Last updated: May 3, 2025
Understanding the UV Index is crucial for residents and visitors in Boulder, Colorado, where the picturesque mountain backdrop comes with high levels of ultraviolet radiation. Awareness of UV levels is essential for preventing sunburn, protecting skin health, and reducing the risk of skin cancer. Boulder typically experiences a dry, semi-arid climate (BSk), leading to intensified sun exposure, especially during midday hours when UV ratings peak at a striking 11. By knowing the daily UV Index, you can take proactive steps to enjoy the outdoors safely while safeguarding your skin against harmful rays.
As the sun rises, the UV Index begins its gradual ascent, starting at a gentle 1 at 07:00. By 08:00, it climbs to 2, signaling a mild increase in UV exposure. The intensity peaks dramatically at 12:00, where it reaches a striking 10, indicating a high risk for harm from unprotected sun exposure. 13:00 sees an even higher point of 11, making it crucial for sun safety measures. As the afternoon progresses, the index begins to descend—falling to 8 by 15:00 and tapering off to a more manageable 3 by 17:00. The day concludes with a soft decline, dropping back to 1 by 19:00. This hourly UVI data underscores the importance of protecting skin during the midday sun to prevent damage and ensure safety.
| 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 | 10 | very high | 15 min |
| 13:00 | 11 | extreme | 10 min |
| 14:00 | 10 | very high | 15 min |
| 15:00 | 8 | very high | 15 min |
| 16:00 | 6 | high | 25 min |
| 17:00 | 3 | moderate | 30 min |
| 18:00 | 2 | low | 45 min |
| 19:00 | 1 | low | 45 min |
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.
UV Risk Categories
| 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 |