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UV index weighs UVB more heavily than UVA. UVB falls much faster than UVA and visible sunlight as the sun gets lower because photons travel through increasingly more ozone and atmosphere. So the higher in the sky the sun is, minus cloud clover, the higher the UV index is.

What your describing sounds a lot like the effect of humidity in the atmosphere. You can't see the humidity itself, but it affects how a smaller percentage of light is scattered, and that you can see, to the point that, when looking out from a high vantage point like a mountain or airplane, on clear dry days you can see further than on clear humid days.

You can use simple heuristics to get a very good idea.

1: How high in the sky is the sun? (Many other factors like time of year, time of day, and latitude feed into this) 2: What is the chance for you at a given time of the sun being blocked by clouds, buildings, etc?

I don't think so. If you controlled for temperature and season I bet you wouldn't be able to tell.

Maybe how different things look with sunglasses on vs off?