UVA at altitude: mountains, hiking and flying

Everyone knows UV is stronger in the mountains. Rather fewer know that the familiar "10% more per 1000 metres" is an erythemal figure — it describes the sunburning band. UVA climbs with altitude too, but more gently, for reasons that say something useful about how the atmosphere works. And at altitude the gradient is rarely the biggest effect anyway.

Why altitude raises UV at all

Go up and there is simply less atmosphere between you and the sun. At 3000 m roughly 30% of the atmosphere's mass is already below you. That reduces four separate things at once:

• Ozone in the path above you — most of it is in the stratosphere, but the tropospheric fraction you climb above still counts.
• Rayleigh scattering by air molecules, which scatters some light back to space.
• Aerosol loading — haze, dust and pollution concentrate in the lower troposphere, so mountain air is optically much cleaner.
• Water vapour and cloud, which you frequently climb above entirely.

Why UVA's gradient is gentler

The gradient differs by wavelength because the things you leave behind are not wavelength-neutral.

Ozone absorbs strongly in the UVB and is almost transparent in the UVA (the same asymmetry that flattens UVA's seasonal cycle). Losing ozone from your path is a large gain for UVB and a negligible one for UVA.

Rayleigh scattering scales roughly as λ⁻⁴ — it is dramatically stronger at short wavelengths. That factor alone makes molecular scattering about 2.5 times stronger at 300 nm than at 380 nm. Climbing above some of the atmosphere therefore recovers much more UVB than UVA.

What is left is aerosol, which is far more wavelength-neutral across the UV, and it is the mechanism that UVA benefits from most. That has a practical implication: in a hazy or polluted region the UVA altitude gradient is steeper than in clean air, because there is more haze to escape. A climb from a smoggy valley floor is a bigger UVA jump than the same climb in the Arctic.

What the numbers look like

Using the model's +6%/km, and adding the surface and air effects that travel with altitude in practice:

The altitude term on its own is real but not dramatic. The reason mountains are a high-UVA environment is that altitude never arrives alone. At a ski summit you have +18% from elevation, +10% from snow reflecting from below, and a clean, dry, low-aerosol atmosphere that costs you less than sea-level haze would. The compound effect is what makes a clear February day at 3000 m rival a summer day at sea level — despite a solar elevation that ought to make it four times weaker.

Aircraft: cockpits, cabins and the pilot data

At a cruise altitude of 10–12 km you are above roughly three quarters of the atmosphere, and UV intensity is several times its sea-level value. What reaches the people inside depends entirely on the glazing.

• Cockpit windshields are laminated glass or glass-acrylic composites and block UVB completely and most UVA — the same laminate physics as a car windshield. Not all of it, though: measurements in a cockpit at 30,000 feet found that about an hour of flight delivered a UVA dose comparable to a short tanning-bed session. Side windows, often acrylic, vary more, and flying above a cloud deck adds a strong reflected component from below.
• Cabin windows are typically stretched acrylic, which absorbs most UVA — passenger exposure is far lower than a pilot's. It is not zero, though, and a sunlit window seat on a long daytime flight accumulates hours at an elevated baseline.

The epidemiology is what pushed this into the literature. A meta-analysis of aircrew studies found roughly twice the melanoma incidence in both pilots and cabin crew compared with the general population, and higher melanoma mortality in pilots. The finding is consistent across studies and hard to dismiss.

It is worth being careful about what it proves. Aircrew differ from the general population in more than their working altitude: they have high incomes, frequent access to sunny destinations, disrupted circadian rhythms, cosmic-radiation exposure and — importantly — better-than-average medical surveillance, which raises detected incidence on its own. Most authors regard occupational UV as a plausible contributor rather than a demonstrated sole cause. The cockpit UVA measurements make the mechanism credible; they do not by themselves close the case.

How the UVA Index handles elevation

The calculator does not ask you for your altitude. It looks up the terrain elevation for your coordinates from Open-Meteo and applies:

A linear approximation is a simplification — the real relationship flattens somewhat at very high elevations — but it is well behaved across the range where people actually stand, and the coefficient is UVA-specific rather than borrowed from erythemal tables. You can see the factor itself in the calculator's Model breakdown, alongside the cloud, aerosol and albedo terms; the derivation for all of them is in how the UVA Index is calculated.

Two limits to keep in mind. The elevation used is the ground elevation at your coordinates, so the model has nothing sensible to say about being in an aircraft. And it does not know that mountain air is typically cleaner than the aerosol data suggests for the wider grid cell, so on a clear alpine day the estimate is more likely to run low than high.

Practical takeaways

• Discount the 10%-per-1000 m rule for UVA — it is a sunburn figure. Around 6% is the better working number.
• But do not relax, because on snow the albedo term is larger than the altitude term, and the two multiply.
• Cover upward-facing skin in the mountains: under the chin, the nose, the underside of the jaw, the ears.
• Wrap-around eye protection is not optional on snow — see UVA and your eyes.
• If you fly for a living, treat the flight deck as a sunny window rather than an indoor space: broad-spectrum sunscreen on the face and hands, and film on side windows where operators permit it.