UV at altitude, on snow and under clouds
Hikers often get their worst sunburns on days that don’t feel hot: a cool ridge, a snowfield, a hazy sky. Altitude, reflection and clouds all change how much UV reaches you.
Altitude: less air, more UV
The atmosphere filters part of the UV in sunlight. The higher you go, the less air is above you, so more UV gets through. The World Health Organization says UV levels increase by 10% to 12% for every 1,000 metres (about 3,300 feet) of elevation.
Real-world gains vary. Some measurements in very clean air show smaller increases, closer to 5% per kilometre, and polluted valleys can show larger gradients. Treat 10% per 1,000 metres as a useful rule of thumb rather than an exact figure.
| Elevation gain | Roughly how much more UV |
|---|---|
| 500 m (1,640 ft) | 5% to 6% |
| 1,000 m (3,280 ft) | 10% to 12% |
| 2,000 m (6,560 ft) | 20% to 24% |
| 3,000 m (9,840 ft) | 30% to 36% |
| 4,000 m (13,120 ft) | 40% to 48% |
So hiking from a trailhead at 500 metres to a pass at 3,000 metres (a 2,500 m gain) puts you in roughly a quarter to a third more UV than you had at the start, and the cool air makes it easy to underestimate.
A note on forecasts
UV forecasts are for the coordinates you look up, and models don’t resolve individual peaks. If your trail climbs well above that point, expect more UV than the forecast shows. Our destination pages spell out the extra UV at well-known summits and passes.
Snow and ice: UV from above and below
Fresh snow can reflect as much as 80% of UV. On a snowfield you get UV from the sky and again from the ground, which is why people burn under the chin, inside the nostrils and on the underside of the nose and ears.
- Wear glacier or mountaineering sunglasses with side protection. Overexposure can cause snow blindness (photokeratitis), a painful sunburn of the eye surface that often appears hours later.
- Put sunscreen on under the chin and around the nose, and use SPF lip balm.
- Treat a snow day as at least one UV level higher than the forecast says.
The combination hikers should fear most is spring: snow still covers the ground, the sun is already high, and the air is thin.
Water, sand and rock
Other surfaces reflect less UV than snow, but it still counts. The WHO gives dry sand about 15% and sea foam about 25%. Pale rock and light gravel bounce some too. If you’re hiking along a lake, a beach or a bright talus slope, you’re getting extra UV that a hat brim alone won’t block. UV also penetrates below the surface of water, so being in the water isn’t protection either.
Clouds: not the shield you think
UV is highest under cloudless skies, but the WHO notes it can be high even with cloud cover. Thick, dark cloud blocks a large share. Thin or broken cloud often blocks very little, and you can burn on a hazy or partly cloudy day without feeling the sun. Under scattered cloud, UV can even briefly rise above clear-sky levels when the sun comes out from behind the edge of a cloud.
The safe habit: use the day’s UV forecast, not how bright or warm it feels.
Shade helps, but doesn’t solve it
Trees, rock overhangs and buildings reduce the direct beam, but UV scatters through the sky and reflects off the ground, so some reaches you even in shade. Dappled forest shade blocks less than solid shade. Shade is a great tool at midday, but pair it with clothing and a hat.
Season, latitude and ozone
Places nearer the equator and times of year with a higher sun get stronger UV. Put those together with altitude and you get the highest UV on Earth, which is why high tropical peaks like Kilimanjaro and the Andes can see extreme readings. The ozone layer also absorbs some UV, so a thinner ozone layer lets more through. Ozone varies with the season, and it is lowest over the polar regions in spring.
Worked examples
- Half Dome, California. About 1,470 m above Yosemite Valley, so roughly 15% to 18% more UV than the valley floor. See UV at Yosemite Valley.
- Gornergrat, near Zermatt. About 1,480 m above the village, with snow around most of the year. See UV at Zermatt.
- Uhuru Peak, Kilimanjaro. Roughly 4,000 m of climbing on most routes, at 3°S, with glaciers around the summit. See UV at Kilimanjaro.
How to estimate the UV on your route
You can adjust any forecast for elevation in your head. Take the peak UV for your trailhead or nearest town, work out how much higher you’ll climb, and add about 10% for every 1,000 metres.
- Look up the peak UV at the trailhead on the home page. Say it’s 6.
- Work out the gain. A summit 1,500 m above the trailhead adds roughly 15% to 18%.
- Multiply: 6 × 1.15 to 1.18 gives about 6.9 to 7.1, so plan for a 7.
- Add snow or bright rock on top if your route has them.
The categories are what matter. Moving from 6 to 7 keeps you in “High”, but a forecast of 7 at the trailhead can become an 8 (“Very high”) at the top, where unprotected skin can burn quickly.
A high-altitude day checklist
- Long sleeves, long pants and a wide-brim hat, put on before the sun is up.
- Sunglasses with side shields, and glacier glasses if you’ll be on snow.
- SPF 30+ (50+ for a long day) on every bit of exposed skin, plus lip balm with SPF.
- A reapplication plan: every two hours, and at each summit or pass.
- Shade breaks for the middle of the day.
Quick answers
Do I need sun protection when it’s below freezing?
Yes. UV has nothing to do with temperature, and cold air at altitude often means clear skies and strong UV.
Is a glacier worse than a beach?
Usually, yes. Fresh snow can reflect up to 80% of UV, while dry sand reflects about 15%, and glaciers are also high up where the air is thin.
Can I burn through fog or haze?
You can. Thin fog and haze scatter UV rather than block it, so the sun can feel weak while your skin is still taking a hit.
Putting it together on the trail
The riskiest mix for hikers is a clear day, high elevation and snow: the sun is strong, the air is thin and the ground reflects it back. On days like that, go for full coverage, a wide-brim hat, glacier glasses and lots of sunscreen, and reapply through the day. For a full kit list, see what to wear hiking for sun protection.
Sources and further reading
The 10% to 12% per 1,000 m figure is from WHO educational material; the smaller clean-air figure is from DermNet NZ.
- WHO: Ultraviolet radiation fact sheet
- WHO: Global Solar UV Index, a practical guide
- DermNet NZ: Global solar ultraviolet index
- Skin Cancer Foundation: Sun-protective clothing
- U.S. EPA: UV index scale
Check today’s UV for your place
This guide is general information about sun safety and isn’t medical advice. Talk to a doctor or dermatologist about your own skin.