Climate averages 1991–2020
Boulder weather
in September.
A warm, moderately wet month: the 4th warmest and the 7th driest of the year.
Long-term climate patterns, not a forecast for a particular trip.
September is a warm month in Boulder: daytime highs average 23.7°C and nights cool to about 9.5°C. By daytime temperature it is the 4th warmest of the 12 months.
Expect about 54 mm of rain on roughly 10 days, which makes September a moderately wet month (the 7th driest of the 12 months). Daylight, from sunrise to sunset, lasts about 12 h 30 min on an average September day.
The nearest other city in this atlas is Longmont (22 km away). In September its highs average 24.2°C, about as warm as Boulder, with 39 mm of rain.
Among the 498 cities listed in the United States, Boulder ranks joint 374th of 498 for the warmest September highs and 176th of 498 for the driest September (rainfall, lowest first). The mean September high of those 498 cities is 26.9°C; Boulder is 3.2°C cooler than that mean.
From September to October, average highs fall by 7.2°C to 16.5°C, and monthly rainfall falls from 54 mm to 48 mm.
The annual picture
September in context
Monthly rainfall
Average total · mmAverages from hourly ERA5 reanalysis data. Actual weather varies from year to year. How these numbers are calculated
Every month, side by side
Boulder climate table
| Month | Average highHigh / low | Average low | Rainfall mm/month | Rain days ≥1 mm/day | Daylight hours/dayDaylight h/day |
|---|---|---|---|---|---|
| January | 4.2°C-8.7°C | -8.7°C | 24 | 5.7 | 9.710 h |
| February | 5.4°C-8.3°C | -8.3°C | 26 | 6.7 | 10.711 h |
| March | 10.0°C-4.5°C | -4.5°C | 50 | 8.8 | 11.912 h |
| April | 13.7°C-0.4°C | -0.4°C | 78 | 11.1 | 13.213 h |
| May | 19.2°C5.1°C | 5.1°C | 103 | 14.9 | 14.414 h |
| June | 25.6°C10.8°C | 10.8°C | 77 | 14.2 | 15.015 h |
| July | 28.7°C14.4°C | 14.4°C | 75 | 14.8 | 14.715 h |
| August | 27.5°C13.7°C | 13.7°C | 71 | 14.8 | 13.814 h |
| September, selected | 23.7°C9.5°C | 9.5°C | 54 | 9.6 | 12.512 h |
| October | 16.5°C3.1°C | 3.1°C | 48 | 7.4 | 11.211 h |
| November | 9.1°C-3.9°C | -3.9°C | 32 | 6.4 | 10.010 h |
| December | 4.0°C-8.8°C | -8.8°C | 23 | 5.3 | 9.49 h |
Rainfall is the average of the 30 separate monthly totals. A rain day is a local calendar day with at least 1 mm of precipitation. Daylight is the calculated sunrise-to-sunset length, not measured sunshine.
Compare places
September in nearby cities
- Longmont in SeptemberUnited States · 22 kmHigh 24.2°C · Low 9.9°C · 39 mm rain
- Westminster (Colorado), United States in SeptemberUnited States · 28 kmHigh 24.1°C · Low 9.4°C · 36 mm rain
- Arvada in SeptemberUnited States · 28 kmHigh 24.1°C · Low 9.4°C · 36 mm rain
- Thornton in SeptemberUnited States · 30 kmHigh 24.0°C · Low 9.4°C · 36 mm rain
- Lakewood in SeptemberUnited States · 38 kmHigh 24.1°C · Low 9.4°C · 36 mm rain
- Denver in SeptemberUnited States · 39 kmHigh 24.3°C · Low 9.5°C · 36 mm rain
- Loveland in SeptemberUnited States · 46 kmHigh 24.0°C · Low 9.6°C · 39 mm rain
- Aurora (Colorado), United States in SeptemberUnited States · 49 kmHigh 25.1°C · Low 10.8°C · 27 mm rain
Read the numbers correctly
Climate, not a promise.
These patterns help compare months. They cannot tell you the weather on a specific day of your trip.
Sources, method and limitsHow these numbers are made (what the highs, lows and rain days mean, the sources and the model cell used for Boulder) is explained once on the Boulder year overview and on the Sources, method and limits page.