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White out v Flat light (physics)

 Poster: A snowHead
Poster: A snowHead
I am writing a short paper on “stuff”.

I have all the physics down and was just looking for some definitions.

I want them to be
Correct
Simple to understand
Concise

I have looked all over Google (wiki is simply wrong on this topic, so I may edit it later when I have time) and can’t find 2 simple definitions.

So I have come up with my own Shocked

Any comments? (Oh god, shouldn’t have asked should I wink )

This is what I have come up with.



see last post

What do ya think - about right - load of rubbish - too long - waffle - just wrong - anything else
Puzzled


Last edited by Poster: A snowHead on Sun 13-05-12 7:53; edited 1 time in total
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 Obviously A snowHead isn't a real person
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Well those descriptions don't exactly fit in with your objectives of 'Simple to understand' or 'Concise'. I'm not convinced about their correctness either. In layman's terms I would have said something along the lines of.

White-out = dense fog where you can't see anything more than a few feet away.

Flat-light = Grey overcast conditions where shadows are diffused and there is a general lack of contrast.

The more I read your descriptions, the less like real life they sound. Especially your flat light description. I'd also go as far as to say that it is pretty much impossible to have a white-out without flat light.
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I think the 'lack of clarity' bit is somewhat woolly. Having it as the result of both definitions makes them sound the same in result but with different causes when that isn't the case.

More simply, more oriented towards skiing but less sciencey sounding:

Flat-light: Lack of visible definition in the snow surface.
White-out: Lack of visibility often accompanied by trouble making out the horizon line and accurately determining where the fall line is.

As a bonus here's a picture of the white out I was in last Sunday:
http://i.imgur.com/BM3S7.png
And skiing with no one in front to reference:
http://i.imgur.com/qunWa.png


Last edited by Well, the person's real but it's just a made up name, see? on Fri 11-05-12 9:42; edited 1 time in total
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I'd have said that a white out is poor visability caused by obscured vision by either heavy precipitation or mist/fog.
Flat light is poor visable definition due to diffuse light, as the eye/brain can't differentiate between near and far objects due to a lack of contrast

Although one may lead into the other, and you can have both at the same time, i'd say they were completely separate phenomenom.

Sorry.
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meh,
Can you delete or resize the pictures please.
They are way too big and messing up my screen

Cheers
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Wayne, sure thing. Smile
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uktrailmonster,
Thanks for that. In the paper I have already covered the difference between cloud and fog.


meh,
I wasn't looking for a description of the results (I have covered that in some details), I was looking for a definition of the conditions - thanks for the feedback though
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Richard_Sideways, Agree, I've always associated white-outs with fog, but could also be caused by heavy snowfall. Basically obscured vision in one form or another. Flat light is entirely different and caused by diffused lighting which occurs whenever conditions are overcast with no direct sunlight. With flat-light you lose shadows i.e. reduction in contrast. Very commonly seen in skiing and photography.
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Wayne wrote:
uktrailmonster,
Thanks for that. In the paper I have already covered the difference between cloud and fog.


You're welcome, but not sure how that affects your descriptions above. I think your definition of a white-out is quite vague and doesn't convey the severity of such conditions or give any clue as to what atmospheric/meteorological (what's the difference in these terms by the way) conditions would cause it. For example visibility is not simply reduced in a white-out, it is severely reduced. Same with clarity, lack of clarity is really no clarity at all.

As for flat light, I've no idea what reflections have got to do with it or multiple colours. It's really just a diffuse lighting issue due to an overcast day no?
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uktrailmonster,
Yep, you may be right.

I have gone along the lines that anything (fog, cloud, falling snow, spindrift, some types of rain, etc) that will scatter the spectrum (basically so we see white) will cause a white-out.

