What is the UV index?
The UV index is an international scale that measures how strong the sun’s ultraviolet (UV) radiation is at a given place and time. The higher the number, the faster unprotected skin and eyes can be damaged. In the U.S., the EPA publishes a daily forecast of the peak UV index for each ZIP code.
What changes the UV index?
| Factor | Effect |
|---|---|
| Time of day | Highest around solar noon, roughly 10 a.m. to 4 p.m. |
| Season and latitude | Higher in summer and closer to the equator |
| Altitude | UV rises with elevation because there is less air above you |
| Clouds | Thick cloud lowers UV, but thin or broken cloud can still let a lot through |
| Ozone layer | Thinner ozone lets more UV reach the ground |
| Surface | Snow, sand and water reflect UV and raise exposure |
Why it matters
UV radiation causes sunburn and contributes to skin ageing, eye damage and skin cancer. The index turns a complicated measurement into one number you can act on.
Look up today’s number for New York, Los Angeles or any ZIP code on the home page, and read the UV index scale for what to do at each level.
Questions
Can you get sunburned on a cloudy day?
Yes. A large share of UV passes through cloud, so you can burn on an overcast day, especially when the index is 3 or higher.
When is the UV index highest?
Around solar noon, when the sun is highest in the sky. That is typically between 10 a.m. and 4 p.m. in the summer.
Who sets the UV index scale?
The scale is an international standard developed with the World Health Organization and others. In the U.S., the EPA publishes forecasts using it.
Sources: U.S. EPA SunWise and the UV Index; World Health Organization, Global Solar UV Index: A Practical Guide. Last reviewed 2026-10-27.