# Why don't nearby atmosphere look blue.

1. Jan 29, 2014

### ovais

Hi all

We know white sun light consist of seven colours and that when it passes through the atmosphere then due atmosphere it scatters. Amount of scattering is more for shorter wavelengths( maximum for voilet). Due to multiple scattering the effect of voilet scattering washes away and blue scattering dominates at large distance.

The atmospheric particles absorbing blue light radiate it ( after some time) in all,directions( this is what scattering is, right?). Text says this scattered blue light reach our eyes and therefore sky appear blue to us(when we look up or at far open distance).

My question is that if particles ( gas molecules) scatters blue light, why don't we see every thing(empty space between my room window and the tree in the lawn) blue, ( after all it is atmosphere only between my window and tree) rather the atmosphere in between appear some what pale yellow. Why only atmosphere of above height(I.e. Sky) appear blue.

Regards

Last edited: Jan 29, 2014
2. Jan 29, 2014

### A.T.

There is not enough air on this short distance for the effect to be noticeable. Distant mountains appear blue.

3. Jan 29, 2014

### ovais

Thanks A.T. for your reply, but the light(blue) has already been scattered, from huge atmosphere(from where it is comming).

It does not make any sense that the scatterers must be far away to observe the scattering taking place( I don't know what I am missing still).

4. Jan 29, 2014

### sophiecentaur

You are missing the fact that the nearby atmosphere is scattering blue light. It's just that there are 100km+ of more distant atmosphere in the same direction which are contributing equal amounts (per metre of path).
Also, you need to look again at the 'Blue' sky. It is only 'Bluish'.

If you can get hold of a photo of a scene with the bluest sky you can find and look, with a photo-processing program, at the actual RGB values in that blue patch. If the Blue were saturated, R and G values would be zero. They are far from zero.

We can be very sensitive to some de-saturated shades. Do the same RGB test on pictures of faces, they may appear very different 'colours' but you will find that the ratios of RG and B differ by only a small amount. This is true, even for pale Nordic skins and dark African skins - much less difference than you could ever imagine; it's the luminance that is different, rather than the chrominance because the dark pigment is pretty much a neutral grey..

5. Jan 29, 2014

### ovais

I will highly appreciate if you eleborate

1. why do we need a thick layer (100km +) of atmosphere to observe scattering?

2. When the upper atmosphere already scatter the blue light(which needs to reach our eye here on earth, and is reaching also), how come this blue light changes to yellow or white again?

3. Since the blue light(after scattering) reaching to us(when we look up the sky) it means the lower atmosphere is not hindering the blue light to reach our eye, then why it is not blue in lower atmosphere as in the sky dispite the fact that blue light is reaching the earth?

Last edited: Jan 29, 2014
6. Jan 29, 2014

### D H

Staff Emeritus
What you are missing is distance.

Rayleigh scattering is a low probability event. A clear sky is almost like a vacuum to visible light. The mean free path between scattering events is in the tens of kilometers for blue light and is several hundred kilometers for red light. When you look across a room, the light that is reflected off the far wall essentially is subject to no scattering whatsoever. The distances have to be large (multiple kilometers) before you start seeing scattering effects.

7. Jan 29, 2014

### sophiecentaur

If there were no atmosphere, we would see no scattered light. Hence the pictures in space have a black background. Very high flying aircraft see less scattering and the effect gets progressively less with altitude. The light that you see consists of all wavelengths - just more blue than the longer ones. Go to a distance of, say 100km and you will see a pretty dark sky but it will be detectably 'blue' because the proportions are pretty much the same (slightly different because the gas mix changes with altitude). On the surface, we see the maximum of scattered light,

The blue light doesn't 'change colour' to white of yellow, with altitude.
If you consider the light in the direct path between Sun and your eye, the more atmosphere it passes through, the bigger proportion of blue light is lost (scattered away) and what is left will be progressively 'redder and redder'. Hence the Sun looks reddish when lower in the sky because its light is passing through a longer path through the atmosphere. What has been lost from that reddish light that we are seeing is visible to other people as a blue sky.
As D H says, scattering is a rare probability event so the scattered light that we see from a blue sky is a small fraction of the Sun's brightness (say 1 millionth). Some if this will be scattered again, of course (but 1 millionth of 1 millionth is just not detectable)

8. Jan 29, 2014

### ovais

Many of my doubts about scattering are getting cleared due to you guys, and now I understand how distance(continues scattering over long distance) is important to,observe an appreciable amount of scattering.