Most of the pictures I see of cloudy days that are shown as flat-light I would say are really white out.
Just trying to keep them separate (the conditions) not the effects and these are sometimes quite similar
You can have flat light on a sunny day but not a white out (below the dew point)


For flat light I have gone with the increase in color (normally around 430nm to around 500nm / blue end of the spectrum) due to a vast number of small particles being present at the right size to reflect small wavelengths. The basics being that this will sort of - but not quite - form a Reighley effect but with a particulate base
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Wayne, mmm... I think you've gone along the wrong lines on both in an effort to fit some physics theory to the situations. For example 'cloud' does not cause a skiing white-out unless you are physically in it i.e. 'fog'. Also scattering of the spectrum doesn't seem like quite the right terminology in this context, although may or may not be technically correct.

For flat light, I think you've gone way over the top. Any fully overcast conditions will cause flat light simply because the sunlight is diffuse. Think lack of shadows and contrast, anything else seems largely irrelevant to me on that one.
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Wayne, fair enough although that isn't really a definition of flat-light/white-out but the conditions that lead to flat-light/white-out, amended:

Flat-light: Lack of visible definition in the snow surface caused by diffusion of light, often by high cloud. (just for reference I've seen it in ash cloud as well after the Grimsvotn eruption last year, although it's less obvious without the uniform coloured surface)
White-out: Lack of any visible definition caused by diffusion of light and reduced visibility, often by cloud at ground level. (although you can be in a white out with pretty good visibility and high cloud as long as there isn't anything to provide contrast, as per the photo on the wiki)
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uktrailmonster,

The reason for all this is that I am going to give my 1st year physics student a 2000 word paper to write on “reasons for viability reductions in an alpine situation” and I thought I'd have a go myself
In the their papers I "expect" Shocked to see a completed run down from the solar hydrogen cycle, to propagation variations (with a bent on wave/particle duality) all the way to definitions of possible wavelength interactions with different crystal facets (in the case of snow) right through to some nutter falling over on a ski slope as they didn't see the mogul.

And they though all they had to do was join load of clubs and drink cider – ani’t uni life brill. Sooner or later you may just have to do some work. Laughing Laughing Laughing

OK, (thanks to everyone) I have changed it to this. Wad-da-ya-recon

see last post.


Last edited by So if you're just off somewhere snowy come back and post a snow report of your own and we'll all love you very much on Sun 13-05-12 7:54; edited 2 times in total
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 You know it makes sense.
You know it makes sense.
Have to say I like meh's description a lot better.
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clarky999 wrote:
Have to say I like meh's description a lot better.


But diffused light is, by definition, a white-out
Flat light is (may be wrong about this) light that cause objects to appear flat and uniform and without discernible features
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Hmm. A white-out, to me, is a very severe case of flat light where almost no references are available. So bad that if a cliff edge were in front, I would not know. I've very rarely encountered it, fortunately. Flat light is unpleasant - though coping with it can be satisfying. A whiteout is downright dangerous.
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From my POV and I might be wrong about this:

White-out - Everything looks white as in the picture I posted before.

Flat-light - Still lots of vis thanks to contrasting objects like cliffs and trees.

So they are both the same thing but flat light is a white out where there is still enough visibility to make out features.

There is a bit of a grey area between the two where you could argue over how much vis there needs to be to differentiate the two. For example in the picture posted before vis isn't terrible, you can see the two following skiers pretty clearly which I'm using to my advantage to give them something to aim at and an idea of slope angle. It's just we're on top of a glacier and everything is white.

This on the other hand is flat-light:
http://i.imgur.com/nL6MB.jpg

It could well be a white out if the cliffs were not there to give enough contrast and depth information.

Perhaps if you have a picture of what you think flat-light is then it might be easier to tell what you mean Wayne.
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Nope, changed my mind.
What I put above is just wrong as “resulting in a reduction of vision” has a number of implications. It “could” suggest that a White-Out is a perceived condition and it’s not. Even if no-one is there to perceive it, the condition can still be present. Also “resulting in a reduction of vision” is a result not a definition.
Maybe I need another job Toofy Grin



What about this Puzzled (we're getting there)

see last post.

I am trying to give the deffinition of the condidtion, not the results.
I have other sections of the paper that go into the results of the conditions and the causes of em.


