Hiking UV Index Calculator — UV Increase by Altitude
Check UV exposure risk for your hike. UV intensity increases roughly 10–12% per 1000m of altitude, and snow reflects up to 80% more UV from below. Enter your region, month, elevation, and sky conditions for a UV index estimate and sun protection recommendations. Based on the WHO UV Index scale.
Quick Answer
How much does UV increase with altitude? UV radiation intensity increases by approximately 10–12% for every 1000 metres (3280 ft) of altitude gain, according to the World Health Organization. At 3000m, UV exposure is 30–36% higher than at sea level. Combined with thinner atmosphere, snow reflection (up to 80% of UV), and reduced cloud cover at altitude, mountain hikers face significantly higher UV risk than lowland walkers.
UV Index Calculator
UV Index
8
Very High
Extra protection needed. Avoid midday sun.
SPF needed
SPF 50+
Skin burns in
25 min
Recommendation
Extra protection needed
Peak UV is typically between 10:00–14:00. Altitude, snow reflection, and cloud breaks can significantly increase exposure.
For reference only. UV estimates are based on monthly averages and may differ from actual conditions. Cloud cover, smog, and local geography affect UV significantly. Always use appropriate sun protection when hiking at altitude. UV burns occur even on cold, cloudy days.
How UV intensity scales with altitude (clear sky, no snow reflection). WHO data.
Altitude
UV Increase
Effective UV Index (if sea level = 6)
Risk Level
Sea level (0m)
Baseline
6.0
🟡 Moderate
500m / 1640 ft
+5–6%
6.3
🟡 Moderate
1000m / 3280 ft
+10–12%
6.7
🟠 High
1500m / 4920 ft
+15–18%
7.1
🟠 High
2000m / 6560 ft
+20–24%
7.4
🟠 High
2500m / 8200 ft
+25–30%
7.8
🔴 Very high
3000m / 9840 ft
+30–36%
8.2
🔴 Very high
4000m / 13120 ft
+40–48%
8.9
🟣 Extreme
5000m / 16400 ft
+50–60%
9.6
🟣 Extreme
Based on WHO data: UV increases ~10–12% per 1000m. Snow reflection adds up to 80% more UV exposure. Values shown for clear sky conditions. Cloud cover reduces UV but not completely — overcast skies still transmit 50–80% of UV.
Sun Protection Recommendations by UV Index
UV Index
Risk
Protection Needed
1–2
🟢 Low
No protection needed for most people
3–5
🟡 Moderate
Hat + sunscreen SPF30
6–7
🟠 High
Hat + sunscreen SPF50 + sunglasses
8–10
🔴 Very high
Full protection: hat, SPF50, sunglasses, cover skin, avoid midday
11+
🟣 Extreme
Avoid outdoor exposure 10am–4pm. Full cover essential
Why UV Is Dangerous at Altitude
The thinner atmosphere at altitude absorbs less UV — at 3000m, around 30% of the protective atmospheric column is gone. Snow reflects up to 80% of incoming UV, compared to about 10% for grass and 25% for sand. Above treeline there is no shade, and mountain hikes often involve longer continuous sun exposure than walks at lower altitude. Hikers often get their worst sunburns at altitude precisely because UV doesn't feel proportional to the temperature — cool wind and high UV is the dangerous combination. Plan your water intake for sun exposure with our hiking water calculator — dehydration risk increases with UV exposure.
Snow Reflection and UV
Snow reflects up to 80% of UV radiation, meaning you receive UV from above AND below. Ski goggles and mountaineering sunglasses with side shields are essential above snowline — regular sunglasses let UV in around the edges. Sunburn under the chin and inside the nostrils is common from reflected UV on snow — apply sunscreen everywhere, including the underside of the nose and ears. Estimate hiking time to plan sun exposure windows with our Naismith's Rule calculator. For high-altitude hikes, also check altitude sickness risk.
How it works
Altitude amplifies UV exposure
UV radiation increases by approximately 10% for every 1,000m of elevation gain. At 3,000m, UV intensity is 30% higher than at sea level, and the thinner atmosphere provides less natural filtering. This is why severe sunburn at altitude can occur on cool, overcast-looking days when hikers are not expecting it.
Snow reflection compounds the effect dramatically. Fresh snow reflects 80–90% of UV radiation back upward, meaning hikers and skiers can be exposed from both above and below simultaneously. This is the cause of snow blindness (photokeratitis): UV damage to the eyes.
