Denver UV Index by Month: Three Years of Data

Denver's UV index peaks in July, averaging 9.5 on a typical day and 10.6 when the sky is clear. It stays at 3 or higher, the level where sun protection is recommended, on about 77 percent of days in the year, including plenty of cold ones. We pulled three years of hourly data to get these numbers rather than repeat the usual estimates. Below is the month-by-month table, why the mile-high air pushes it up, and what to do about it.

Denver UV index by month

Each day's highest UV index reading, averaged by month, from 1,096 days between January 2023 and December 2025. "Typical day" includes whatever clouds showed up. "Clear sky" is the cloud-free value for the same day.

Source: hourly UV index for Denver from the Open-Meteo CAMS air quality dataset, Jan 1 2023 to Dec 31 2025, daily maximums averaged by month. For today's reading, the live Denver UV index runs in the header of our homepage.

What the numbers mean in practice

• UV 3 or higher on 77 percent of days. 839 of those 1,096 days hit 3 or above, the point at which sun protection is generally recommended. That includes days in February and November.
• UV 8 or higher on 27 percent of days. 296 days landed in the very high range, which is roughly one day in four, year-round.
• The highest single reading in three years was 12.5, in summer, under a clear sky.
• September still averages 6.9. Denver's UV does not switch off when the weather turns. October averages 4.9, which is a higher reading than many coastal cities see in June.

Why Denver runs high

Three things stack up here, and only one of them is obvious.

• Elevation. Denver sits at 5,280 feet, and the thinner column of air above you absorbs less UV on the way down. The EPA's figure is about 2 percent more UV per 1,000 feet of elevation. At the ski resorts, 9,000 to 11,000 feet, that compounds.
• Clear days. Colorado gets a lot of them, and the table shows how little the typical-day and clear-sky numbers diverge in winter. Cloud cover is the main thing that pulls UV down, and Denver does not get much of it.
• Snow. Snow reflects as much as 80 percent of UV back at you, so a bright February day on a white surface delivers far more than the index alone suggests. Sand reflects about 15 percent and water about 10.

UV increases about 2% for every 1,000-foot increase in elevation due to thinner mountain air. Snow may reflect as much as 80% of UV, sand 15%, and water 10%.

Source: EPA, A Guide to the UV Index

The full altitude breakdown, including how Colorado compares to cities far closer to the equator, is in Colorado UV index: why sun protection is non-negotiable at altitude.

What to wear, by season

• May through August, index 7.5 to 9.5. Broad spectrum daily, reapplied every two hours outdoors and after swimming or sweating. A hat earns its place.
• March, April, September, October, index 4.9 to 6.9. The same daily habit. This is the stretch people skip, and it is the stretch that quietly adds up.
• November through February, index 2.0 to 3.7. Low on paper, except on snow. If you are skiing, riding or shoveling in sun, the reflected load is the real number. See sunscreen for skiing and snowboarding.
• Year-round: UVA barely changes with the season and passes through window glass, which is the case for a daily habit rather than a summer one. More in UVA is the real winter sun problem.

Built in Denver, for this

Swellies was formulated here, under these numbers. Five ingredients, non-nano zinc oxide at 21 percent as the only active, Broad Spectrum SPF 46, water resistant 40 minutes, in a 50ml airless pump that fits a jacket pocket. SPF 46 rather than 30 is a margin choice, because applying less than the tested amount is normal and the index here is not.

Frequently asked questions

What is the UV index in Denver?

It peaks in July, averaging 9.5 on a typical day and 10.6 under clear skies, and bottoms out in December around 2.0. Across 2023 to 2025, Denver reached UV 3 or higher on 77 percent of days and UV 8 or higher on 27 percent of days. The live reading is in the header of our homepage.

Which month has the highest UV index in Denver?

July, averaging 9.5 for the daily maximum, with June close behind at 9.0 and August at 8.6. The highest single clear-sky reading in the three-year dataset was 12.5.

Why is Colorado's UV index so high?

Elevation, clear skies and snow. Denver sits at 5,280 feet, and UV rises about 2 percent per 1,000 feet because there is less atmosphere overhead to absorb it. Colorado also gets few cloudy days, and snow reflects as much as 80 percent of UV back up at you.

Does elevation affect the UV index?

Yes. The EPA puts it at roughly 2 percent more UV for every 1,000 feet of elevation gain. Denver at 5,280 feet gets more than a sea-level city at the same latitude, and a 10,000-foot trailhead gets more again.

Do I need sunscreen in Denver in winter?

The index averages 2.0 to 3.7 from November through February, which is low, but two things change that. UVA stays relatively steady year-round and passes through glass, and snow reflects up to 80 percent of UV, which is why ski days burn faces.

What UV index requires sunscreen?

Protection is generally recommended from UV 3 upward. Denver is at or above that on roughly three days out of four across the year.

Built in Denver, under a UV index that peaks near 10. Five ingredients, SPF 46. Shop Swellies SPF 46 →