Cooler air is not weaker sun. Gain elevation and you leave more of the atmosphere below you. Less air means less filtering. The UV dose climbs even when the breeze feels easy.

How much altitude adds

Public health guidance puts a percentage on the climb:

• WHO: UV levels rise about 10% to 12% with every 1,000 meters (about 3,280 feet) of elevation
• EPA: UV intensity increases about 6% per kilometer above sea level

Those are planning ranges, not a promise for every ridge on every day. Ozone, aerosols, and cloud cover still move the number. The direction is consistent: higher trailheads need more cover than the same forecast at the coast.

Rough illustrations from the WHO band (sea-level peak of 8 as a starting point):

• ~1,000 m / 3,280 ft (many foothill towns) → roughly +10–12% → about 9
• ~1,600 m / 5,280 ft (Denver-class elevation) → on the order of +16–20% → high 9 to low 10
• ~3,000 m / 9,800 ft (high alpine passes) → about +30–36% → into the 10–11 band

Treat those as order-of-magnitude checks, not a substitute for today’s reading at your location.

Cool air fools you

Mountain days break the usual comfort cue. Thin air and wind strip heat off skin. You feel fine. UV does not care.

Three traps stack at elevation:

1. Temperature as a green light. A 55°F summit can still sit under Very High or Extreme UV near solar noon.
2. “I’m only up for a few hours.” Burn time shortens as the index climbs. A short ridgeline lunch is not short in UV terms.
3. Bright ground. Fresh snow can reflect up to about 80% of UV and nearly double total exposure (WHO). That bounce hits the underside of the chin, nose, and eyelids—the same problem as beach sand, louder. For shade and reflection in general, see shade does not cancel UV.

Timing still matters: peak UV sits near solar noon, often before the day feels warmest. That clock is covered in UV peaks before the heat. Altitude raises the whole curve; it does not move solar noon.

Before you hike or ski

Use the live UV index for your area, then add elevation judgment:

1. Read today’s peak and time—not only whether the trailhead feels chilly.
2. Name your elevation band. A few hundred meters is a nudge. Above about 1,500–2,000 m, plan as if the sea-level story understates your dose.
3. Cover for bounce on snowfields and pale rock—sunscreen on face and neck, brimmed hat, UV-blocking sunglasses or goggles. Clothing beats hoping the breeze is protection.
4. Recheck when you gain a lot of height mid-day. Driving from a valley floor to a high pass is a UV step-change, not the same day you packed for at breakfast.
5. Do not wait for heat stress to decide. On mountains, heat and UV often disagree. Protect for the index, not the sweater weather.

Ski resorts and summer fourteeners share the rule. Cold does not cancel UV. Thin air does the opposite.

Limits

Altitude percentages above summarize WHO and EPA public guidance. Local UV still depends on sun angle, ozone, aerosols, and clouds. Snow reflection figures are WHO surface-albedo guidance; wetness and slope change the bounce.

UV readings on UV Index Today are estimates for everyday sun-safety planning, not medical advice. For SPF and related Q&A, see the Guide. For your location’s current numbers, open the homepage.