Climate averages 1991–2020
Longmont weather
in September.
A warm, fairly dry month: the 4th warmest and the joint 5th driest of the year. Meets the pleasant-weather rule
Long-term climate patterns, not a forecast for a particular trip.
September is a warm month in Longmont: daytime highs average 24.2°C and nights cool to about 9.9°C. By daytime temperature it is the 4th warmest of the 12 months.
Expect about 39 mm of rain on roughly 6 days, which makes September a fairly dry month (the joint 5th 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 Boulder (22 km away). In September its highs average 23.7°C, about as warm as Longmont, with 54 mm of rain.
Among the 498 cities listed in the United States, Longmont ranks joint 342nd of 498 for the warmest September highs and joint 158th of 498 for the driest September (rainfall, lowest first). The mean September high of those 498 cities is 26.9°C; Longmont is 2.7°C cooler than that mean.
From September to October, average highs fall by 7.4°C to 16.8°C, and monthly rainfall rises from 39 mm to 41 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
Longmont climate table
| Month | Average highHigh / low | Average low | Rainfall mm/month | Rain days ≥1 mm/day | Daylight hours/dayDaylight h/day |
|---|---|---|---|---|---|
| January | 4.5°C-8.4°C | -8.4°C | 19 | 4.7 | 9.710 h |
| February | 5.9°C-7.6°C | -7.6°C | 20 | 5.3 | 10.611 h |
| March | 11.0°C-3.9°C | -3.9°C | 42 | 7.3 | 11.912 h |
| April | 14.8°C0.4°C | 0.4°C | 67 | 9.5 | 13.213 h |
| May | 19.9°C5.9°C | 5.9°C | 87 | 11.7 | 14.414 h |
| June | 26.0°C11.4°C | 11.4°C | 53 | 8.4 | 15.015 h |
| July | 29.5°C15.0°C | 15.0°C | 42 | 7.3 | 14.715 h |
| August | 28.3°C14.3°C | 14.3°C | 39 | 6.7 | 13.814 h |
| September, selected | 24.2°C9.9°C | 9.9°C | 39 | 5.9 | 12.512 h |
| October | 16.8°C3.2°C | 3.2°C | 41 | 6.5 | 11.211 h |
| November | 9.4°C-3.6°C | -3.6°C | 28 | 5.8 | 10.010 h |
| December | 4.2°C-8.1°C | -8.1°C | 18 | 4.5 | 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
- Boulder in SeptemberUnited States · 22 kmHigh 23.7°C · Low 9.5°C · 54 mm rain
- Loveland in SeptemberUnited States · 26 kmHigh 24.0°C · Low 9.6°C · 39 mm rain
- Thornton in SeptemberUnited States · 35 kmHigh 24.0°C · Low 9.4°C · 36 mm rain
- Westminster (Colorado), United States in SeptemberUnited States · 37 kmHigh 24.1°C · Low 9.4°C · 36 mm rain
- Arvada in SeptemberUnited States · 41 kmHigh 24.1°C · Low 9.4°C · 36 mm rain
- Greeley in SeptemberUnited States · 44 kmHigh 26.0°C · Low 11.3°C · 27 mm rain
- Fort Collins in SeptemberUnited States · 47 kmHigh 23.8°C · Low 9.6°C · 39 mm rain
- Denver in SeptemberUnited States · 49 kmHigh 24.3°C · Low 9.5°C · 36 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 Longmont) is explained once on the Longmont year overview and on the Sources, method and limits page.