UV Safety
The UV Index Explained: What the Numbers Actually Mean
A clear, plain-English guide to the WHO UV Index — what each level means for your skin, when to act, and how to read a forecast.
The UV Index is a single number — usually between 0 and 11+ — that tells you how strong the sun's ultraviolet radiation is at your location. The World Health Organization standardized it so weather services around the world report it the same way.
But what does “UV 7” actually mean for how long your skin can be outside before burning? This guide breaks the scale down into real, actionable categories.
How the scale works
Each whole number on the UV Index represents about 25 milliwatts per square meter of erythemally weighted UV — the kind that causes sunburn. The scale is open-ended, but in practice anything above 11 is rare outside the tropics and high altitudes.
- 0–2 Low — minimal protection needed for most people.
- 3–5 Moderate — wear SPF 30+, sunglasses, and seek shade near midday.
- 6–7 High — protection is essential. Reapply SPF every 2 hours.
- 8–10 Very High — extra precautions. Avoid sun 10am–4pm.
- 11+ Extreme — unprotected skin can burn in minutes.
Why the UV Index changes through the day
UV peaks when the sun is highest in the sky — typically between 11am and 3pm local time. Cloud cover, altitude, latitude, and surface reflection (snow, sand, water) all shift the number.
A good rule of thumb: if your shadow is shorter than you are tall, the UV is strong enough to burn unprotected skin.
How to use the forecast
Check both the current UV and today's maximum. The current value tells you what to do right now; the maximum tells you when to plan around. Hourly forecasts are most useful for planning outdoor activity, exercise, and childcare.
Key takeaways
- UV 3+ is when sun protection becomes important for most skin types.
- Burn time depends on UV strength and your Fitzpatrick skin type.
- Plan outdoor activity around the daily peak — usually 11am to 3pm.