UV Index 🇺🇸United States

The United States spans an exceptional range of latitudes, from Hawaii at roughly 19–22°N near the tropics to northern Alaska beyond 71°N in the Arctic. Hawaii sits in the year-round high-UV tropical band, while northern Alaska receives almost no direct sun for parts of winter. The contiguous 48 states fall in the mid-latitude band where UV peaks strongly in summer and falls sharply in winter, with regional differences driven mainly by latitude, elevation, and local cloud cover.

Elevation amplifies UV across much of the western United States. The Rocky Mountains, Colorado Plateau, and the Sierra Nevada place large populated areas above 1,500 m, where the thinner atmosphere lets more UV reach the ground; the official UV index forecast model treats elevation, snow and ice reflectivity, and ozone as the main drivers of surface UV. The U.S. Environmental Protection Agency publishes the daily UV index nationwide on a 1–11+ scale, providing the public-facing forecast Americans see in weather reports.

The United States sits at #144 of 191 in the WHO's global ambient-UV ranking with about 2,736 J/m² of average daily UV, around the 25th percentile worldwide.