# Boulder, CO UV Index Today — hourly UV forecast > Source: https://breathecheck.com/uv-index/us/colorado/boulder/ > UV index in Boulder, CO right now, today's peak and the hour it arrives, on the EPA scale. Modelled data, labelled as modelled. # Boulder, CO UV Index Today Modelled for 17 Aug 2026, 02:00 UTC · GMT-6 ## Right now 0 Low No protection needed for most people. Today's peak is 9 (Very High) at about 1pm local time. ## Hour by hour today Local time, midnight to midnight. Bar colour is the EPA UV band, never restyled. ## Boulder today, in numbers - **Latitude**: 40.0°N — 52 of 121 from the north - **Today's peak**: 9 (Very High) — 27 of 121 nationally - **Peak arrives**: 1pm local (GMT-6) - **At or above 6**: 4 hours, 11am to 3pm - **At or above 3**: 6 hours, 10am to 4pm The window from 11am to 3pm is the practical answer for Boulder today. Nationally today the spread runs 11.7 in Honolulu (21°N) to 0.8 in Anchorage (61°N). Why latitude and the clock decide this → ## A mile of atmosphere already below you Boulder sits at 5,430 feet, about 1,655 meters, where the plains meet the Flatirons. Using the World Health Organization’s rule of roughly ten percent more UV per 1,000 meters of altitude, that puts the city on the order of fifteen percent above a sea-level town at the same latitude on the same day. It is the largest factor at work here and a constant one, applying in February as much as in July. Snow is the second factor and it is intermittent. The WHO puts fresh snow’s UV reflectance as high as 80 percent, more than any other common natural surface, so a clear day over fresh Front Range snow delivers more than the sun angle alone accounts for. Boulder does not hold that snow. Chinook downslope winds strip the foothills repeatedly through the winter, so the reflected boost comes and goes instead of running December to March. The daily shape has a local wrinkle. Cumulus builds over the Continental Divide through the morning and drifts east over the city in the afternoon, which reverses the usual assumption about when a summer day is clearest: in July the open part is often the hours before midday, and the buildup arrives about when the sun is highest. ## Nearest metros we model UV changes with latitude, not with a city boundary, so a neighbour a hundred miles north reads differently for a reason. ## Where Boulder sits on the scale 0 is Low. No protection needed for most people. Today Boulder also reaches Very High at its peak — a different band from the one it is in right now, which is why the hour matters as much as the day. The scale is open-ended: 11 is where the named bands stop, not the maximum. All five bands, with the EPA guidance for each → ## Modelled, not measured The UV figures for Boulder are computed by the Copernicus atmosphere model, not read off an instrument here — unlike the AQI on our air-quality pages, which comes from a regulatory monitor. Cloud is where a UV model is weakest. What that means in practice → ## Questions about UV in Boulder **What is the UV index in Boulder today?** 0 right now (Low), peaking at 9 around 1pm local time. UV stays at 6 or above for 4 hours, from about 11am to 3pm. This is a modelled value, not a ground measurement. **When is UV strongest in Boulder today?** Around 1pm local time (GMT-6), not at noon — solar noon drifts across a timezone and daylight saving moves it later on the clock. **Is this measured or modelled?** Modelled. The figures come from the Copernicus atmosphere model via Open-Meteo, unlike the AQI on our air-quality pages, which is read from an EPA regulatory monitor. Cloud cover is where a UV model is weakest. ## Related On this page Air quality alerts We alert on AQI, not UV — UV is predictable from the calendar, air quality is not. Updated 17 Aug 2026, 02:00 UTC