What the UV index means
The UV index is a unitless number created by the World Health Organization to turn the ultraviolet radiation reaching the ground into something usable day to day.
Clear sky value. Thick cloud lowers the real index, but ultraviolet goes through thin cloud and reflects off sand, snow and water.
Index
| Range | What to do |
|---|---|
| Low (0 to 2) | You can stay outside with no special protection. |
| Moderate (3 to 5) | Shade around midday, hat, sunglasses and sunscreen. |
| High (6 to 7) | Avoid the sun between 10 am and 4 pm; SPF 30 or higher. |
| Very high (8 to 10) | Unprotected skin burns fast; cover up and stay in the shade. |
| Extreme (11 and above) | Avoid midday sun. Sunburn in minutes. |
Choose a city
- ShanghaiShanghai, CN
- ChongqingChongqing, CN
- ChengduSichuan, CN
- BeijingBeijing, CN
- GuangzhouGuangdong, CN
- ShenzhenGuangdong, CN
- KinshasaKinshasa, CD
- IstanbulIstanbul, TR
- LagosLagos, NG
- Ho Chi Minh CityHo Chi Minh City (HCMC), VN
- TianjinTianjin, CN
- WuhanHubei, CN
- LahorePunjab, PK
- Xi’anShaanxi, CN
- MumbaiMaharashtra, IN
- São PauloSão Paulo, BR
- Mexico CityMexico City, MX
- HangzhouZhejiang, CN
- KarachiSindh, PK
- DelhiDelhi, IN
- MoscowMoscow, RU
- DhakaDhaka Division, BD
- SeoulSeoul, KR
- HarbinHeilongjiang, CN
- TokyoTokyo, JP
- DongguanGuangdong, CN
- CairoCairo, EG
- HefeiAnhui, CN
- JohannesburgGauteng, ZA
- WeifangShandong, CN
- NanjingJiangsu, CN
- ShenyangLiaoning, CN
- FoshanGuangdong, CN
- LondonEngland, GB
- New York CityNew York, US
- JakartaJakarta, ID
- BengaluruKarnataka, IN
- JinanShandong, CN
- HanoiHanoi, VN
- TaipeiTaiwan, TW
- LimaLima Province, PE
- BogotáBogota D.C., CO
- DalianLiaoning, CN
- Hong Kong, HK
- BaghdadBaghdad, IQ
- WuzhongNingxia, CN
- QingdaoShandong, CN
- TehranTehran, IR
- YantaiShandong, CN
- HyderabadTelangana, IN
- Rio de JaneiroRio de Janeiro, BR
- SuzhouJiangsu, CN
- PuxiShanghai, CN
- ZhengzhouHenan, CN
- AhmedabadGujarat, IN
- AbidjanAbidjan Autonomous District, CI
- NanningGuangxi, CN
- KunmingYunnan, CN
- ShijiazhuangHebei, CN
- ChangchunJilin, CN
- MET Norway — Locationforecast 2.0 (ultraviolet_index_clear_sky)Licence: CC BY 4.0 e NLOD 2.0 (uso comercial permitido) · Credit: Dados do MET Norway (Instituto Meteorológico da Noruega)O índice é calculado para céu limpo: com nuvens espessas o valor real pode ser menor.
- GeoNames cities15000Licence: CC BY 4.0 · Credit: Dados de cidades: GeoNames (CC BY 4.0)
- Dados de cidades: GeoNames (CC BY 4.0)
How to read these numbers
The calculation weights each wavelength by its power to cause erythema, that is, reddening of the skin. The result is divided by 25 mW/m², which makes an index of 1 equal to 25 milliwatts per square metre of effective radiation.
The scale has no ceiling. Values above 16 have been measured in the Andes. Radiation rises about 10% per thousand metres of altitude, and fresh snow reflects up to 80% of what hits it.
Protection that works
- Shade in the middle of the day beats any product.
- A tightly woven shirt protects more than badly applied sunscreen.
- SPF 30 or higher, reapplied every two hours and after swimming.
- UV rated sunglasses: the cornea burns too.
- Extra care on water, sand and snow, which reflect the radiation.
Frequently asked questions
- Does SPF 50 protect twice as much as SPF 25?
- No. SPF 30 blocks about 97% of UVB and SPF 50 about 98%. The real difference lies in how much you apply and whether you reapply.
- Does the UV index change with heat?
- No. It depends on the sun’s elevation, ozone, altitude and cloud. A cold clear day at altitude can carry an extreme index.