You hit the nail on the head here. It may have something to do with the atmosphere or even barometric pressure. I don't believe it is related to being closer to the sun.
KM
It has to do with the fact that as you get closer to sea level the air pressure goes up, thus the air is thicker. Thicker air will absorb more solar energy than thinner air.
Example. Got burned by the sun after 1 hour at midday when sun was directly overhead.
A week before i was out for four hours in the afternoon 4pm -8pm didn't get as burned
With the sun overhead the column of air that the sun is passing through is at its thinnest ie only the air above your head, if that air is filled with particulate matter ie smoke it will block out even more solar energy.
In the early morning or afternoon the sun is at a low angle and as such the solar energy has a longer way to travel through the atmosphere so more of its energy has a chance to be absorbed.
Imagine it this way, you stick a pin into an orange at a 90 degree angle (perpindicular) to the peel. The pin doesn't have to go in very far before it sticks through the peel into the flesh of the orange.
Then you take the same pin and carefully insert it on a low angle, almost flat to the peel. you could probably get the entire pin to dissapear into the peel without it hitting the flesh.
More exposure to the air in the atmosphere means more chances to absorb UV radiation, translates into less fading at lower altitudes and less sunburn in the morning or afternoon.
Make sense?
I may try one of these as a cheap practice basket once I bank enough league funds.