Boulder weather in September
September in Boulder: warm, a little rain, with clean air — one of the best months to visit.
#1 of 12 months in Boulder · #61 of 897 destinations to visit in September · comfort 93/100 (tier S)
Updated June 2026 · climate normals from ERA5 reanalysis
Traveller notes on Boulder in September
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Boulder climate normals — September
| Daytime high | 27°C / 81°F | 🌤 warm |
|---|---|---|
| Nighttime low | 10°C / 51°F | |
| Rainfall | 40 mm / 1.6 in | 🌦 some rain |
| Rainy days | 8 days | |
| Humidity | 54% | 🍃 comfortable |
| Dew point | 6°C / 43°F | |
| Sunshine | 11.2 h/day | |
| Daylight | 12.4 h | |
| Wind | 13 km/h / 8 mph | |
| Snowfall | 3.8 cm / 1.5 in | |
| Air quality (US AQI) | 36 | 🍃 good |
Monthly averages from ERA5 reanalysis. AQI is the mean US Air Quality Index for the month.
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Boulder in September — FAQ
- Is September a good time to visit Boulder?
- September in Boulder: warm, a little rain, with clean air — one of the best months to visit. It ranks #1 of 12 months for Boulder on SeasonMap's comfort score (93/100).
- What is the weather like in Boulder in September?
- In September, Boulder averages a daily high of 27°C and a low of 10.3°C, with about 39.9 mm of rain over roughly 8 rainy days.
- What is the air quality in Boulder in September?
- Boulder has a mean US Air Quality Index of about 36 in September.
- Does it snow in Boulder in September?
- Boulder sees roughly 3.8 cm of snowfall in September.