View source for Colors of the world around us

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==The sky==
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tl;dr:

* '''The sky''' 
: ...blue due to the nitrogen and oxygen. Their molecular size just ''happens'' to have a more significant effect on the EM frequency in the visible region, specifically what humans call blue (via [https://en.wikipedia.org/wiki/Rayleigh_scattering Rayleigh scattering]).
: Since it's scattered, repeatedly, it's a diffuse effect throughout the whole sky
: there are other related effects, and when looking away from the light source, those other effects may be less pronounced, and the result a little bluer yet.
:: and the simplest of the other effect is "...clouds"


* '''Sunsets''' are red, and seem more so directly around the sun, actually for much the same reason
: that is, when you look at a sunset, you are looking at the horizon
:: seeing light that has passed through ''more'' atmosphere - so has ''more'' of its blue scattered out, leaving relatively more red
:: it helps that we're looking almost directly along the original light path; that's the direction where mostly the unscattered rays are visible
: pollution increases the effect a little






When you first hear this question, and have a middle school physics background, 
it is quite sensible to theorize that the reason that the sky is blue might be refraction.
Water droplets, perhaps.


It turns out it's not, but it's a sensible theory, water droplets are absolutely there, and are doing a thing, and in fact they are more interesting than you might think.

It's one of those cases where it's hard to know which mix of effects are there in the first place ''and'' hard to estimate which would be the strongest one.

So even if you figure that it can't be refraction, you wouldn't be left with a lot.


...a reason it can't be refraction is that if it were, 
then direction would like matter a lot more than it does - we would see different colors depending on where we are, 
and it would probably be almost entirely dark from behind us, opposite the sun.

Yet it's blue everywhere, fairly evenly.
{{comment|(And even photos from space stations reveal a blue sheen around earth)}}


It's also relevant that sunlight ''mostly'' makes it through, apparently mostly undeterred,
though we wouldn't necessarily know those numbers - if only a small part made it through that would also just a case of be "it is what it is".


Even arguments like "if it didn't make it through, then sunlight couldn't be particularly white" would be sort of true, but also ''really'' messy -- but let's be distracted by one thing at a time).


Also, direct sunlight and hard shadows probably wouldn't happen, 
because if most light were reflected it would probably be diffuse, it would come from lots of different places.
 

Refraction does happen in the sky, e.g. within cloud or mist droplets doing their rainbow thing, 
but you see very little of this, 
for few different reasons.
There are a few cool related effects that do show up sometimes, though - see e.g. {{search|iridescent cloud}} and [https://en.wikipedia.org/wiki/Glory_(optical_phenomenon) glory] (similar to a rainbow but different mechanics).



Turns out the blue-sky effect works at a smaller scale than refraction.

Light interacts with all particles, but larger ones just block it.

It turns out at a particularly tiny scale, that interaction is more interesting.

Looking at interactions just within the human-visible light range (~400-700nm) 
it turns out that interaction happens ''roughly'' the same scale of the smallest of molecules.


The physics of such interactions involves electric polarizability and a handful of other things we could spend a lot of time on, 
but when we simplify that a bunch, it turns out that ''if the size of wave is similar to a particle's size, interesting things happen''.

There are a few different effects happening at the same time, 
but with any one substance one of them tends to dominate,
and which one that is depends on whether wavelengths are on the same order, much larger, or much smaller.

Physics has different sets of equations to approximate the larger-scale effects, which is we can refer to these effects with a single, named formula.


When particles are at least a little larger, you use [http://en.wikipedia.org/wiki/Mie_scattering Mie scattering].
: which (unlike Rayleigh) is also a directional effect, with some more in front and some more to the side
: the amount of frequency selectivity falls off with increased particle size, {{verify}} and after a while it starts to be just generic scattering
:: ...which is a good part of why clouds appear white (gray just means thick clouds, or shadows from higher clouds)
:: and why the sky near the horizon often looks lighter blue - there's almost always a little cloud (or pollution)

For about the same size (often meaning colloids), [https://en.wikipedia.org/wiki/Tyndall_effect Tyndall effect] is significant (related to Rayleigh scattering)
: which is e.g. why fine smoke appears slightly blue.
: since there is a narrow band where this applies, we don't see this a lot.
: It's also subtle enough that you may assume you're just seeing it wrong.

