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It is long time that I am looking for what physical reason(s) explains this observation:

the sun ambient light between sunrise and midday is crispy, somehow whiter than that of the afternoon hours. In other words it is like that morning outdoor light is more similar to that of a white lamp, and that of afternoon more yellow and incandescent bulb -'like.

Even in more physics term, it seems that the morning light has a higher Temperature than that of afternoon. Note that I refer to the T of an emitting black body, not the temperature as colloquially used in fashion, art, etc that goes right the other way around.

For those just a bit familiar with photography, is as the golden hours of morning and afternoon are not identical (the obvious opposite casting of shadow is obviously not part of the matter), like if the source of lighting changes (obviously the sun doesn't, so it is an atmospheric phenomenon or so).

Everything I came up would have a basically symmetric behaviour around midday so can't explain this.

The only asymmetry I can see is that " morning is light through a night chilled atmosphere", while "afternoon is light going around in an already warmed up atmosphere". However "this explanation" - as a ground where to plug optical phenomena of refraction or Mie scattering - would require about the same temperatures of the air, in summer as in winter, or day after day.

Another possible asymmetry is due to our long scale adaptation to luminosity or brightness, but the change I refer to is relatively abrupt around noon.

A possibility might be related to temperature of the atmosphere perhaps via refraction between layers of it..... it is the last idea that came to my mind. In this case same layering might occur independent of seasons.

Please note that I am not asking for Rayleigh scattering alone and why yellow and red are dominant near horizon at sunrise or sunset.

I am asking why the light of a clear day is not symmetrically changing centered at noon.

Please also note that before telling me that the difference I am asking about does not exist, ask yourself if you feel able to tell if it is morning or afternoon without knowing anything else but looking at the environmental light in a clear day. Most persons I've talked to about this, didn't realise it before (again a big surprise).

Alchimista
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  • It might help if you gave your location, so that any local phenomena can be taken into account. – f.thorpe Feb 03 '18 at 22:13
  • @farrenthoroe. If I would have noticed a relation ok. No this is based in my observation since I am on Earth, it nothing specific to a location. Just I never approached the poles, and never I was very close to equator. Thanks for the advice. I hope you notice different lighting as I described. I took it for granted all along my life. ... – Alchimista Feb 03 '18 at 22:19
  • @farrenthorpe I made a few typos above ;) – Alchimista Feb 03 '18 at 22:32
  • The temperature of the air is warmer in the day, which has effects. Also, pristine vs human influenced air will make a difference. For instance, daily agricultural or urban activity will change the regional atmosphere during the day. The morning will be more pristine due to little activity at night. There are other factors where location is important. Have you spent your life at sea? In cities? In agricultural areas? – f.thorpe Feb 03 '18 at 23:53
  • @farrenthorpe. Sure and I agree. I ve try to signal it in the Q. The change seem to sharp to me. I wanted to say before that myself I grew up in a big town right on the see. With lot of natural rock and green just at my back. This could have influenced my perception. Because for the fact I believe to see, I saw in five years in an alpine country than years and years in inner flatland in France. Visited lot of places worldwide. – Alchimista Feb 04 '18 at 00:10
  • @farrenthorpe I would be already satisfied is some others notice that difference. In a better season I will phitograph a blank paper in the same conditions but when sun is symmetric around 12 h. If I see well I should get different "shade of white". But I don't know how to suppress the auto correction. The few thinking like me also describe morning as bluish abd afternoon yellowish or cold and warmer respectively, but they use the photography dictionary no the Wien displacement. Thanks a lot for your interest. – Alchimista Feb 04 '18 at 00:16
  • Perhaps is Mie forward. ....? – Alchimista Feb 04 '18 at 00:30
  • related and intensely beautiful https://en.wikipedia.org/wiki/Blue_hour – trond hansen Nov 02 '19 at 06:33
  • Your question is rather long winded. Better to abbreviate it and add some evidence that morning light is different from afternoon light when the sun is at the same angle above the horizon in both cases. The saying " Red sky at morning, shepherd's warning], red sky at night shepherd's delight" suggests that morning and evening light is the same. – Michael Walsby Nov 02 '19 at 15:15
  • I am aware of difference between individuals but I see it. Are you really sure that being in an open place at 10 am has the same light temperature as at 2 pm. I am not really interested at sunrise and sunset as for in that case Rayleigh diffusion dominates. Proof is that photographing asphalt or a white paper leads to different photographs. But having just digital cameras I am afraid of processes I am not aware of. Perhaps I ask in photography how to get the proper blank and update this. I can also delete the Q, though for me the difference is dramatic. @Michael Walsby – Alchimista Nov 03 '19 at 08:12
  • It isn't invariably the same,but it is sometimes. There are too many variables. For example there are sometimes morning mists which affect the light,but are gone in the afternoon. perhaps you should stipulate the atmospheric conditions so that people can compare like with like. – Michael Walsby Nov 03 '19 at 09:05
  • That is straightforward. Even living in places where a full clear day is rare. – Alchimista Nov 03 '19 at 09:07
  • Could it just be a psychological phenomenon, where you get used to relatively dull morning twilight and see the brilliance of sunlight in the morning, and on the other side, you get used to the crispy noon and going further in time seems to make it duller? As for myself, I have watched lots of time lapses (both in real life and in photo sequences) and never noticed this difference. – Ruslan Sep 26 '21 at 20:09
  • Yes there could be that component, like adaptation (physio-psychological). But to me the change is so centered at the sun midday that I am sure the angle is at play somewhere... Tough it can be that a progressive augmentation of particles of every sort, natural and man-made, progressively yellows the light. Once I was reading an article somewhat related to my observation. It even claimed that shadow are more defined in the morning (due to light spectrum impinging at ground) but I can't find it anymore. It was the most related to my Q, unfortunately I did not save it don't know why :( – Alchimista Sep 27 '21 at 09:07

