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The UV Index was developed by WHO, the World Meteorological Organization, UNEP, and ICNIRP as a standardized international scale, designed to communicate the intensity of UV radiation at ground level without requiring technical knowledge — before its adoption, every country used different, incomparable scales.

Actual intensity depends on several factors together: the sun's height in the sky (highest at solar noon and in summer), latitude (stronger near the equator), altitude (increases roughly 10% per 1000 meters, since there's less filtering atmosphere), the thickness of the ozone layer, and cloud cover — this last one is deceptive: even under overcast skies, most UV still gets through, and a common mistake is assuming cloudy weather makes protection unnecessary.

The number shown in this app is calculated in real time by combining these factors with weather models — that's why it changes hour by hour, peaking near solar noon and dropping close to zero at dawn and dusk, as you can see in the chart above.

UV intensity increases by roughly 10% for every 1,000 meters of altitude — there's simply less atmosphere above you to filter it. A ski resort at 2,000m can see meaningfully higher UV than the valley below it, on the same clear day.

The sun isn't the only source, either — surfaces bounce a real share of it back up at you. Fresh snow reflects most of it; sand and sea foam matter too, just less dramatically. It's why reflected sunburn (and snow-blindness) are real risks even in the shade.

UV exposure isn't only a risk to manage: the same UVB radiation that causes sunburn is also what triggers the body to synthesize vitamin D, essential for absorbing calcium and keeping bones healthy — which is why total, round-the-clock protection isn't this app's goal.

A 2024 study numerically modeled how much midday sun (10:00–16:00, the March–October active season in the Mediterranean basin) is needed to produce 1000 IU of vitamin D, for each Fitzpatrick skin type — lighter skin synthesizes it faster, darker skin needs more time for the same amount.

These are seasonal averages, not scaled to today's UV the way the burn-time estimate above is — the app shows them side by side in the Forecast panel, so you can get a sense of the real margin between the two.

SPF (Sun Protection Factor) measures how much longer protected skin takes to burn compared to unprotected skin, under lab conditions using a thick, generously applied layer (2 mg per cm²) — in practice, almost no one applies that much, so real-world protection is usually lower than the labeled number.

The relationship between SPF and protection isn't linear: SPF30 blocks about 97% of UVB rays, SPF50 about 98% — a small numerical gap that misleads many people into assuming SPF50 offers roughly double the protection of SPF30. It doesn't. What actually makes the difference is reapplying often and using enough of it, not chasing an ever-higher number.

SPF only measures protection against UVB rays, the ones that cause visible sunburn. A "broad spectrum" label means the product also protects against UVA rays, which penetrate deeper and contribute to skin aging and cancer risk even without causing a visible burn — checking for "broad spectrum" matters as much as the SPF number itself.

Alongside UV, this app also shows the European Air Quality Index (European AQI) for your location — a single number that summarizes five separately measured pollutants: PM2.5, PM10, ground-level ozone, nitrogen dioxide, and sulphur dioxide.

The index always reports whichever pollutant is worst at that moment, not an average — and its six bands (Good, Fair, Moderate, Poor, Very Poor, Extremely Poor) are calibrated against the WHO's 2021 air quality guidelines, so each band corresponds to a comparable level of health risk regardless of which pollutant is driving it.

Long-term exposure to these pollutants is linked to respiratory and cardiovascular disease — the European Environment Agency estimates that most of Europe's urban population breathes levels above WHO guidelines for at least one of these five pollutants.

Solar UV reaching the ground depends heavily on how much ozone sits between you and space. Industrial chemicals (CFCs) tore a seasonal hole in it over Antarctica starting in the 1980s — by the mid-2000s it had grown to nearly 30 million km², roughly the size of Africa.

The 1987 Montreal Protocol phased those chemicals out worldwide, and it's working: the hole has been shrinking since around 2000, and current science projects it back to 1980 levels by 2066 over Antarctica.