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When skiing, if the weather is good, I often come back with a better tan than if I have been at the beach. Obviously with altitude, there is less blocking of UV rays by the atmosphere.

But how much is blocked?

Is there any guidance that says, "For every x thousand metres your exposure is doubled" or your sun factor should be increased by y?

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And yes, I realise this could also be ontopic on Physics.SE, but the answer I'd get there would likely be a physics answer as opposed to a people answer :-) –  Rory Alsop Aug 20 '12 at 8:54
    
Definitely on topic here as far as I'm concerned! –  berry120 Aug 20 '12 at 10:02
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1 Answer

up vote 7 down vote accepted

As found here:

"With every 1000 m in altitude, UV levels increase by approximately 10 per cent."

Percentages are tricky to work with, so here is a worked-out example. Suppose you start out at sea level (0m), and you climb all the way up to Mt. Everest's summit (8848m). Suppose also that at sealevel, you normally need to apply sun block factor 15. Then, climbing up,

   0   ->   1.00  ( = times UV exposure at sea level)
 500   ->   1.05   ->  apply factor 15.73
1000   ->   1.10   ->  apply factor 16.50
1500   ->   1.15   ->  apply factor 17.31
2000   ->   1.21   ->  apply factor 18.15
2500   ->   1.27   ->  apply factor 19.04
3000   ->   1.33   ->  apply factor 19.97
3500   ->   1.40   ->  apply factor 20.94
4000   ->   1.46   ->  apply factor 21.96
4500   ->   1.54   ->  apply factor 23.03
5000   ->   1.61   ->  apply factor 24.16
5500   ->   1.69   ->  apply factor 25.34
6000   ->   1.77   ->  apply factor 26.57
6500   ->   1.86   ->  apply factor 27.87
7000   ->   1.95   ->  apply factor 29.23
7500   ->   2.04   ->  apply factor 30.66
8000   ->   2.14   ->  apply factor 32.15
8500   ->   2.25   ->  apply factor 33.72

But it gets worse. If you travel higher and higher, you're bound to hit snow at some point. As can be found here, some types of snow can reflect up to 80% of UV light. This means that if there is 100% snow cover, you're exposed to (roughly) 1.8 times the amounts above:

   0   ->   1.80 (times UV exposure at snow-less sea level)
 500   ->   1.89   ->  apply factor 28.32
1000   ->   1.98   ->  apply factor 29.70
1500   ->   2.08   ->  apply factor 31.15
2000   ->   2.18   ->  apply factor 32.67
2500   ->   2.28   ->  apply factor 34.26
3000   ->   2.40   ->  apply factor 35.94
3500   ->   2.51   ->  apply factor 37.69
4000   ->   2.64   ->  apply factor 39.53
4500   ->   2.76   ->  apply factor 41.46
5000   ->   2.90   ->  apply factor 43.48
5500   ->   3.04   ->  apply factor 45.61
6000   ->   3.19   ->  apply factor 47.83
6500   ->   3.34   ->  apply factor 50.17
7000   ->   3.51   ->  apply factor 52.62
7500   ->   3.68   ->  apply factor 55.18
8000   ->   3.86   ->  apply factor 57.88
8500   ->   4.05   ->  apply factor 60.70

To top it all off: there's psychological aspects to consider. As can be found here, clothing also protects against sunburn to a certain extent. If you go higher, you're bound to need more clothing, so less skin will be exposed. It will of course also get colder. These two factors make many people feel that using sunblock is not that important anymore, while in fact, the skin that still is exposed actually needs to be protected, and with a much higher factor than usual too.

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Superb answer - thanks for that! –  Rory Alsop Aug 20 '12 at 12:42
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