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I am asking why the light of a clear day is not symmetrically changing centered at noon.

Trying to attribute this to a single cause misses the point. There are multiple causes. Other than lighting itself (i.e., the Sun angle), most aspects of the atmosphere are not symmetric about noon.

The low on a clear day typically occurs right around sunrise (not midnight), while the high typically occurs hours after noon. Assuming a front is not passing through, relative humidity is typically highest in the hours around sunrise, lowest a few hours before sunset, roughly coinciding with the low and high temperatures. Atmospheric stability is high in the early morning on a clear calm day, low in the afternoon and evening. During weekdays, pollution oftentimes is at a minimum around 6 to 8 AM and a peak around 5 to 7 PM.

All of these can contribute to the effects you are seeing, plus other factors I did not list.

David Hammen
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  • So all this could point to Mie forward scattering leading to some more blue forwards at morning ? I was already thinking it might be something related to water fine particles. Can I assume that roughly in clear weather the pattern stay the same independent of season? Coming from night and coming from day is indeed the only asymmetry I can really on. My feeling of a life doesn't fit with pollution, tough I totally agree that this lead to a yellowing of light. Thanks. We are going in the right direction. Stability is also of interest. Any suggestions how it can be related to diffusion whateve – Alchimista Feb 05 '18 at 17:31
  • Whatever regime and or refraction, I.e. anything can alter the spectral distribution? Like a different relative weight of wavelength in different atmospheric "shells". Depending on T and its gradient blu or red can bend differently so the colour temp of environmental light might also change as I describe. – Alchimista Feb 05 '18 at 17:35
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    "plus other factors I did not list." why didn't you list them? You know them but choose not to list them? Or do you ignore them? – Santropedro Nov 24 '18 at 23:19
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    @Santropedro - Weather. Time of year. The time at which a Sahara dust storm that blew west across the Atlantic or the time at which a dust storm that blew east from Oklahoma arrived, All kinds of things. – David Hammen Nov 24 '18 at 23:49
  • @DavidHammen I am not referring to any of the change you have in mind. But I don't pretend to be right. – Alchimista Sep 23 '21 at 18:11
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I believe the effects you are talking about are due to scattering by dust and water vapor in the atmosphere. During the night winds are lower and wet areas don't dry out as fast as during the day. Both of these mean that at the start of the day there is less dust and water in the atmosphere at a given location. As the day progresses winds pick up (caused by heating from the sun) and the winds pick up dust and the amount of water held by the atmosphere increases due to warmer temperatures. During the middle of the day you don't see the effect of this in the light because the sun's rays are coming directly down through the atmosphere. The sky will be a brighter blue due to the blue-selective scattering of rays that are not coming directly towards you. In the late afternoon the slanting rays come through a much longer path through the atmosphere accentuating the scattering effect, resulting in redder light reaching you.

