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The UV index scale
The UV index measures how strong the sun's burning ultraviolet rays are, on a scale that starts at 0 and has no upper limit. The higher the number, the faster unprotected skin and eyes are damaged. The scale and colors are the same worldwide (World Health Organization); in the United States the forecast is issued by the EPA and the National Weather Service.
| UV index | Level |
|---|---|
| 0–2 | Low Low danger from the sun's UV rays for the average person. Wear sunglasses on bright days. If you burn easily, cover up and use broad-spectrum SPF 30+ sunscreen. Sand, water and snow reflect UV and add to your exposure. |
| 3–5 | Moderate Moderate risk from unprotected sun exposure. Stay in the shade near midday, when the sun is strongest. Wear protective clothing, a wide-brimmed hat and UV-blocking sunglasses, and apply broad-spectrum SPF 30+ sunscreen every 2 hours, even on cloudy days, and after swimming or sweating. |
| 6–7 | High High risk from unprotected sun exposure: skin and eyes need protection. Reduce time in the sun around the middle of the day, seek shade, and wear protective clothing, a wide-brimmed hat, UV-blocking sunglasses and broad-spectrum SPF 30+ sunscreen, reapplied every 2 hours. |
| 8–10 | Very high Very high risk: unprotected skin and eyes can be damaged and burn quickly. Minimize time in the sun around the middle of the day, seek shade, and cover up with clothing, a wide-brimmed hat, UV-blocking sunglasses and broad-spectrum SPF 30+ sunscreen, reapplied every 2 hours. |
| 11+ | Extreme Extreme risk: unprotected skin and eyes can burn in minutes. Try to avoid the sun around the middle of the day, stay in the shade, and use every protection: clothing, a wide-brimmed hat, UV-blocking sunglasses and broad-spectrum SPF 30+ sunscreen, reapplied every 2 hours. |
When to protect yourself
Health agencies advise protection whenever the UV index is 3 or higher. Our city pages show the exact hours when today's forecast reaches 3 in each city.
The shadow rule: if your shadow is shorter than you are, the sun's UV is at its strongest — protect yourself.
What changes the UV index
The sun's height matters most: UV is strongest around solar noon, in summer and closer to the equator. Thin ozone lets more UV through. Clouds usually lower it but can raise it for short moments at their edges. Altitude adds roughly 10% per 1,000 m. Snow reflects up to 80% of UV, sand and sea foam about 15–25%, so you can burn in the shade on a beach or on a ski slope.