UV Index Lookup by City and State

Understanding the UV Index

The UV Index is a measure of the strength of ultraviolet radiation from the sun at a particular place and time. Developed by the World Health Organization in 1994 and adopted by the US Environmental Protection Agency, it ranges from 0 (nighttime) to 11+ (extreme tropical midday sun). The index is a key tool for understanding your risk of sunburn, skin damage, and long-term skin cancer risk from sun exposure.

UV radiation is influenced by several factors: latitude (closer to the equator means stronger UV), altitude (UV increases about 10-12% per 1,000 meters of elevation), time of year (summer solstice peaks), cloud cover, ozone layer thickness, and surface reflection (snow, sand, and water reflect UV and increase your effective exposure). Cities at high altitude, like Denver and Santa Fe, experience significantly higher UV than cities at the same latitude near sea level.

How UV Index Is Measured

The UV index is calculated from measurements of solar spectral irradiance — the intensity of UV radiation at different wavelengths reaching the earth's surface. Instruments called spectroradiometers measure UVA (315-400 nm) and UVB (280-315 nm) radiation, which is then weighted by the McKinlay-Diffey erythemal action spectrum that models human skin sensitivity. The result is normalized so that 1 UV index unit equals 25 milliwatts per square meter of erythemal UV radiation.

In the United States, the EPA and National Weather Service issue daily UV index forecasts using satellite-based ozone measurements, atmospheric modeling, and surface observations. The forecasts represent the peak UV expected at solar noon under clear-sky conditions. You can check the live daily forecast for your zip code using the EPA UV Index API. The monthly averages on this site are derived from historical data accounting for typical cloud cover, ozone levels, latitude, and altitude.

UV Index and Vitamin D

While excessive UV exposure is harmful, moderate sun exposure is essential for vitamin D synthesis. Your skin produces vitamin D3 when UVB radiation converts 7-dehydrocholesterol in the epidermis. The amount of time needed varies by skin type, UV index, and body area exposed. At UV 3, fair-skinned individuals need approximately 10-15 minutes of midday sun on arms and face to produce adequate daily vitamin D. At UV 8+, just 5-8 minutes may be sufficient. Darker skin types need 3-5 times longer. During winter months in northern states, UV levels may be too low for meaningful vitamin D production, which is why supplements are commonly recommended.

UV Index Scale

Sunburn Time by Skin Type

How quickly you burn depends on both the UV index and your skin type. The Fitzpatrick scale classifies skin into six types based on response to UV exposure. Below are approximate minutes to first redness (minimal erythemal dose) at various UV levels without sunscreen.

UV Exposure and Outdoor Activities

Different outdoor activities carry varying levels of UV risk. Water-based activities like swimming, surfing, and fishing expose you to reflected UV from the water surface (up to 25% reflection), while sand reflects 15-20% and concrete 10-12%. Snow is the most reflective surface, bouncing back up to 80% of UV radiation.

Runners and cyclists face extended exposure during training. A 2-hour outdoor run at UV 7 delivers roughly the same cumulative UV as 30 minutes at UV 14 — well into the extreme range. Golfers, hikers, and construction workers also accumulate significant doses over the course of a day. Using the UV index data for your city, you can plan outdoor activities for lower-UV times of day and apply appropriate sun protection.