UV Index rises measurably with altitude — roughly a 10-12% increase for every 1,000 meters (about 3,280 feet) of elevation gain — because there's simply less atmosphere above you to absorb and scatter UV radiation.
Thinner atmosphere, less filtering
The atmosphere absorbs and scatters a meaningful portion of incoming UV radiation — at higher elevation, UV rays have less atmospheric depth to pass through before reaching your skin, so more of the original intensity survives.
Why this catches people off guard
Cooler mountain temperatures create a false sense of lower sun intensity, when the opposite is often true — a ski trip or high-altitude hike can involve genuinely higher UV than a beach day at the same latitude and season.
Snow and altitude compound the effect
Fresh snow reflects a large percentage of UV back upward, adding a second exposure pathway on top of the altitude increase — part of why snow-related sunburn (including on the underside of the chin and nose) is a well-documented phenomenon.
Should sun protection habits change specifically for mountain trips?
Yes — checking the actual local UV Index rather than assuming cooler weather means lower UV, and accounting for snow reflection if applicable, are both worth doing specifically for higher-altitude travel.