UV Index Checker

The WHO scale runs open-ended from 0: 1–2 low (minimal risk), 3–5 moderate (SPF for extended time out), 6–7 high (protection needed, midday shade), 8–10 very high (extra protection, minimize 10am–4pm), 11+ extreme (unprotected fair skin burns in under 15 minutes). Each unit roughly scales burning intensity, so index 8 delivers about twice the burning radiation of index 4 — meaning safe exposure time halves. Snow, sand, and water reflection add meaningfully on top; altitude adds ~10% per 1,000m.

Yes — it's the mechanism behind most surprise burns. UV is unrelated to temperature (a 60°F June day can carry the same UV as a 95°F one; the sun's angle is what matters), and typical cloud cover blocks only a modest fraction of UV — thin clouds 10–20%, and broken clouds can even briefly amplify it via edge reflection. Wind and cool air suppress the warmth cues that normally tell you you're cooking. The reliable signals are the calendar, the clock, and this index — never how hot it feels.

The gap between tested and real-world SPF is application: labs use 2mg/cm² — roughly a shot glass for a body, a full teaspoon for face and neck — while typical users apply a quarter to half that, turning SPF 50 into effective SPF 10–15. So: apply generously 15 minutes before exposure, reapply every 2 hours and after water or heavy sweat regardless of the bottle's claims, and pick broad-spectrum SPF 30+ (above 50 the marginal gain is small; the number matters less than the amount). Shade during the peak hour shown beats any lotion.

The index peaks at solar noon — roughly 1pm during daylight saving time, not the 3–4pm temperature peak people intuit — with the danger window spanning about two hours either side. The zero-equipment heuristic that actually works: the shadow rule — when your shadow is shorter than you are, UV is strong enough to burn; shorter than half your height, you're in the peak band. Seasonally, UV in May equals UV in August at most latitudes, which is why the year's first sunny weekends produce so many burns on winter-pale skin.