verisimilidude
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  • This was what I was thinking of working together into an answer. Honestly, I've never noticed such a discrepancy, and solar radiation measurements (such as from the Oklahoma mesonet) don't show notably large difference, at least in energy being scattered....... but if there were a reason, would think wind (more in the evening) would be the cause. – JeopardyTempest Feb 05 '18 at 19:12
  • And @David Hammen too. The point is that Rayleigh scattering is insensitive to big particles and it caused by molecular sizes scatters. Mie and particulate can let more blue down along the raypath and can explain more blue reaching the surface layer of atmosphere when sun is low. If I could affirm that more water DROPLETS are at morning than in afternoon than bingo. This could explain why morning is more white abd crispy than afternoon. However I never saw an afternoon with the same lightning of an afternoon. – Alchimista Feb 05 '18 at 19:38
  • To all. I am lso thinking about possible refraction effect vs T. At least I can assume that the vertical T gradient gets higher along the day. As water and terrain will faster rise T . Searching for documention I even found this! :)) https://www.sciencedirect.com/science/article/pii/S0161642096307112 – Alchimista Feb 05 '18 at 19:41
  • I'm hesitant to think it's water droplets on typical mornings. Certainly relative humidity tends to be higher in the morning than the afternoon, but unless conditions approach the dew point (and you start in towards fog formation), doubt there's significant droplets in the air (and besides, wouldn't more droplets lead to more scattering, and thus less white??). But don't know. Dew does often condensate onto the ground many mornings, perhaps it has something to do with that??? – JeopardyTempest Feb 18 '18 at 21:49
  • @JeopardyTempest droplets do not need to be big. In all cases there will be more effects . It is the overall results. Opposite to common scattering Rayleigh, other regimes behave different. I am also thinking of different bending by refraction due to vertical T gradient. At least the pattern will be independent of season if I assume that lower atmosphere is warmer than high one and this different grow along the day passing. – Alchimista Feb 20 '18 at 10:39
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I completely understand what your asking. I too have observed the "difference" in the light of the day. Morning light and afternoon light have a completely different look to them. I have no real way to prove this, but I believe this effect is due to the rotation of the Earth in relation to the Sun's position to the observer.

What I mean is this. In the morning the rotation of the Earth is towards the Sun, increasing the frequency of the wavelengths of light we observe. Shorter wavelengths tend toward the blue hues, and the light of the day appears blueish. In the afternoon the rotation of the earth is away from the sun causing longer observed wavelengths or wavelengths that tend toward the reddish hues causing the light of the day to appear to be reddish. During mid-day since the sun is directly overhead there is no distinction either way, and the light of the day appears bright and white. This is just how I have justified this phenomenon.

Fred
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Michael E. Magnes
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  • I am glad of hearing this as for I am still astonished by the fact that what is evident to me it is not so for the most. I am afraid that the speed involved can't justify a Doppler effect. Still, I appreciate the effort. I once conjectured that - speaking about refraction now - cold sky might tilt to the surface more blue, so that afternoon relatively more yellow would be around. But no idea of the correctness. This comment is valid only if you also find the afternoon "warmer" or "more yellow" than morning. :) if you see the opposite I will be even more surprised ;) – Alchimista Sep 23 '21 at 18:04
  • https://sciencenotes.org/fast-go-make-red-light-look-green-relativistic-doppler-effect/ – Alchimista Sep 23 '21 at 18:14
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This is an excerpt from a paper written on the subject... Why is sunset more reddish than sunrise? By Neerad Tanvi, Bikaner

I never considered the settlement of dust at night, but that makes total sense. ... The scattering is also related to the size and quantity of the scattering particles. During the night time the atmosphere is cool and the aerial particles and dust particles settle by morning whereas by evening they get dispersed. Thus, the scattering is more by evening than in the morning. Lastly, since the earth is spinning from West to East, relatively we move towards the Sun during mornings (eastward) and away from the Sun during evenings (westward). There is one phenomenon, called, Doppler Effect, which adds to this differential. In other words, there is an inherent natural cause for this difference between morning and evening sunlight. Prof. A. Ramachandraiah NIT, Warangal

user24506
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  • Yes likely it is scattering due to particulate of various nature, including very fine mist. Except that I notice it along the day. I must ignore the suggestion regarding Doppler effect as for our tangential speed on Earth isn't comparable to the speed of light. See the link under M. E. Magnes answer. – Alchimista Sep 24 '21 at 09:22
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Nobody has mentioned the fact that on most summer days cumulus clouds will start appearing mid to late morning and come to occupy a significant proportion of the sky by mid afternoon, inevitably altering the perceived light quality

Delta Juliet
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  • It sounds like they're saying yearround. And of course not everywhere sees such cumulus. But do agree that has a definite contribution to differences (especially as I live in Florida!) – JeopardyTempest Aug 05 '23 at 20:57