Originally Posted by David Lyga
Yes, this is fairly self-evident. One factor that you haven't touched on is that the spectral composition of sunlight is also influenced by Rayleigh scattering of light, and the frequency affected by scattering depends on the particle size of the matter doing the scattering. The blue colour of the open sky is caused by light scattering by oxygen and nitrogen molecules. We are actually seeing the light scattered away from the direction it is moving in. In early or late hours, the distance the light travels through air is sufficient to deplete the blue spectrum enough to turn the light substantially yellow, hence the "golden hour". Golden hour is not quite relevant to your discussion, but it is worth noting that latitude has a marked effect on spectral composition for the same reason. Smoke, dust and mist in the higher layers of the atmosphere may have similar effects on spectral composition, and may vary due to changes in weather, volcanic erruptions, seasons etc. All things considered, "Sunny-16" is a guideline with a narrow set of parameters, and probably never was intended as a rule or a law.