On Mont Blanc's summit at 4,807m, UV radiation intensity is roughly 50% higher than at sea level. On Aconcagua at 6,960m, it's more than double. On Denali or Everest, it's extreme. Most climbers underestimate both the intensity and the speed at which damage occurs. I've seen climbers get severe sunburn in under an hour at 4,000m in partly cloudy conditions โ they thought because it was cold and cloudy, they didn't need protection. They were wrong, and they suffered for it.
Why Altitude Makes UV Worse
Two factors combine at altitude: first, the atmosphere is thinner, so it filters less UV. For every 1,000m of altitude gain, UV intensity increases by about 10-12%. Second, snow reflects up to 80-90% of UV radiation โ compared to grass (about 3%), water (about 10%), or sand (about 15%). On a sunny day on a snow-covered glacier at 4,000m, you're receiving direct UV from above and reflected UV from below simultaneously. Your effective UV exposure can be double what you'd receive on bare ground.
Cloud cover provides a false sense of security. Clouds reduce visible sunlight, making conditions feel less sunny, but they block very little UV โ thin clouds reduce visible light by 50-80% while reducing UV by only 10-30%. I've seen the worst sunburns occur in apparently overcast conditions.
Snow Blindness: Prevention and Treatment
Snow blindness (photokeratitis) is sunburn of the cornea โ the eye's surface. It's intensely painful and typically develops 6-12 hours after exposure. Symptoms: gritty feeling in eyes, excessive tearing, redness, swelling of eyelids, and severe light sensitivity. In extreme cases, temporary vision loss can occur. The condition is usually self-limiting and resolves within 24-72 hours, but it's incapacitating while it lasts.
Prevention: wear UV-protective glacier glasses or goggles with side shields. The key specification is: 100% UV protection (UV 400), category 3 or 4 lens for high altitude. Dark lenses alone are insufficient โ they cause the pupil to dilate, allowing more UV in. Wraparound glasses that block peripheral light are essential. In blizzard conditions where normal glasses fog, goggles with the same UV specification are necessary. Never look directly at the sun, even briefly โ this can cause permanent retinal damage.
Sunscreen Strategy
Use a high-SPF broad-spectrum sunscreen (SPF 50+, blocking both UVA and UVB). Apply generously before you start โ most people apply far too little. A full face and neck requires about 1/4 teaspoon. Reapply every 2 hours of exposure, or more often if you're sweating heavily or wiping your face. Pay particular attention to: the backs of hands, the back of the neck (often missed), ears, the nose (which catches the sun regardless of head angle), and the lips (use SPF lip balm, not regular balm โ most lip balms contain no UV protection at all).
For multi-day trips, factor sunscreen into your food resupply calculations. A week of serious alpine sun protection requires roughly 2-3 full tubes of SPF 50+. Carry more than you think you need โ there's no resupply at 5,000m.
After-Sun Damage
Even with protection, cumulative UV exposure over multiple days causes damage. Signs of over-exposure: skin that is unusually red and tender after the climb, Peeling skin on face and hands in the days following the climb, unusual skin darkening (tanning is your skin's emergency response to UV damage). If you've burned badly, cool compresses and aloe vera can soothe the skin. If you have blistering sunburn, seek medical attention โ second-degree burns require professional treatment.
Related Articles
- Cold Weather Layering Guide โ Full clothing system for alpine conditions
- First Aid for Climbers โ Treating sunburn and eye damage
- Winter Summit Preparation โ Planning for high-altitude UV exposure