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).

  • $\begingroup$ It might help if you gave your location, so that any local phenomena can be taken into account. $\endgroup$ – f.thorpe Feb 3 '18 at 22:13
  • $\begingroup$ @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. ... $\endgroup$ – Alchimista Feb 3 '18 at 22:19
  • $\begingroup$ @farrenthorpe I made a few typos above ;) $\endgroup$ – Alchimista Feb 3 '18 at 22:32
  • $\begingroup$ 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? $\endgroup$ – f.thorpe Feb 3 '18 at 23:53
  • $\begingroup$ @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. $\endgroup$ – Alchimista Feb 4 '18 at 0:10

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.

  • $\begingroup$ 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 $\endgroup$ – Alchimista Feb 5 '18 at 17:31
  • $\begingroup$ 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. $\endgroup$ – Alchimista Feb 5 '18 at 17:35
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    $\begingroup$ "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? $\endgroup$ – Santropedro Nov 24 '18 at 23:19
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    $\begingroup$ @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. $\endgroup$ – David Hammen Nov 24 '18 at 23:49

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.

  • $\begingroup$ 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. $\endgroup$ – JeopardyTempest Feb 5 '18 at 19:12
  • $\begingroup$ 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. $\endgroup$ – Alchimista Feb 5 '18 at 19:38
  • $\begingroup$ 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! :)) sciencedirect.com/science/article/pii/S0161642096307112 $\endgroup$ – Alchimista Feb 5 '18 at 19:41
  • $\begingroup$ 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??? $\endgroup$ – JeopardyTempest Feb 18 '18 at 21:49
  • $\begingroup$ @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. $\endgroup$ – Alchimista Feb 20 '18 at 10:39

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