The scale · WHO standard
How our UV data works
uvindex.now shows one number and asks you to trust it. This page explains exactly where that number comes from, how fresh it is, what our estimates can tell you — and what they cannot. Last updated: September 30, 2026.
The data source
All UV values on this site come from Open-Meteo, which combines satellite observations (NASA, EUMETSAT) with atmospheric weather models to compute the UV index per hour and per location. These are modelled estimates, not readings from a local UV sensor. We operate no measurement hardware of our own.
What “refreshed every 15 minutes” means
Our pages re-fetch data from Open-Meteo every 15 minutes, and current values are interpolated between hourly model points. The underlying weather models have their own update cycles — typically every one to six hours depending on the model and region. So “every 15 minutes” describes how often we check, not how often a new physical measurement is taken.
What the UV index is
The UV index is the WHO’s standard measure of erythemally weighted ultraviolet radiation — a weighted sum across both UV-B and UV-A wavelengths (roughly 290–400 nm), where shorter wavelengths count far more heavily because they are much more effective at causing sunburn. The same number means the same risk anywhere on Earth.
Protection thresholds
We follow the WHO’s guidance: from UV 3, protection is recommended — shade around solar noon, covering clothing, a hat, sunglasses and broad-spectrum SPF 30+; from UV 8, extra protection is essential. Sunscreen is one layer of protection among several, not the whole answer.
How burn-time estimates work
The sunburn calculator combines the current UV index with your self-selected Fitzpatrick skin type to estimate the time to visible sunburn on unprotected skin. It is an indication for awareness — not a safe exposure time. Individual sensitivity varies with medication, skin condition, reflection from water, sand or snow, and altitude. We deliberately do not multiply burn times by SPF factors: the WHO advises against using SPF to calculate how long you can stay in the sun. Whatever the number says, reapply sunscreen at least every 2 hours and after swimming or sweating.
Sun Session reminders
Sun Session is a reapplication reminder, not a protection guarantee: it pings you when it’s time to reapply based on your skin type, SPF and the current UV — sooner is always fine, and swimming, sweating or towelling means reapplying immediately.
Location handling
City pages use fixed coordinates for each city centre. “My location” uses your browser’s geolocation (with your permission) and passes coordinates directly to Open-Meteo; we do not store them.
What the tool cannot predict
Local cloud breaks, reflection (fresh snow can nearly double exposure), altitude (roughly +10% UV per 1,000 m), ozone anomalies and your personal sensitivity. Treat every number on this site as decision support — for medical advice about your skin, see a professional.