Please have a look at the introduction part of the following link and especially the references mentioned therein...

https://www.google.de/url?sa=t&rct=j&q=&esrc=s&source=web&cd=25&ved=2ahUKEwjG_vPviuTkAhUJLlAKHc_NAEsQFjAYegQIBBAC&url=http%3A%2F%2Fcolorimaginglab.ugr.es%2Fpages%2Fpdfs%2Fjosaa_2016_1049%2F%2521&usg=AOvVaw3KcpTCNitO0lvpAfFIWkx6

good luck

Thank you Gerhard Martens

Dear Ms Putran

There are two clear types of analytical study on these phenomena. The first is colorimetric as cited by the previous answer; and the second is spectral.

Using Spectral analysis and line spectra more detailed chemical studies become possible,

See:- "Variation of outdoor illumination as a function of solar elevation and light pollution" – Nature https://www.nature.com › scientific reports › articles

and

"Spectral recovery of outdoor illumination by an extension of the Bayesian inverse approach to the Gaussian mixture model" October 2012Journal of the Optical Society of America A 29(10):2181-9 DOI: 10.1364/JOSAA.29.002181

Thank you for these references Mr. Oulton.

Sreeja:

I've had this question in the back of my mind for some time now. I want to eventually have a database of spectral measurements organized by geographic location, time of day, weather (sky conditions), and with images. This would require a monumental effort, however. In the meantime, check out these two thing:

1. Daylight and It's Spectrum by Henderson (book, out of print)

https://www.amazon.com/Daylight-Its-Spectrum-S-Henderson/dp/B007EQJP8Q/ref=sr_1_9?keywords=henderson+daylight&qid=1569414577&s=gateway&sr=8-9

2. Spectral Distribution of Typical Daylight as a Function of Correlated Color Temperature (Paper, Judd et al)

https://www.osapublishing.org/josa/abstract.cfm?uri=josa-54-8-1031

I am happy to chat further about this topic.

Thank you so much for these @Tony. I'll be in touch.

While I have not read all of the references (thank you, everyone), I suspect that they address the average spectral power distribution of daylight under clear skies. Another reference that may be important is therefore:

Lee, R. L., and J. Hernández-Andrés. 2006. "Colors of the Daytime Overcast Sky," Applied Optics 44(27):5712-5722.

A related question is whether you are interested in the spatially-averaged SPD or the spatial distribution of SPD under clear skies. If the latter, then you might be interested in:

Article A Qualitative and Quantitative Evaluation of 8 Clear Sky Models

Hi Ian Ashdown , thanks for the reference. I'm interested in the spatially-averaged distribution of SPD (at ground level) at different geographical locations. However, if significant differences are present spatially at a single location, I would be interested in understanding that as well.

Hi, in fact, we want to do it, but we have not recources (human, machines, finance...etc) enough for many regions. If we have international cooperations, we will have the databank. It is very useful. RG - Vinh

Yes I agree.