-atmosp])
+
Also, pools of water and seas near the shore are usually be the color of what's suspended in there[http://en.wikipedia.org/wiki/Colored_dissolved_organic_matter].
  
 +
Very clear and at least moderately deep water will be dominated by, well, the absence of that.
 +
Less light makes it out, and that which does will be more heavily biased to blue.
 +
So deeper blues are deeper (clear) water.
  
Ch4, CO2, Freon
+
You don't see these deeper blues much because water still enough to let things sink down, and be wave-less so you can find a spot with less reflection (or a polarization filter has a uniform effect), is rare.
  
  
 +
This effect is stronger from straight up, which is why deep clear water can appear nearly black on satellite images, though there's always some white reflection, particularly on regular photos from space.
  
Pressure has an infuence, but again, not so much in the range where human bodies want to live.
 
  
 +
https://earthobservatory.nasa.gov/features/ColorImage/page2.php
  
 +
---
 +
One reason we consider the sea to be blue is because we see most of it from a very shallow angle, at which it reflects the sky.
  
https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19770012747.pdf
 
  
  
 +
Water absorbs some light, but relatively little of it,
 +
which means that if there's anything in it reflecting light, that will be the color you see most.
  
See also:
 
* http://hyperphysics.phy-astr.gsu.edu/hbase/atmos/blusky.html
 
  
* http://en.wikipedia.org/wiki/Mie_scattering
+
Near the coast, you'll also see the surface under it - near beaches usually yellowish sand, and being so near is also lit fairly well, so this amounts to water a blue filter on top of a relatively bright yellow, which combines to a cyan for shallow water, down .
* http://en.wikipedia.org/wiki/Rayleigh_scattering
+
  
* http://science.howstuffworks.com/nature/climate-weather/atmospheric/sky.htm
 
* http://optics.kulgun.net/Blue-Sky/
 
* http://sci.odu.edu/sci/Scire/05Edition/whitecloud.html
 
* http://www.sciencemadesimple.com/sky_blue.html
 
* http://mb-soft.com/public/bluesky.html
 
  
* http://www.spc.noaa.gov/publications/corfidi/sunset/
+
Seas and oceans also have a bunch of sediment, particularly near costs, where there is more to kick up.
  
* http://en.wikipedia.org/wiki/Color_of_water
+
Different sediments can help add different tints, often green (also from plankton chlorophyll), red, and a few others.
  
  
  
  
 +
Even pure water absorbs light, and a little more on the red end, meaning that water has a very subtle blue tint.
  
The moon
+
In smaller amounts this is nearly invisible, and it is rare to look through a ''lot'' of pure water for practical reasons.
  
 +
Still, this is why certain still and clear water (e.g. [https://nl.wikipedia.org/wiki/Crater_Lake Crater Lake]) looks like a deep blue.
  
  
 +
There's an additional detail to this - light has to make it back at all, and ''because'' water itself absorbs light, deep enough water will eventually remove all light.
  
Blood moon
+
Little light makes it further down than a few hundred meters, and basically no light beyond a kilometer.
 +
 
 +
So if there's nothing in the first few hundred meters reflecting light back up, little to nothing makes it back up, and it just shows up as black.
 +
 
 +
 
 +
This is quite visible in some satellite images, also due to the straight-down angle.
 +
 
 +
Water will often be darker than you would expect from, well, maps, drawings, and photos of beaches.
 +
a lot of satellite imaging often looking in wider and specific range of light,
 +
even plain photos from space might have water brightened for us because it looks more natural to us.
 +
 
 +
Also, note that pictures of earth will always have some blue tint
 +
 
 +
 
 +
 
 +
* http://en.wikipedia.org/wiki/Color_of_water
  
  
 
-->
 
-->
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 +
==Clouds==
 +
<!--
 +
 +
http://sci.odu.edu/sci/Scire/05Edition/whitecloud.html
 +
 +
https://www.weather.gov/jetstream/color
 +
 +
-->
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 +
  
 
[[Category:Random everyday stuff‏‎]]
 
[[Category:Random everyday stuff‏‎]]
 
[[Category:Nerding around]]
 
[[Category:Nerding around]]

Latest revision as of 16:13, 12 September 2021

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, fix, or tell me)

The sky

On sunsets

See also

Might other planet's skies be other colors?

Aurora

The color of water

Clouds