But one thing more I want to ask to make things crystal clear :

I understand for appericiable scattering to take place distance should be large and that the nearby atmosphere(due to not sufficent thickness or distance from our eye) is not able to do enough scattering. Now my question becomes, why do I need to look at distant atmosphere( Sky I.e. Why do I need to look towards that direction) to observe the scattered blue light, while the fact is that light from any direction during the day is the same scattered light which is in sky above) When the long distance atmosphere is successful in scattering the blue light(upto earth) I should be able to observe it at any angle, whereever light of sun(after scattering from long distance atmosphere) is reaching.

This logic of mine urges me to have blue even in nearby atmosphere, while at the same time I agree with D H, that lower atmosphere(itself) could not provide sufficient scattering to produce blue.

Sorry for my less understanding and weak mind which is slow in learning.

Last edited: Jan 29, 2014
9. Jan 29, 2014

### sophiecentaur

The sky looks pretty much blue in all directions - except directly at the Sun. Even when directly overhead, the Sun will not look exactly the same colour of 'white' that it will appear in space.
This only applies to a perfectly clear sky. Clouds of water droplets (and ice crystals) and dust can change the situation. You very seldom get perfect atmospheric conditions to support what I am saying.

You can do a simple experiment (a scale model of the atmosphere) in a dark room with a very few drops of milk added to water. A lamp or white OH projector beam, viewed directly through the cloudy, scattering, water will look redder and you will see a bluish haze in the water from the side. The walls need to be low reflectivity (blackout fabric preferably), if you want a good result. Being so much more dense than the air, the effect shows with only as few tens of cm of cloudy water. A fish tank is quite good for this and the water cloudiness needs to be barely detectable.

10. Jan 29, 2014

### ovais

***scatterred away*** ....***left*****

sopiecentanur I think a part of my confusion is that what really happens after scattering does the coloura filter out(as you say sactter away) as they pass and only unscattered liight could cross the curtains(after scattering) of atmosphere and each time a colour scatter away the white light loses its colours starting from blue(or voilet)? If this is the case how can we say blue sky when blue has filtered out and is not reaching the earth(in our eye).

And another case may be scattering may mean that colours just came in visibilty as they scatter. Each time when when an particular colour scattered by sufficient amount we can observe it. But why do we need to look at a particular angle(I.e. towards sky) to observe the scattered blue light(scattered by long distance atmosphere) as I questioned in prevous post.

Thanks a Bunch

Last edited: Jan 29, 2014
11. Jan 29, 2014

### A.T.

12. Jan 29, 2014

### sophiecentaur

Take a beam of sunlight that is passing through the atmosphere 50km over your head. You will never see that beam. All you will see is a tiny portion of light that's been scattered out of that beam and in your direction. The spectrum of that scattered light is slightly tilted up on the short wavelength direction so it looks bluish. The 'colour' of the scattered light (i.e. relates to its spectrum) is always the same. As I said before, the blue from the 'blue sky' may be scattered but that's only 1/millionth of the millionth that came from the Sun (in any particular direction) The secondary scattering you refer to is just not relevant.

The only 'direct' light you will see from the sun comes in a straight line from the Sun to you. (That's obvious but you need to bear that in mind.) You only see the sun at one spot in the sky. All the rest is scattered light.

Have you read the wikipedia article on 'why is the sky blue'?

