Last updated: May 3, 2025
Understanding the UV Index is crucial for residents of Boulder, Colorado, where the sunny BSk climate significantly elevates exposure to harmful ultraviolet rays. With UV levels soaring as high as 9 during midday, awareness of these readings is key to preventing sunburn and promoting long-term skin health. Whether you're hiking the stunning trails or enjoying a sunny day in the park, knowing the UV Index helps you take the necessary precautions to protect your skin from potential damage. Stay informed, stay safe, and enjoy all that Boulder’s beautiful outdoors have to offer!
As the sun rises, the UV Index begins its ascent, starting at a mild 1 at 07:00. By 08:00, it climbs to a 2, indicating a low risk of harm from unprotected sun exposure. However, from 09:00 onwards, the intensity builds, peaking at 9 between 13:00 and 14:00—a time when precautions are essential to protect skin from damage. The afternoon hours see a slight decrease, dropping to 8 at 15:00 and further declining to 4 by 17:00. As the sun sets, the UV Index falls back down to 1 by 19:00, marking a return to safer levels. Stay vigilant during those midday hours to enjoy the sunshine while keeping your skin healthy!
| Time | UVI | Category | Time to Burn |
|---|---|---|---|
| 07:00 | 1 | low | 45 min |
| 08:00 | 2 | low | 45 min |
| 09:00 | 3 | moderate | 30 min |
| 10:00 | 5 | moderate | 30 min |
| 11:00 | 7 | high | 25 min |
| 12:00 | 8 | very high | 15 min |
| 13:00 | 9 | very high | 15 min |
| 14:00 | 9 | very high | 15 min |
| 15:00 | 8 | 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 |
Throughout the year, the UV Index experiences significant fluctuations, with the highest risk typically occurring from March to September. Visitors and residents should be particularly cautious during the high-risk months of April (UV Index 8), May (UV Index 9), June (UV Index 11), and July (UV Index 10), where the potential for skin damage is at its peak. During these months, burn times decrease dramatically, with the extreme risk in June allowing for only 10 minutes of unprotected sun exposure. As a guideline, it's advisable to wear protective clothing, apply broad-spectrum sunscreen, and seek shade during peak sun hours to minimize UV exposure. In contrast, October to February sees much lower UV levels, allowing for greater sun safety with recommended burn times ranging from 30 to 45 minutes.
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 |
| Boulder | 3 | 4 | 6 | 8 | 9 | 11 | 10 | 9 | 8 | 5 | 3 | 2 |
| Los Angeles UVI | 4 | 6 | 8 | 9 | 10 | 11 | 11 | 10 | 10 | 7 | 5 | 4 |
| Houston UVI | 5 | 8 | 9 | 11 | 11 | 12 | 12 | 11 | 10 | 9 | 7 | 5 |