Last edited by Well, the person's real but it's just a made up name, see? on Sun 13-05-12 7:54; edited 2 times in total
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I think it is misleading to use the word colours (note that there is a 'u' in colours even in Wigan wink ). As far as I am concerned you must use the word contrast in your definition of flat light possibly even white-out) and several other posters have also used the word contrast.

In what circumstances does a white-out cause flat-light Puzzled

Keep trying though Toofy Grin
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meh, sorry, I missed your whiteout photo links. They tie in well with my whiteout experiences.
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meh, I am with you on this.
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I'm with everyone else here - colour has nothing to do with flat light to me.

Cloud cover above you, meaning ambient light and no shadows = flat light
Cloud or heavy snow around you, meaning you can't see very far = white out
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halfhand, yer I'm thinking this as well from Wayne's description of flat-light it seems to me he means that it would have to be a property of the snow surface and the frequencies of the spectrum it absorbs or reflects. It confuses me because his description seems to mean the snow would look a different colour!?
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halfhand wrote:
In what circumstances does a white-out cause flat-light

If you have falling snow any light hitting the crystals will scatter so you have a white-out as the light is diffused (scattered) to such an extent that the colors merge into white; the falling snow will also physically block your view increasing the severity of the white-out. This white-out can cause flat-light as, assuming that the temperature is low enough (which it is quite often in ski resorts) to keep the crystals small, they will reflect only a section of the white light with a small wave-length, this will also distort (overpower) any light reflecting from trees, posts, cafes, etc and make it hard to see whats going on around you.

It’s a bit light splitting the whited-out light with a prism but only getting a bit of the resultant light on the other side of the prism.

This has the effect of a reduction is overall visibility of features due to the white-out and also a reduction in definition of the feature (that may be still visible) due to the flat light

There are lots of other ways in which these 2 conditions can be combined.
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Wayne, are snow crystals really small enough given they are typically a lot larger than the wavelength of the light? For example 10 micrometers in diameter versus 0.75 micrometers for the largest wavelength of visible light. Rayleigh scattering only occurs with particles smaller than the wavelength right? Rather it's Mie scattering in this case (which is why clouds look grey)?
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Whiteout - Fog/cloud/blizzard etc

Flat light - lack of contrast caused by sunlight being blocked

Degrees of both but in extremis a blizzard where sun was still shining through would probably not be a whiteout (and may not technically be a real blizzard)

You can easily have flat light on a clear day after sunset.
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meh wrote:
Wayne, are snow crystals really small enough


Yes, Mie would be applicable in some conditions IF crystals were smooth or (dependant on the conditions they formed and amalgamated in) there was no faceting. You will find this simplistic answer all over the internet, but it’s not the full story as that’s not how snow/ice is in reality.

Remember the surface area of an ice crystal consists of A and C facets. A Bergeron progression will normal cause the C axis to be larger (initially) but in virtually all cases the A axis will grow to a larger size.

This will produce many surfaces small enough for R/S. Mind you this assumes that you agree with the Mulins- Sekerka Instability theory (which I tend to think is about right - but what do I know wink ) and if it is then you can get reflection of specific ranges of wavelength from a crystal that is larger than would normally be associated with R/S.


Last edited by And love to help out and answer questions and of course, read each other's snow reports. on Fri 11-05-12 12:11; edited 1 time in total
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Oh well Madeye-Smiley .

I have looked with a great deal of interest at everything that's been posted - cheers for that everyone. It’s always too easy to come up with what you think is wonderful only then to put it out and see what people think, only to be told to go away and do it again.

I am one of the few physicists that you'll come across who will always willing to admit he cocked up (it happens so often, got no choice Toofy Grin ). I am going to sit and think for a day or so and see if I can come up with another definition - as it seems my first stab at it was way out.

Thanks again


PS. They are similar but Fog is different from Cloud - but that's another story Madeye-Smiley
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Wayne, interesting!
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Wayne, Not sure if it's too late, but I'd have thought the simplest way to differentiate between the two could be:

White Out - result of precipitation, i.e. snow.

