Blue Shadows on the Snow

[wonders9]

The most recent update (179) showed images of winter maps where there are blueish shadows on the snow. Some people tend to think that shadows should not be blue. 

 

In fact the real shadows on the snow are blue or at least bluish. Both art and science will tell us that they are so. The reason is that the sky is blue, whose light hits the snow and is reflected by it. The parts of the snow that are not in shadows are lit by the sun and the sky together, but sunlight is usually much much brighter than light from the blue sky, so these parts are mainly of the colour of sunlight. The parts of snow in the shadows, however, are lit only by the blue sky, hence the blue/bluish shadows. (Oh, one can always see what colours they are by direct observation.)

 

The same goes with shadows on aircraft with a white skin (but the belly is also lit by the ground, so things are more complicated in this case).

 

Several pictures are attached to illustrate what has been said.

 

Some might argue that photos may not exactly represent real life colours on the earth. For them I would point out that unedited photos are reliable with regard to at least one fact, the fact that the colour of the shadows is very similar to that of the sky. So if the sky is blue, the shadows on the snow will be blue; if the sky is not very blue, the shadows won't be either. 

 

 

It is interesting to note that the shadows on thick clouds (the dark side of the clouds not lit by the sun) should reflect to some degree the main colour of the ground, for similar reasons. This effect does not seem to be present in the current BoS graphics. If it is present, scenery in the sim will be much more harmonious.

 

_

Edited November 15, 2017 by wonders9

[CanadaOne]

Your post, though excellent is incomplete.

 

You have to mention where you live, how cold it gets, and brag about the bigness of your snow.

[wonders9]

[Luftschiff]

Capital work, I came here expecting nonsense - Instead I got the opposite. Great post! 

[xvii-Dietrich]

In fact the real shadows on the snow are blue or at least bluish.

 

Under certain conditions.

 

But, yes... shadows on snow, on a clear day, with the sun not at any low elevation will appear bluish to the human eye.

 

 

 

Both art and science will tell us that they are so.

 

Science is a methodology, not an authority.    Don't get me started...

 

 

 

The reason is that the sky is blue, whose light hits the snow and is reflected by it. The parts of the snow that are not in shadows are lit by the sun and the sky together, but sunlight is usually much much brighter than light from the blue sky, so these parts are mainly of the colour of sunlight. The parts of snow in the shadows, however, are lit only by the blue sky, hence the blue/bluish shadows. (Oh, one can always see what colours they are by direct observation.)

 

 

... are lit by mie-scattered and rayleigh-scattered light from the sky, and other scattered light from surrounding objects.

 

Additionally, the O-H vibrational mode of water in the snow itself results in the absorption of red components of the spectrum, enhancing the "blueness" (especially under packed conditions; c.f. glacial blue ice).

 

 

 

The same goes with shadows on aircraft with a white skin.

Only partly. White paint will certainly reflect ambient light, but it has no vibrational absorption or partial transmission properties.

 

 

 

 

Some might argue that photos may not exactly represent real life colours on the earth. For them I would point out that unedited photos are reliable with regard to at least one fact, the fact that the colour of the shadows is very similar to that of the sky. So if the sky is blue, the shadows on the snow will be blue; if the sky is not very blue, the shadows won't be either.

 

Photos (especially digital) are a re-representation of the wavelength sensitivity of the sensor (= camera pixel) convolved with the wavelength spectrum of the output pixel (= monitor pixel) convolved with the response of cone cells in the human eye. For broadband emission, this works fine (sunlight is essentially broad-band emission). For narrow band emission (e.g. aurorae), it usually fails due to mono-chromaticism. However, all photos are a "real" representation, in that their response functions are real effects. It is just that some people have different definitions of real.

 

 

 

 

It is interesting to note that the shadows on thick clouds (the dark side of the clouds not lit by the sun) should reflect to some degree the main colour of the ground, for similar reasons.

Partially.

 

However, cloud bases also have collective multi-path scatter from within the cloud itself. This is solar white-light, although also convolved with Rayleigh scatter in the atmosphere and internal O-H vibrational absorption.

 

 

 

 

 

 

 

References

 

• https://en.wikipedia.org/wiki/Blue_ice_(glacial)
• http://www.dartmouth.edu/~etrnsfer/water.htm
• http://hyperphysics.phy-astr.gsu.edu/hbase/atmos/blusky.html
• http://hyperphysics.phy-astr.gsu.edu/hbase/atmos/scattercon.html
• https://scied.ucar.edu/longcontent/snow-white-maybe-or-maybe-not

[wonders9]

Capital work, I came here expecting nonsense - Instead I got the opposite. Great post! 

Glad that you like it. 

[wonders9]

Under certain conditions.

 

But, yes... shadows on snow, on a clear day, with the sun not at any low elevation will appear bluish to the human eye.

