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.
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
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.