Sun Protection

UV Is Radiation, Not Weather

Most sunburns happen on the day you didn't see coming. They show up at 72°F (22°C) under May overcast, on a long highway drive, on a cold ski day at 14°F (−10°C), on a 10:30 AM walk under tree shade. The pattern: UV radiation doesn't follow temperature, doesn't follow comfort, and doesn't follow how bright the day looks. It follows different rules.

This section explains those rules. Which share of the radiation does which kind of damage. Why clouds mislead and why car glass only tells half the truth. How snow, water, and altitude can double the dose. And what "broad spectrum" or the UVA logo on a bottle actually promises. The specific dose-and-timing answers live in the two calculators below — this page covers the physics behind them.

UVA and UVB — Two Wavelengths, Two Damage Profiles

UV reaching the Earth's surface arrives in two main wavelength bands. UVB (280–315 nm) is the shorter, more energetic kind — it burns skin within hours, drives the classic sunburn, and is critical for DNA damage in the outer skin layer. UVA (315–400 nm) is longer-wavelength, penetrates deeper into the dermis, and is the primary driver of visible photoaging (wrinkles, pigment spots, loss of elasticity) — without causing acute pain at the time.

Roughly 95% of the UV reaching the ground is UVA. UVB is only about 5%, but it's significantly more aggressive per photon. Both independently contribute to skin cancer risk — the American Academy of Dermatology and the WHO both list them as established risk factors.

Why Clouds Mislead and Glass Only Tells Half the Story

Clouds filter visible light, but barely filter UV. The Skin Cancer Foundation has cited the same number for years: up to 80% of UV gets through a normal cloud cover. Thin, hazy clouds let through closer to 90%. What clouds do is dim the visible brightness and lower the heat — both signals the brain interprets as "less sun." The UV doesn't get the memo.

Glass filters one band, not both. Standard window glass blocks about 97% of UVB but only 25–50% of UVA. A well-known case published in the New England Journal of Medicine documents a 69-year-old American truck driver with dramatic one-sided photoaging on his left cheek — 28 years at the driver's window. Most car windshields are laminated and block UVA almost completely, but the side windows usually aren't. Long-distance commuters get years of UVA on the left arm and left cheek.

Even under a beach umbrella, about half the UV dose still arrives — scattered through the sand, the water, and the atmosphere. Full shade isn't full protection.

Altitude, Snow, Water — The Reflection Multipliers

UV intensity climbs roughly 10–12% per 1,000 m (3,300 ft) of elevation, because the atmosphere thins and absorbs less. At 8,200 ft (2,500 m) — typical Alpine ski altitude — that's already 25–30% more UV than at sea level. On top of that, the surrounding surface reflects an additional fraction back upward. The WHO publishes the standard values:

• Fresh snow: reflects up to 80% of UV. Faces on a ski day get effectively double the dose — half from above, half from the slope.
• Water: 10–30%, depending on angle. Midday sun on flat water reflects more than morning light at a low angle.
• Sand: 15–25%. Light-colored beaches reflect noticeably more than dark volcanic rock.
• Concrete, asphalt: 10–12%. Part of the reason urban heat days produce sunburns.
• Grass, lawn: 2–5%. Lower than every other surface.

Skiing on a July glacier is the maximum combination: a 30% altitude bonus plus 80% snow reflection produces an effective UV load most European beaches never reach — at 23°F (−5°C) and snowing.

Why May Burns More Than People Expect

In May, the UV index across most of the temperate Northern Hemisphere already hits 6 to 8 under a clear sky — that's 70 to 80% of the July peak. Yet most first sunburns of the year happen on a sunny weekend in May or early June, not at the height of summer. Two effects stack on each other:

First: winter skin has lost its own protective layer. The natural self-protection time of average European or North American skin sits at 10–15 minutes in April; by August, after weeks of gradual pigmentation, it's closer to 20–30. The same UV index hits more sensitive skin in May than it does in August.

Second: the mental model. "May" reads as spring in the brain; "August" reads as summer. At 72°F (22°C) and sunshine in May, people stay outdoors for four hours without sunscreen; at 86°F (30°C) in August, the tube is already in the bag. The EPA's UV index forecast and the German Federal Office for Radiation Protection (BfS) both publish daily UV index values starting in early spring for exactly this reason — physically the season is already on, but most people haven't switched mode yet.

What "Broad Spectrum" on the Bottle Means

The SPF number on a sunscreen measures UVB protection only. An SPF 50 product without UVA protection would be half-finished — it blocks sunburn but not photoaging and not the UVA-related share of skin cancer risk. Three labels regulate this differently around the world:

• EU standard (UVA logo). Products sold in the EU may carry the circular "UVA" mark only if the UVA protection factor reaches at least one-third of the labeled SPF. For SPF 30, that means a UVA-PF of at least 10. The circular UVA stamp is the visible sign of a genuine broad-spectrum product.
• US standard ("Broad Spectrum"). The FDA allows the label only if the product passes the critical wavelength test (critical wavelength ≥ 370 nm). The threshold is less strict than the EU version, but still meaningful.
• PA system (Asia, some EU products). Scale from PA+ (low UVA protection) to PA++++ (very high). For Mediterranean or tropical travel, PA+++ or higher is the sensible floor.

What's commonly missing: cheap supermarket sunscreens without the UVA logo. The bottle can carry SPF 50 and effectively no UVA protection at all. With dermatologist brands like La Roche-Posay Anthelios, Eucerin Sun, Avène, Bioderma Photoderm, or ISDIN Fotoprotector, the UVA marking is standard.

When the Calculators Are the Right Tool

This page covers the physics — when UV hits hardest, what actually reaches the skin, and what the label has to promise. For the two practical questions, there are dedicated tools:

• "How long does this sunscreen protect me?" — the sunscreen calculator combines skin type, SPF, UV index, and reapplication into a real protection time.
• "How much sunscreen do I need for my vacation?" — the sunscreen vacation calculator returns bottle counts per person, day, and destination type.

Common Questions About UV Radiation

Adjacent Areas

• Sunscreen Calculator – protection time by skin type, SPF, and UV index.
• Sunscreen Vacation Calculator – bottle counts for an entire trip.
• Hydration – heat, sun, and fluid loss land on the same afternoon.
• Festival & Camping – planning UV exposure across multi-day outdoor events.
• Health & Fitness overview – all everyday-health tools in one place.