Flat light - result of 'overhead' conditions, cloud \ fog.
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Pour moi:

White Out: viz is impared severely because of non-transparent material in the air - ie. it's an emulsion.

Flat Light: definition is impared because sunlight is so diffuse that the brain has little or no shade / shadow information - hence everything appears 'flat'.

In contrast, false shading in this 3D map gives it lots of 'shape':


And indeed here:


Last edited by Poster: A snowHead on Fri 11-05-12 13:57; edited 4 times in total
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AndAnotherThing.. wrote:
Wayne, Not sure if it's too late, but I'd have thought the simplest way to differentiate between the two could be:
White Out - result of precipitation, i.e. snow.
Flat light - result of 'overhead' conditions, cloud \ fog.

I Like that


altis,
Some mod's gona shout at you Shocked

meh,
I still keep falling over on skis though Toofy Grin
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fatbob wrote:
You can easily have flat light on a clear day after sunset.

Before sunrise too, had that problem last month.
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Quote:

So they are both the same thing but flat light is a white out where there is still enough visibility to make out features

Don't agree with that. You can encounter flat light in conditions where the air is very clear and you can see for miles and miles, not just "make out a few features". For me, a white out is not being able to see the next piste marker post (and sometimes not even your feet, so the issue of whether the light is "flat" is immaterial). That's why I have previously argued that trees are no help in a proper white out - as you won't see a tree till shortly before you smack into it. In flat light they are indeed helpful, giving a clue to the lie of the land, etc.

I have never personally encountered flat light in sunny conditions. How can you have a lack of shadow if the sun is shining? Poor visibility, however (e.g. when you can see from one piste marker post to the next) invariably seems to be accompanied by flat light, where you look 10 metres ahead, where you expect to see features on the snow (ruts, bumps etc) and see just flat whiteness.

There may be a "commonsense versus physics" issue here.

If I am in sunshine, looking up the mountain at heavy clouds sitting on the top, I have a shrewd suspicion that it will be "white out" conditions at the top. ie I shall be right in the middle of the cloud and see nowt, just like looking out of the window of a plane in heavy cloud. What's the difference?
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altis, haha, subtle edit! Laughing
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pam w, yer I also didn't think of the example given by fatbob where the direct sunlight is occluded giving flat light conditions in the shadow under some circumstances. So they aren't quite the same thing as flat-light can occur without the need for clouds.

The image I link in that same post is what I meant by still enough visibility. You could actually see a long way it's just I happen to be looking down a 45 degree slope taking off my crampons so it doesn't really show.
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Flat light - light that results in an early afternoon beer.

White out - light that results in an early morning beer. Invariably followed by a black out.
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Wayne, I think on the flat light bits, there's a pretty big difference in what you define and what the rest of us think.

For most of us, flat light has nothing to do with color!

Reading through most of the posts, that's the impression I got. Flat light is purely and simply due to too many light sources from all directions. So all shadows are washed out. And one can't see the moguls or even the surface sloping down vs up. But one can still see quite clearly a lift tower, or the piste marking post from a good distance away.

In the white out, one may not be able to see even the piste marker till one's almost right in front of it.
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Quote:

And one can't see the moguls

very good point. Was once skiing with son in Tignes. He sent me down a blue and said he was going to do some spiffing moguls he'd enjoyed the day before and would see me at the bottom. He soon caught up with me, on piste. He's stood at the top of the mogul field, where he knew there were huge bumps, and it was all just - well - flat! So he chickened out. But there was no overall visibility problem that day. You could see for miles.
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AndAnotherThing.. wrote:
Wayne, Not sure if it's too late, but I'd have thought the simplest way to differentiate between the two could be:

White Out - result of precipitation, i.e. snow.

Flat light - result of 'overhead' conditions, cloud \ fog.


To expand a bit,

White Out - result of precipitation, i.e. snow - reduction in visibility (while fog reduces visibility, for me, in a ski context 'White Out' would be about snow\blizzard.)

Flat light - result of 'overhead' conditions, i.e. cloud \ fog - reduction in contrast
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