About the data

UV index forecasts. Every UV number on this site is the U.S. Environmental Protection Agency's hourly UV index forecast (Envirofacts UV Index service), which the National Weather Service's Climate Prediction Center calculates from the forecast of total ozone, the sun's angle, elevation and expected cloud cover. We download it for the ZIP code of each city every morning and publish it as it is. It is public domain.

Forecast, not measurement. Real UV can be higher or lower: broken clouds, haze, altitude and reflective surfaces such as snow, sand and water change it. Protect yourself whenever the UV index is 3 or higher.

Cities. All U.S. places with 10,000 or more people from GeoNames (CC BY 4.0), each matched to a ZIP code from the GeoNames postal code list. Sunrise, solar noon and sunset are computed for the city's coordinates with the PyEphem astronomy library (U.S. Naval Observatory conventions), in the city's time zone.

Advice on each level follows the EPA and World Health Organization guidance for the UV index. It is general information, not medical advice.

We are not affiliated with the EPA, NOAA or the National Weather Service.