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Public health desks do not quote the same percentage. Flag the split before you use either one:

• WHO: UV levels rise about 10% to 12% with every 1,000 meters (about 3,280 feet) of elevation. The shorter WHO Q&A rounds that to about 10% per 1,000 m.
• EPA: UV intensity increases about 6% per kilometer above sea level. NOAA’s Climate Prediction Center uses the same about 6% per kilometer step in its UV Index computation.

Those are different estimates, not a typing error. WHO’s band is the public-health planning range in the Global Solar UV Index guide. EPA’s 6% is the elevation tweak inside the U.S. forecast model. Ozone, aerosols, and cloud 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—not a forecast):

• ~1,000 m / 3,280 ft (many foothill towns) → roughly +10–12% → about 9
• ~1,600 m / 5,280 ft (Denver’s mile-high elevation at the State Capitol) → on the order of +16–20% → high 9 to low 10. EPA’s 6%/km on the same 1.6 km is closer to +10%.
• ~3,000 m / 9,800 ft (high alpine passes) → about +30–36% → into the 10–11 band. EPA’s model would call the same climb about +18%.

A Colorado fourteener—a summit at 14,000 ft, about 4,270 m—is on the order of four WHO altitude steps from sea level, +40–50% on that band, or about +26% on EPA’s. Treat those as order-of-magnitude checks, not a substitute for today’s reading. Local ozone and cloud still write the index.

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. Heat is infrared. Altitude is missing air.
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). CPC’s forecast model lets snow albedo raise the dose by 30% to 100%. That bounce hits the underside of the chin, nose, and eyelids—the same problem as beach sand, louder. Dry rock and grass bounce much less; WHO puts grass, soil, or water usually under 10%. 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.

Ski resorts and summer fourteeners share the altitude rule and split on the floor. A ski day stacks thin air plus that 80% snow bounce. A dry summer summit stacks thin air without the white reflector—unless you still cross a snowfield. Cold does not cancel UV. Snow makes it louder.

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. A valley-floor reading is the valley’s air column, not the pass.
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. Say which percentage you are using—WHO’s 10–12% or EPA’s 6%.
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. A 1,000 m climb is a WHO 10–12% step—or an EPA 6% step—on top of whatever the index already was.
5. Do not wait for heat stress to decide. On mountains, heat and UV often disagree. A fourteener in July and a ski hill in March both earn cover. Only the snow bounce changes.

Sources

The 10–12% per 1,000 m figure is WHO, Global Solar UV Index: a practical guide; the shorter ~10% per 1,000 m line is WHO UV radiation Q&A. The ~6% per km figure is EPA, Learn About the UV Index; NOAA CPC uses the same step in How the UV Index is computed. Denver’s 5,280 ft / 1,609 m Capitol elevation: Britannica. Snow bounce up to 80%, nearly double exposure, and grass/soil/water under 10%: WHO practical guide and WHO UV Q&A. CPC snow-albedo increase 30–100%: CPC compute. Those percentages are planning ranges, not a forecast for your ridge.