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The UV index
The UV index is a standard scale, developed with the World Health Organization, for how strong the sun's ultraviolet radiation is at your location. The higher the number, the faster unprotected skin can be harmed — and the less time it takes.
General sun-safety information, not medical advice. Burn times vary enormously with skin, the UV index, altitude, reflection off snow or water, and some medicines — when in doubt, cover up. Any new, changing or unusual mole or spot should be checked by a doctor.
Reading the number
The index runs from low, single figures up into the double figures near the equator, at altitude or in strong summer sun. It is reported in weather apps and forecasts. Because it reflects real conditions, it is a better guide than air temperature — you can burn on a cool, bright, breezy day when the UV is high and you do not feel hot.
It is amplified by reflection and altitude
Snow, water, sand and concrete bounce UV back at you, and UV rises with altitude, so a mountain or a beach can expose you to far more than the flat number suggests. Cloud reduces UV only partly — you can still burn through light cloud.
| UV index | Category | What to do |
|---|---|---|
| 1–2 | Low | Minimal protection needed for most people. |
| 3–5 | Moderate | Seek shade around midday; hat, sunglasses and SPF 30. |
| 6–7 | High | Protection needed; cover up, SPF 30+, shade from late morning to mid-afternoon. |
| 8–10 | Very high | Extra care; avoid the sun in the middle of the day. |
| 11+ | Extreme | Avoid being outside at midday; full protection essential. |
Related
The vitamin D window
Your skin only makes vitamin D when the sun is high enough — which, up north, it is not for months.
How SPF works
What the SPF number means, why SPF 50 is not double SPF 30, and the amount almost everyone gets wrong.
Find your skin type
The Fitzpatrick scale, from always-burns to never-burns, with advice for each.
Source: WHO Global Solar UV Index and general dermatology and public-health guidance. General information, not medical advice. · checked 2026-09-04
Frequently asked questions
Can I burn when it is cloudy?
Yes. Cloud filters only part of the UV, so on a high-UV day you can still burn through light or broken cloud. Check the UV index, not just the sunshine.
Why can I burn when it is not hot?
Burning is caused by UV, not heat. A cool, bright day — or a windy one that hides the heat — can carry a high UV index, especially near snow or water.
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