UV Index Scale Table
| Index | Level | Fair-skin burn | What it asks for |
|---|---|---|---|
| 0-2 | Low | ~60+ min | sun protection optional |
| 3-5 | Moderate | ~45 min | SPF 30+, hat, shades |
| 6-7 | High | ~30 min | SPF every 2 h, shade midday |
| 8-10 | Very High | ~15-20 min | SPF 50, avoid 10-4, real shade |
| 11+ | Extreme | ~10-15 min | avoid midday, full coverage |
| Multiplier | Effect on burn time |
|---|---|
| Fresh snow | ~80% more UV reflected up at you |
| Sand | ~20% more reflected |
| Altitude | ~10% stronger UV per 1,000 m |
| Thin cloud | still passes most of the index |
The UV index is a single number for the day's harshest hour - not an average - running 0 at night to 11+ in tropical and high-altitude sun, and the US version conforms with the international WHO guidelines: 0-2 low, 3-5 moderate, 6-7 high, 8-10 very high, 11+ extreme.
Bottom line: the bands roughly track burn time for fair skin - hours at low, around half an hour at high, ten to fifteen minutes at extreme - which is why the protection advice steps up with the number: shade and sunglasses from 3, SPF 30+ re-applied from 6, and from 8, avoid the midday window outright.
The honest part: the index ignores you personally - skin type, altitude (about 10% more UV per 1,000 m), snow and water reflection, and ozone swings can move the real dose far off the forecast number. The scale is a planning floor, not a permission slip; the shadow rule beats the app when the forecast is stale.
How to use
- Type today's forecast UV number and read the band - or read the table straight: each row's protection column is what the number is asking of you that day.
- Use the shadow rule to check the index in real time: a shadow shorter than you means UV is in the high range or above, whatever the app claimed at sunrise.
- Pair the band with the burn-time column and your own history: if you redden in 20 minutes at moderate, the same sun at very high needs layers and shade, not more sunscreen.
Frequently asked questions
What UV index is dangerous?
6 and above is where unprotected damage accelerates - high (6-7) burns fair skin in well under an hour, very high (8-10) in about 15-25 minutes, and extreme (11+) in roughly 10-15 minutes around solar noon. At 3-5 the risk is real over a full outdoor afternoon; below 3, ordinary activity needs little deliberate protection.
Does a high UV index mean hotter weather?
No - UV and temperature are different quantities. A thin-air high-altitude spring day can post UV 10 while feeling cold, and a hazy warm day can run lower UV because cloud and pollution scatter the rays. That is why skiers burn worse than beachgoers at the same latitude in winter: altitude and snow reflection raise UV while the temperature drops.
How fast can I burn at UV 8?
For fair skin (Fitzpatrick I-II), roughly 15-20 minutes of direct midday sun; medium skin about 25-30; dark skin an hour or more, though UVA damage accumulates without visible burns. Sunscreen labeled SPF 30 theoretically multiplies those numbers by thirty - in practice reapplication every two hours, or after swimming, is what delivers the protection.
What time of day is UV highest?
Solar noon window - roughly 10 a.m. to 4 p.m., sharpest 11-1 - when the sun's angle minimizes the atmosphere the rays cross. UV tracks the sun angle more than the clock: it peaks near the summer solstice, and the daylight-saving shift moves human exposure later into the already-high afternoon UV hours.
Is UV 11 possible outside the tropics?
Yes, in the right geometry: high summer noon at altitude (Denver posts 11+ days; equatorial-altitude sites like Quito approach 15-16), over snowfields, and during ozone-hole episodes in southern South America and Australasia, where springtime UV can reach extreme bands at mid-latitudes.