 

Good post!   Very exact and interesting. It is always good to know some new knowledge. It's long so I am not quoting all the post.

Well, my post was not intended to be rigorous, but I hope it has stated the major factor that makes the shadows have the colour they are. I guess it has.

 

the O-H vibrational mode of water in the snow itself results in the absorption of red components of the spectrum...

 

I like this part the most. 

Edited November 15, 2017 by wonders9

[Tone71]

Most importantly, it looks cool (pun intended). In all seriousness, great post, thanks! 

[Lensman]

I've seen yellow shadows on snow ... not blue, YELLOW!

At least I think they were shadows ...

[CIA_Yankee_]

Your post, though excellent is incomplete.

 

You have to mention where you live, how cold it gets, and brag about the bigness of your snow.

 

Agreed. How can we trust the OP as an authority on the subject if he lives in Texas, for example?

 

Bigness of Snow is an important determining factor for whether or not we believe the OP.

 

I live in VT, where snow has much bigness, for example, and I do find blueish shadows on that bigsnow at times.

[IckyATLAS]

I learned something. Good post.

[Gambit21]

Agreed. How can we trust the OP as an authority on the subject if he lives in Texas, for example?

 

Bigness of Snow is an important determining factor for whether or not we believe the OP.

 

I live in VT, where snow has much bigness, for example, and I do find blueish shadows on that bigsnow at times.

Yet other times you will I imagine.

I live in Wa State where we get snow from wee bigness all the way to heap big...uh...bigness.

As a trained artist though I dare not attempt to render snow, as I'm a lowly American who couldn't possibly grasp how to pull this off.

 

Did you know that only someone from Nebraska can paint or design grasslands? Also only a person from Arizona or Africa can render sunshine in a scene - even if you're an artist.

It's a fact - also you can only illustrate a hockey scene if you're Canadian or Russian.

 

I wish I'd learned of these limitations before I went to art school back in the day or spent all that time learning 3D software.

[CanadaOne]

Agreed. How can we trust the OP as an authority on the subject if he lives in Texas, for example?

 

Bigness of Snow is an important determining factor for whether or not we believe the OP.

 

I live in VT, where snow has much bigness, fo r example, and I do find blueish shadows on that bigsnow at times.

 

I love Vermont!  

 

We had a cottage on Lake Chaplain for years and I still have family in the area. Great place and great people.

 

That said, your snow does not compare to ours.

[Gambit21]

You guys need to stop crossing the border just to get milk at Costco - seriously. It's a zoo.

[II/JG17_HerrMurf]

There are no Costco’s in Canada? Say whaaaaaaaaaaa?

 

Also, does Canadian milk have a bluish hue in winter?

 

And just as importantly, can anyone explain why Aunt Beru’s milk was blue? They were in a desert with hardly any moisture much less snow!

Edited November 15, 2017 by II/JG17_HerrMurf

[sniperton]

Until someone from the Antarctic shares his/her opinion regarding snow and chilly weather, I don't believe anyone.  
 

[ShamrockOneFive]

There are no Costco’s in Canada? Say whaaaaaaaaaaa?

 

Also, does Canadian milk have a bluish hue in winter?

 

And just as importantly, can anyone explain why Aunt Beru’s milk was blue? They were in a desert with hardly any moisture much less snow!

 

Oh we totally have Costco. They were all Price Club before that.

 

Milk only takes on a blueish hue when its in blue bags (yes, milk in bags, also they can be blue sometimes).

[Gambit21]

Yeah the Costco near us is just the closest one for our Vancouver peeps just to the north.

[Livai]

Depends on the Sun, current Weather, Environment and Time how it reflects the Colors or your Monitor how you imagine things ->>> "Imagination is the beginning of creation"..........................    
 

[Boneboys]

I've seen yellow shadows on snow ... not blue, YELLOW!

At least I think they were shadows ...

Don't eat the yellow snow, Nanook rubs it...

 

Ice blue is a color not related to rayleigh scattering (or so I'm led to believe).

[Mesha44]

Also, does Canadian milk have a bluish hue in winter?

 

Wrong question.  We are discussing snow, so the correct question would be does Canadian Icecream have a bluish hue?

 

and the answer is "Depends on the Flavor"

 

 

but seriously this is a great thread, and nice comment by Livai also.

Edited November 16, 2017 by Mesha44

[wonders9]

I forgot to mention this: 

 

Generally speaking it is not easy for the untrained eye to recognize the colours of shadows in real life (very difficult when the shadow is on an object that is not white). Instead, the human eye tends to see an object as having the same colour (which is the answer to the question "what colour is that object?") whether or not it is in shadow. The human visual perception system is biologically programmed to be like this. That is why some (perhaps many) people tend to think that the shadows on snow should not be bluish.

 

Yet it seems that the colours of shadows in a photograph are easier to recognize, so the photos are a good means of showing the colours of shadows - at least one can use a colour-picker to analyse them.