13. Jan 29, 2014

### sophiecentaur

Don't believe what you read there. They seem to believe in antigravity. lol

14. Jan 29, 2014

### FactChecker

My 2 cents:
The scattered light from the sky is the blue component of light that otherwise would not reach your eye. The rest goes right on through to space. So you see more blue from the sky than other colors. Sunset is what remains after the blue has been scattered away and the remaining light clipped the earth and didn't quite go through to space. Suppose you look at something on the ground. Some blue light from it is scattered away from your eyes. Based on that alone, it should look less blue. But that missing blue is replaced by scattered blue light from other objects near it. So it evens out. The mind compensates for any small differences and you do not notice them.

Last edited: Jan 29, 2014
15. Jan 30, 2014

### sophiecentaur

Be careful with your words here. Light of all wavelengths is scattered; it's just that more blue than red etc. is scattered. Your computer RGB values will show you that, even if you don't like the theory.

16. Jan 30, 2014

### FactChecker

You made a good point that when you look at the sky away from the sun it is primarily the scattered light that you see, whereas looking directly at the sun shows you more of the original light (although some higher frequency light has been scattered.) However, that doesn't address the original question of why that does not appear to happen to light from other objects. The computer RGB values only measure, not explain. I was trying to explain without introducing unnecessary complications.

17. Jan 30, 2014

### sophiecentaur

It does. If you take a photograph of something in shade (no direct sunlight) it looks a different colour from how it looks in the sun. (White Balance) Different but not very different (more RGB values would show the amount) because the light from the sky is pretty much White - with a strong hint of Blue. All digital cameras, these days, have an auto colour balance setting that adjusts the RGB gains so that the whole picture balances out to an average 'grey', helping to reduce the colour balance problem. Old film cameras were much more 'honest' and a roll of film gives a vast range of reproductions of someone's face because it just says what it sees.

The problem is that the intuition about colour vision (even down to the "seven colours" in the OP) is so strong that there are a lot of misconceptions that need to be dispelled before anyone can get a proper grasp.

18. Jan 30, 2014

### ovais

Ok so it means we just see in sky(in general other than the case of watching sun directly) the scattered light only, this now makes me well understood as to why actually sky is blue, i read wikipedia also.

Now let me put what I understand.

Both scattered and unscattered(white) light is reaching the earth and are stricking the particles of(lower atmosphere) to view atmosphere(with whatever colour) light again have to scatter from these particles of lower atmosphere. Thing is that the scattering of the particles of lower atmosphere is not sufficient enough(due to short distance between them and us) to get blue in amount to reach our eye. And almost unscattered light(from lower atmosphere) is reaching our eye when look at lower atmosphere.

Is this what you people are saying? Is my interpretation Ok?

Last edited: Jan 30, 2014
19. Jan 30, 2014

### FactChecker

The blue tint of a photograph taken in "open shade" is the opposite effect that would be expected when the blue light from an object is scattered. When the blue is scattered, one would expect a red tint in the remaining direct light, sort of like a sunset, but not so extreme. The point I am trying to make is that the scattered blue light from other objects can make up for the reduction of blue directly from the object you are looking at. In fact, the case of open shade, there is such a large component of scattered blue and so little direct light, that the overall effect is a blue tint to everything. The effect is clear and measurable "even if you don't like the theory".

20. Jan 30, 2014

### sophiecentaur

What do you mean by that? The only unscattered light that gets to you is directly from the Sun. All the other light from the 'sky' has been scattered.

The effect of secondary scattering (you seem to keep ignoring this) is absolutely minimal and not part of this explanation. (1/1,000,000 X 1/1,000,000 = Nothing)

Could you draw a picture showing where you think this "un-scattered light from the atmosphere" is supposed to come into existence? It either goes directly in a straight line from the Sun or it is scattered.

21. Jan 30, 2014

### A.T.

It has nothing to do with lower and upper atmosphere : distant mountains are blue because of the lower atmosphere. In fact the lower atmosphere scatters more because it is denser.

