And an extra thank you to the poster who called out attention to UV Strength above 11,000 feet - that is something I was not aware of. Though I recall a bad sunburn in the late 90s when I climbed half-dome with a tank top...
I have a small portable UV meter.
As stated earlier, I have been based in Medellin, Colombia the last 5 years and thus live at 5,000 feet/1500 meters/~ 1 mile high most of the year. I never wear sunglasses, don't even own a pair any more...I used to believe all the anti-UV hyperbole...but have since weaned myself off sunglasses. In recent travels 2010 in Peru - Lake Titicaca (High altitude), and Macchu Pichu, etc..., didn't use any sunglasses nor did I bring them. It was sunny all the time.
I am aware of snow blindness which can occur....
When I lived in SoCal I owned a pair of pricey Maui Jims, lost them, then replaced them with a brand - Rudy Project...so...I don't plead ignorance with fancy sunglass marketing. But lack of knowledge of our bodies is the reason we need so many "tools/gadgets"
I will take into this new information into consideration. But -- one benefit of not eating is a reduced IRON content over time. Our urine is yellow because of HEME removal. The presence of iron and other trace metals in our body is what causes sunburn (and red blood). It has to do with (electromagnetic) properties of these metals in our body reflecting the UV rays...
So...no eat...gradually less iron and B-12? (Which is cobalt I think)...among other vitamins / unnecessary minerals.
Now, someone will remark about anemia...but that is not a concern...but again, one needs to learn certain things to overcome this fear.
Another bonus..less sweet blood (sugar)...no bug bites / bug attraction -- no insect repellent needed. No need for bear box then due to food or fragrant repellents. But we're getting ahead of ourselves here.
So how much more UV radiation am I receiving in Flagstaff, versus in Phoenix?
As mentioned earlier, UV radiation increases with altitude. This increase has been shown to be between 4-5% for every 1000 feet ascended. Thus, for the same day in June, the amount of UV radiation would be approximately 27% higher in Flagstaff than in Phoenix (about 6000 feet elevation difference). If you were going from Phoenix to the top of the San Francisco Peaks (approximately 12,000 feet), you would experience an increase in UV radiation of about 50%!
If you were to do this on a day in the winter with snowfall on the ground at Flagstaff, you would also have to factor in the affect of the reflection of UV radiation on the snow, and your UV exposure would go from 27% higher (just due to the elevation difference) to approximately 100% higher due to snow on the ground (snow is approximately four times more reflective than desert). While skiing up at 10,000 feet, you would receive about 200% higher levels of UV radiation than you would receive on the same day in the winter in Phoenix.
This underscores the important fact that protection from the sun should be even more of a concern for anyone who is planning activities at higher elevations, even in the winter.