For particles '''much smaller''' than the wavelength of light, there is [http://en.wikipedia.org/wiki/Rayleigh_scattering Rayleigh scattering].


(There are some further related effects, like [https://en.wikipedia.org/wiki/Raman_scattering Raman scattering])



For the sky, the largest components by far are nitrogen and oxygen ([https://en.wikipedia.org/wiki/Atmosphere_of_Earth#Composition together 99%]), and since both of those are roughly 150pm, roughly a factor thousand smaller than the wavelength of visible light, Rayleigh scattering is the effect that applies most.

Rayleigh scattering is [http://en.wikipedia.org/wiki/Elastic_scattering elastic scattering], which means that the energy (so wavelength, color) is preserved, and you can only change its direction.{{verify}}
Each such scattering is very subtle, but since there's a lot of atmosphere, it'll add up. 

This is also why it is quite a diffuse effect, why it looks fairly uniform throughout the sky, 
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 flies by undisturbed)}}




Further notes
* there are other contributing effects, Rayleigh scattering is just the largest.
: For example, Ozone is slightly blue gas by itself - but during the day the intensity of the Rayleigh effect makes that almost negligible. It's only right around sunrise and sunset, when there is little light, that its contribution matters to a degree you might notice at all{{verify}}

* People paying attention to the spectrum will be wondering why the sky isn't a blue-violet mix. 
: It is.
: Just not noticeably, mostly because our eyes are not as sensitive to violet as to blue ''and'' the sun puts out less of it (remember it's at the edge of visible).

* Scattering happens in all of the visible spectrum, but the size of these particular molecules mean it happens significantly more for blue than for red, perhaps by a factor four.
: (This relates to that visible light is a fairly large range, 400nm-700nm, and that the wavelength of red is approximately twice that of blue - this turns out is a large difference in this context).
: It's also not linear - on the red end the difference between colors is smaller, making blue stand out a little more than it would if it were linear.

* That amount of difference is noticeable, but not overpoweringly so, which is also why even through dozens of kilometers of relatively dense atmosphere, the sky is only a pale rather than a deep blue.


* only the sky looks blue, not objects, because most sunlight makes it through anyway
: the light that makes it through is also ''much'' stronger than the amount of scattered blue
: also why things still look white on overcast days (when you can't directly see the blue sky)
: (also, if a strong light source is close enough to white, we easily adapt to consider it white, regardless of its tint)


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===On sunsets===

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In most directions, the above effect works out as blue, most of the light involved is moving in straight lines, and we only sse a small amount of light scattered to the side (without this effect, the sky would look black).

However, when you are looking light that is pointed in your direction,
you see light with a ''less'' blue, since a bunch of it is scattered to the side.

Which makes that light relatively yellow, perhaps orange, and even red.


The effect is stronger around sunrise and sunset mostly because sunlight has to go through more atmosphere to get to you, meaning it scattered away more blue in the process.

The effect also happens during the day, but less pronounced as it's not passing through nearly as much atmosphere, and more importantly, the sun is way too bright to look at in the first place. (also, any clouds in the path will make it easy to see it as ''bright'' more than anything else)


There is also a brief period where the sunlight is lighting the underside of clouds 



Actually, with just nitrogen and oxygen, this amount of scattering usually gets you orange at worst.

To get deep red, you need fairly unusual circumstances.

Most relevant are microscopically dispersed small particles in air.

Which is the definition of an [https://en.wikipedia.org/wiki/Aerosol aerosol], but there are different kinds of aerosols.

Man-made aerosols, primarily pollution, are typically a mix of a ''lot'' of different things,
which means it ''easily'' includes larger particles. As a result, 
: most pollution and smog and such is too large to be a good Rayleigh scatterer, 
: some of it is going to mix in some Mie scattering making for grayer washed out colors,
: and particularly the larger particles just blocks more light so will look washed out and darker.