Key facts
+10% UV per 1,000m of elevation gain
Snow reflects 80–90% of UV radiation upward
Thin cloud cover blocks less UV than most hikers expect
UV burns can occur even when temperatures feel cold
Understanding the 1–11+ scale
The UV Index was developed by the WHO and WMO to provide a simple, standardised measure of solar UV radiation intensity. The scale runs from 0 (no UV) to 11+ (extreme). Each category corresponds to specific recommended protection measures.
Key thresholds
Index 3+: sunscreen recommended
Index 6+: seek shade at solar noon
Index 8+: avoid outdoor exposure 10:00–16:00
In practice, most Alpine destinations in summer reach UV Index 7–10 by late morning, and UV Index can briefly exceed 12 above 3,000m near the summer solstice.
Sun protection for hikers
At UV Index 6 and above, unprotected skin can begin to burn in as little as 15–20 minutes. The four practical layers of protection for mountain hikers:
Sunscreen
SPF 30+ minimum, reapply every 2 hours and after sweating. Apply 20 minutes before exposure.
Clothing
UPF-rated clothing covers more skin than sunscreen alone and doesn't require reapplication. Long sleeves and collar protection are especially effective on long ridgeline days.
Sunglasses
Category 3 or 4 lenses for glacier and high-altitude routes. Snow blindness from reflected UV can occur in under an hour without eye protection.
Timing
Schedule ridge walks and summit attempts for morning. Return to valley or shade by 11:00–12:00 on high UV days.
Frequently asked questions
How much does UV increase per 1000m of altitude?
UV radiation increases approximately 10–12% per 1000 metres of altitude gain (WHO data). At 3000m, UV is 30–36% stronger than at sea level. This is because the thinner atmosphere at altitude absorbs less UV radiation.
Do I need sunscreen while hiking in winter?
Yes — especially at altitude or on snow. Snow reflects up to 80% of UV, effectively doubling your exposure. Winter sun is weaker than summer at low altitude, but at 2000m+ on snow, UV can be as intense as a summer beach.
What SPF should I use for mountain hiking?
SPF 50 minimum for any hike above 1500m or on snow. Reapply every 2 hours and after sweating. Use a sport/water-resistant formula. Don't forget lips (SPF lip balm), ears, and the back of your neck.
Why is UV still high on a cloudy day?
Clouds reduce UV by 20–70% depending on thickness. Light cloud cover still allows 70–80% of UV through. On a partly cloudy day in the mountains, UV can be nearly as high as on a clear day, and many hikers are caught off-guard because it doesn't feel hot. The "overcast" estimate in this calculator uses 30% of clear-sky UV, which represents moderate cloud cover.
What is "snow blindness" and how do I avoid it?
Snow blindness (photokeratitis) is UV sunburn of the cornea. Symptoms (gritty, painful eyes, temporary vision loss) typically appear 6–12 hours after exposure. It can occur in under 1 hour without eye protection at high altitude on snow. Prevention: always wear glacier glasses (Category 4) or wrap-around sunglasses rated UV400 when traveling on snow.
Does SPF 50 protect twice as well as SPF 25?
Not exactly. SPF 25 blocks 96% of UVB rays; SPF 50 blocks 98%. The difference is small, but at very high UV Index (8+) or extended exposure, SPF 50 provides meaningfully better protection. More important than SPF value is reapplication. All sunscreens degrade with sweat and require reapplication every 2 hours.
Why is summer UV higher in southern Europe than northern Europe?
The sun angle is higher (closer to directly overhead) at lower latitudes, meaning sunlight travels through less atmosphere and reaches the ground with more UV intact. Southern Spain, Greece, and Turkey regularly reach UV Index 10–11 in July, while Scotland rarely exceeds UV Index 7.
Does this calculator work for skiing and ski touring?
Yes. Select the appropriate region and month, set altitude to your ski resort elevation, and enable "Snow on route" for accurate reflection adjustment. Ski resorts at 1,500–2,500m in March–April typically reach UV Index 7–9, higher than many people expect when it's cold.
Can UV damage happen in winter?
Yes, particularly at altitude. Snow reflection at high elevation combined with a clear sky can produce UV Index 4–6 even in December and January in Alpine regions. Winter mountaineers and ski tourers should always carry sunscreen and glacier glasses.
Is UVA or UVB more dangerous for hikers?
Both. UVB causes sunburn and is the primary driver of the UV Index scale. UVA penetrates deeper into skin and causes long-term damage and skin aging. Use a broad-spectrum sunscreen that protects against both. Most SPF-rated products sold in the EU provide broad-spectrum protection.
How does this calculator compare to weather app UV forecasts?
Weather app UV forecasts use real-time data from satellites and ground stations, making them more accurate for specific days. This calculator uses monthly averages; it is better suited for trip planning than day-of conditions. For accurate same-day forecasts, check a local weather service with UV Index data.