 

If one does not find it difficult to discern correctly the colours of shadows in real life, then he is gifted for the art of painting.  ​ 

Edited November 16, 2017 by wonders9

[wonders9]

Agreed. How can we trust the OP as an authority on the subject if he lives in Texas, for example?

 

Bigness of Snow is an important determining factor for whether or not we believe the OP.

 

I live in VT, where snow has much bigness, for example, and I do find blueish shadows on that bigsnow at times.

 

I think how big the snow is will not have any noticeable effect on the shadows' colour as long as the snow is white. What really matters is the colour of the sky, for example, how blue the sky is.

[Uufflakke]

 "Imagination is the beginning of creation"..........................

 

 

 

Are you Bob Ross...   

 

[xvii-Dietrich]

 

Yet it seems that the colours of shadows in a photograph are easier to recognize, so the photos are a good means of showing the colours of shadows - at least one can use a colour-picker to analyse them.

 

 

There are two effects here...

 

 

The first is a light-intensity effect. A real-life scene has huge variations in the overall brightness and directionality of the light. A photograph is usually viewed under nearly-uniform lighting conditions.

 

The second effect is scale. The real-life scene covers larger areas, whereas the photograph is condensed. The eye is much better at comparison of nearby-objects, but not so much of disjunct ones. Imagine making your photograph larger, then cut it up and put the shadow parts on one side of the room and the bright parts on the other... now try to compare them. That's difficult. However, on the small photograph, the brain can undergo direct differential comparison.

 

These two things are a major challenge for computer graphics: high-dynamic range (HDR) and disparity of scale. But technology is getting better and our virtual-view of the world is increasing in fidelity.

[wonders9]

Depends on the Sun, current Weather, Environment and Time how it reflects the Colors or your Monitor how you imagine things ->>> "Imagination is the beginning of creation"..........................    

 

 

In the second photo the sky is grey, so the snow looks greyish.

[wonders9]

There are two effects here...

 

 

The first is a light-intensity effect. A real-life scene has huge variations in the overall brightness and directionality of the light. A photograph is usually viewed under nearly-uniform lighting conditions.

 

The second effect is scale. The real-life scene covers larger areas, whereas the photograph is condensed. The eye is much better at comparison of nearby-objects, but not so much of disjunct ones. Imagine making your photograph larger, then cut it up and put the shadow parts on one side of the room and the bright parts on the other... now try to compare them. That's difficult. However, on the small photograph, the brain can undergo direct differential comparison.

 

These two things are a major challenge for computer graphics: high-dynamic range (HDR) and disparity of scale. But technology is getting better and our virtual-view of the world is increasing in fidelity.

 

I agree on both of them. Perhaps there is yet one more reason: the real-life scene is 3D, but a photograph is 2D.

[xvii-Dietrich]

 

In the second photo the sky is grey, so the snow looks greyish.

 

Indeed.

 

Those images are great examples!

 

As an experiment, though, try cutting the pixels out of the images and put them side by side without the context of the scene. The gray shades are not as close a match as you might first expect. (Tip: try averaging them in blocks to reduce pixel noise.)

 

And, as a thought experiment, consider why the clouds appear grey, if above them is blue sky?

 

 

And then you have this effect...

 

 

... still cloudy, but the snow/ice appears blue in places.

[JG1_Daze]

[SvAF/F19_Tomten]

And then you have this effect...

 

blueicesnow.jpg

 

... still cloudy, but the snow/ice appears blue in places.

I don't know much about this subject, but since you posted a picture of a seal and I love seals I agree with everything you say... Just look at that seal, it's so awesome!

[Gambit21]

Thread solved.

You're welcome.

 

[CanadaOne]

Two happy couples.

[wonders9]

 

And, as a thought experiment, consider why the clouds appear grey, if above them is blue sky?

 

 

And then you have this effect...

 

blueicesnow.jpg

 

 

Interesting question , I'm not sure ; what's your answer?

Actually that grey is still a little bit bluish, which is too subtle to be noticeable, but contributes to the tonality (or harmony) of colours of the scene. 

Edited November 17, 2017 by wonders9

[senseispcc]

If you are a painter of the 20th century you know that all shadows can be or should be blue! But some very dark other more transparent. But some painter since then have come back from this absolutism and gone back to more dark browns or greens. It is all a question of light; do you paint with light or with paint?  

[wonders9]

 

If you are a painter of the 20th century you know that all shadows can be or should be blue! But some very dark other more transparent. But some painter since then have come back from this absolutism and gone back to more dark browns or greens. It is all a question of light; do you paint with light or with paint?  

 

Yes the Impressionists showed us the colours of shadows outdoors. 

[xvii-Dietrich]

Interesting question , I'm not sure ; what's your answer?

 

O-H vibrational mode absorption in the water.

 

https://en.wikipedia.org/wiki/Blue_ice_(glacial)