It has nothing to do with distance between us and the particles : light is very fast.

It has nothing to do with lower and upper atmosphere:
- Unscattered light reaches your eye, when you look directly into the sun.
- Scattered light reaches your eye, when you look somewhere else than the sun at the sky or at distant backgrounds.

The amount of scattered light we see, depends on the amount of air along the line of sight. That amount depends on the distance between eye and the background, where the air ends (mountain, space).

Try this starting at 9:00 min:

22. Jan 30, 2014

### sophiecentaur

haha touche! but didn't I make that point already?
I'm not actually sure what you are objecting to.
'Reflected light' from nearby objects (Rayleigh scattering is not from solid objects) will, of course, affect the observed colour of a scene. How is this in any way relevant to the simple model that I am trying to use.? In any case, it will not affect the observed colour of the sky (because it is only passing through a few metres of atmosphere and because of the 1/1,000,000 factor). The colour of the sky is precisely what one would expect. It needs no other compensating effect to explain why pictures taken in the shade look bluer. (Additional coloration from a nearby 'coloured' reflector can be anything you choose).

Did you see the photos taken on the Moon? There was a load of rubbish quoted as evidence that the photos were taken on Earth - partly on the basis that the shadows 'should' have been totally black. That ignored ground reflections, of course. But the sky still looked totally black, in spite of ground reflections. The colour of the sly on Earth has a vanishingly small contribution from scattering of ground reflected light. Quoting that effect is just adding needless complication, imo.

23. Jan 30, 2014

### sophiecentaur

That's a bit sweeping. By an altitude of 3000m, the density is only about 1/3 of that at ground level. So sunlight travelling over a long distance near the horizon will be subject to vastly more scattering for every km than light coming vertically. These guys usually do a good job of explaining things and they bring in the Mie scattering too, which alters the saturation of the blue sky near the sun.

Between the viewer and a distant mountain, there is a lot more dense air than in a similar distance looking upwards. Sunlight falling on the path between viewer and mountain will have more blue light scattered towards the viewer on a (say) 20km path than for a vertical 20km path. So a lot of blue light will be added to what the viewer sees. Of course, to add complication. the colour of the light reflected from the distant mountain will be modified the opposite way due to some lost blue over the path between. But the contrast is still lost due to scattering. I think that what goes on at low level is a bit irrelevant because the air is so seldom clear right next to the ground and that upsets the model.

24. Jan 30, 2014

### ovais

I was infact thinking to send a diagram showing what I suppose but I am not finding a way to send you here on PF. Is there a way please tell me, I will make a diagram by hand, take its photograph and will post it here.

But I appreciate now your statement that all other light coming from sky is scattered light and that it is only 'direct' light which is un-scattered infact I should define direct light as one which is coming un-scattered.

But AT is saying distance has no role to play in scattering. My only question (from the begining) is that if scattered light is reaching the earth why don't air in lower atmosphere appear blue???

The mechanism of seeing is that you can see a colour if that colour wavelenth is reaching your eye and everywhere people agree that blue light is reaching the earth. Then why don't this blue(scattered) light available(after scattering) near us make us look our near(atmosphere) blue.

25. Jan 30, 2014

### sophiecentaur

It does. Each metre cube is only very slightly blue but 100km+ of air has enough molecules in it to scatter enough light in your direction for you to see more blue than green and red light. Itr all adds up. See my last post where I discuss the fact that distant mountains look blue because all the dense air in between is scattering light (more of it is blue) and that is adding to the light that is coming your way from the mountains.

Do an experiment. If you can get hold of a piece of old window glass. It will look clear when you are only looking through the 5mm thickness but look into one edge and it will look pretty green because you are getting the effect of 100 times the thickness of the glass. Also, take a small glass of grotty bath water. It may well look drinkable in a small quantity but the full depth of the bath can make it look brown and much less attractive. (Haha - I'm not saying your bath is any worse than mine but it would be a long way for you to come to see my bath water.)