UV Index in Boulder, United States

Last updated: July 2, 2025

Understanding the UV Index is crucial for residents and visitors in Boulder, Colorado, where the sun's rays can be particularly potent due to the city's high elevation and sunny climate. Awareness of UV levels helps in preventing sunburn and protecting skin health, especially during the peak hours when the UV index reaches a staggering 11 to 12 between noon and early afternoon. With Boulder’s BSk climate, the risk of overexposure increases, making it essential to check the UV index daily to enjoy outdoor activities safely while keeping your skin protected from harmful ultraviolet radiation.

Hourly UV Index in Boulder

Throughout the day, the UV Index displayed a dynamic pattern, peaking at an intense 12 around 1 PM. Starting off gently at 1 by 7 AM, it climbed steadily, reaching 9 by 11 AM, a clear sign to take precautions. As the sun approached its zenith, the index swelled to its maximum at 12 PM, emphasizing the importance of sun safety. Following the midday peak, the index gradually decreased, dropping to 9 by 3 PM, then tapering off further to 2 by 6 PM and 1 by 7 PM. This data underscores the significance of shielding oneself from the sun's harmful rays during the day, especially when the UV Index is at its highest.

Average solar exposure data for the current time of the year. Actual values may vary and depend on many factors, such as cloud cover, vegetation cover, surface type, etc.
Time	UVI	Category	Time to Burn
07:00	1	low	45 min
08:00	2	low	45 min
09:00	4	moderate	30 min
10:00	7	high	25 min
11:00	9	very high	15 min
12:00	11	extreme	10 min
13:00	12	extreme	10 min
14:00	11	extreme	10 min
15:00	9	very high	15 min
16:00	6	high	25 min
17:00	4	moderate	30 min
18:00	2	low	45 min
19:00	1	low	45 min

Monthly UV Index Trends in Boulder

During the year, UV levels vary significantly, with the most dangerous months being March to August, where the UV Index peaks at 12 in June and July, indicating an extreme risk of harm from unprotected sun exposure. April and May also maintain a very high to extreme rating, resulting in a rapid burn time of just 10 minutes. Conversely, during the winter months, such as January and December, the UV Index is at a more manageable 4, categorized as moderate, allowing for longer exposure times of 30 minutes without immediate risk. Residents and visitors are advised to exercise caution during the high-risk months by wearing sunscreen, protective clothing, and seeking shade, especially during peak hours when the sun's rays are most intense.

Monthly Maximum UV Index for Boulder, United States

The monthly maximum UV Index (UVI) represents the highest UV radiation level recorded each month in Boulder. It measures the strength of ultraviolet radiation from the sun and helps assess the risk of sunburn and UV-related health issues.

Data source: Statistical UV Index data for Boulder, based on information from the Copernicus Climate Change Service.

UV Risk Categories

Extreme (11+): Avoid the sun, stay in shade.
Very High (8-10): Limit sun exposure.
High (6-7): Use SPF 30+ and protective clothing.
Moderate (3-5): Midday shade recommended.
Low (0-2): No protection needed.

Compare Boulder UV Index with Other United States Cities

The table above highlights how UV exposure in Boulder compares to other major locations across United States. This comparison helps travelers and residents better understand regional sun intensity and plan outdoor activities with sun safety in mind.

Boulder's solar exposure compared to other cities in United States. Generated using Copernicus Atmosphere Monitoring Service information.
	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Chicago UVI	3	4	6	8	9	10	10	9	8	6	4	2
Brooklyn UVI	3	5	6	8	10	10	9	9	8	6	4	2
New York City UVI	3	5	6	8	10	10	9	9	8	6	4	2
Los Angeles UVI	4	6	8	9	10	11	11	10	10	7	5	4
Boulder	4	6	9	11	11	12	12	11	10	7	5	4
Houston UVI	5	8	9	11	11	12	12	11	10	9	7	5