So while people argue that smog helps red sunset, this depends a lot on what is in there, and it often also works ''against'' it being bright.


It will only help create a specific color when it has fairly uniform 

Some natural aerosols (think forest fires, volcanic eruptions) are actually more uniform,
so ''can'' be more selective for a color.
It also helps that those two cases disperse a lot of it into a lot of atmosphere.
If it's in the right range, it can add more to Rayleigh scattering than to others, and some have specific colors.
(see the NOAA link below for more details)




Less pollution in your path is also the largest reason why on mountains and in airplanes you'll see nicer sunsets.

Temperature affecting how and how long pollution hangs around also explains why winter gives somewhat nicer sunsets.


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===See also===
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* 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/
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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



https://www.spc.noaa.gov/publications/corfidi/sunset/

http://hyperphysics.phy-astr.gsu.edu/hbase/atmos/redsun.html

https://www.loc.gov/rr/scitech/mysteries/weather-sailor.html


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===Might other planet's skies be other colors?===
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Yes, but there's a lot of assumptions in there that seem to add up to "...but not by much in a lot of practice".


The simplest change would be to have a different star[https://en.wikipedia.org/wiki/Stellar_classification].
Even with everything else being equal, a slightly different-colored light source, means different colors.

One question is whether we could survive. 

Another is whether eyes wouldn't naturally be selective in a similar 'looks like uniform spectrum to me' even though it isn't - the same is true for us with our sun.


But let's assume the sun and our eyes as constants, and vary only the atmosphere.


Different gases might lead to other colors, 
but there are relatively few gases likely to be abundant, most will be molecular gases so fairly small,
and relatively few which would have a significantly different effect (TODO: back that).


And if we want to not die from the atmosphere's contents, there's much tighter constraints.

Pressure also has an influence, but again, not so much in the range where human bodies want to live.




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 wild colors within human-visible range.



'''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.


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 - it seems to do the same, but the low pressure makes it less significant{{verify}}), so actually a clear martian sky looks almost black.

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.

Large enough to mainly have color the classical way, in this case because it has has rust mixed in.

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.


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].




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.

Nitrogen and oxygen don't. 
In fact, most gases have little or no color,
and most of the more exciting-colored gases are things we really don't want to breathe.

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






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]



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==The sun==
(TEMPORARILY ELSEWHERE)

==Flames==

==Aurora==
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[https://en.wikipedia.org/wiki/Aurora] is basically an ionization effect,
and the color dependent on the gases present.  

You don't ''want'' this to be very intense on your planet, 
because the main reason aurora is visible is that [[ionizing radiation]] making it further into the atmosphere. 

It happens primarily at 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, as well as our electronics (so varied infrastructure). 
In that regard we're lucky it fluctuates from little to some, not from some to more.

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==The color of water==
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: '''So is the sea blue for the same reason?'''

Well, partly it's just that we almost always look at the sea at such a shallow angle that most of what we see is reflection,
and the sky is blue. 
And since most of that reflection is from near the horizon and there's likely some clouds involved, often a lighter blue.


This is also also why polarization filters help look into water, and make any color of the water seem a little more intense.

...because yes, the water is also blue by itself. 
Which is how it can appear a deeper blue than just reflected sky could explain.

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}}).


Pools of water, and seas near the shore are usually be the color of what's suspended in there, e.g. [http://en.wikipedia.org/wiki/Colored_dissolved_organic_matter dissolved organic matter] or much we put into a nearby river, or just sand. 

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.


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.



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.


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 .


Seas and oceans also have a bunch of sediment, particularly near costs, where there is more to kick up.

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. 

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. 

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==
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http://sci.odu.edu/sci/Scire/05Edition/whitecloud.html

https://www.weather.gov/jetstream/color

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[[Category:Random everyday stuff‏‎]]
[[Category:Nerding around]]