DermUV
Monthly UV Index
Boulder, CO
Monthly UV Index in Boulder, CO
Average UV in Boulder is highest during May, Jun, Jul and lowest during Jan, Dec. Use this month-by-month guide to plan sunscreen, shade, clothing, and outdoor timing.
Boulder UV Index by Month
Average UV in Boulder peaks during May, Jun, Jul, when high sun angles combine with longer days. The deceptive months are usually early spring and early fall, when air temperatures stay mild but UV climbs into the high category by mid-morning. Locals often skip sunscreen on cool, overcast days, which is exactly when UV-A continues to drive long-term skin damage. UV in Boulder is lowest during Jan, Dec.
| Month | Avg UV | Risk |
|---|---|---|
| Jan | 5.0 | Moderate |
| Feb | 6.7 | High |
| Mar | 8.7 | Very High |
| Apr | 10.7 | Very High |
| May | 11.9 | Extreme |
| Jun | 12.3 | Extreme |
| Jul | 12.1 | Extreme |
| Aug | 11.3 | Extreme |
| Sep | 9.6 | Very High |
| Oct | 7.4 | High |
| Nov | 5.5 | Moderate |
| Dec | 4.6 | Moderate |
Highest UV Months in Boulder
Peak monthly UV reaches about 12.3 in May, Jun, Jul. That is the period when unprotected skin burns fastest, especially near solar noon. Check the live UV index in Boulder, CO before outdoor plans because clouds, ozone, altitude, and reflection can move the real-time number above or below the monthly average.
Lowest UV Months in Boulder
Monthly UV is lowest in Jan, Dec, around 4.6 on this model. Lower UV does not mean zero UV. UVA can still pass through cloud and glass, and snow, water, or pale concrete can reflect UV back onto exposed skin.
Monthly Sunscreen Guidance for Boulder
For months that average UV 3 or higher, broad-spectrum SPF 30+, sunglasses, shade, and UPF clothing are the practical baseline. For months in the very-high or extreme range, reapply every two hours outdoors and after swimming or heavy sweat. These are educational estimates, not personal medical advice.