Climate averages 1991–2020
Rapid City weather
in September.
A warm, moderately wet month: the 4th warmest and the 6th 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 Rapid City: daytime highs average 23.3°C and nights cool to about 11.2°C. By daytime temperature it is the 4th warmest of the 12 months.
Expect about 42 mm of rain on roughly 7 days, which makes September a moderately wet month (the 6th driest of the 12 months). Daylight, from sunrise to sunset, lasts about 12 h 34 min on an average September day.
The nearest other city in this atlas is Bismarck (358 km away). In September its highs average 21.4°C, 1.8°C cooler than Rapid City, with 46 mm of rain.
Among the 498 cities listed in the United States, Rapid City ranks joint 389th of 498 for the warmest September highs and joint 166th of 498 for the driest September (rainfall, lowest first). The mean September high of those 498 cities is 26.9°C; Rapid City is 3.6°C cooler than that mean.
From September to October, average highs fall by 8.3°C to 14.9°C, and monthly rainfall rises from 42 mm to 50 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
Rapid City climate table
| Month | Average highHigh / low | Average low | Rainfall mm/month | Rain days ≥1 mm/day | Daylight hours/dayDaylight h/day |
|---|---|---|---|---|---|
| January | 2.5°C-8.1°C | -8.1°C | 13 | 3.9 | 9.39 h |
| February | 3.5°C-7.6°C | -7.6°C | 20 | 5.1 | 10.410 h |
| March | 8.5°C-3.5°C | -3.5°C | 40 | 7.6 | 11.912 h |
| April | 12.8°C1.1°C | 1.1°C | 80 | 11.8 | 13.413 h |
| May | 18.4°C7.2°C | 7.2°C | 112 | 14.1 | 14.715 h |
| June | 24.4°C12.9°C | 12.9°C | 97 | 14.5 | 15.415 h |
| July | 28.9°C16.9°C | 16.9°C | 78 | 13.4 | 15.115 h |
| August | 28.2°C16.1°C | 16.1°C | 60 | 10.8 | 14.014 h |
| September, selected | 23.3°C11.2°C | 11.2°C | 42 | 6.7 | 12.613 h |
| October | 14.9°C3.9°C | 3.9°C | 50 | 7.3 | 11.011 h |
| November | 7.9°C-2.5°C | -2.5°C | 20 | 4.4 | 9.710 h |
| December | 2.8°C-6.9°C | -6.9°C | 15 | 3.6 | 9.09 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
- Bismarck in SeptemberUnited States · 358 kmHigh 21.4°C · Low 10.9°C · 46 mm rain
- Fort Collins in SeptemberUnited States · 417 kmHigh 23.8°C · Low 9.6°C · 39 mm rain
- Greeley in SeptemberUnited States · 424 kmHigh 26.0°C · Low 11.3°C · 27 mm rain
- Loveland in SeptemberUnited States · 437 kmHigh 24.0°C · Low 9.6°C · 39 mm rain
- Billings in SeptemberUnited States · 456 kmHigh 23.7°C · Low 10.4°C · 35 mm rain
- Longmont in SeptemberUnited States · 462 kmHigh 24.2°C · Low 9.9°C · 39 mm rain
- Boulder in SeptemberUnited States · 482 kmHigh 23.7°C · Low 9.5°C · 54 mm rain
- Thornton in SeptemberUnited States · 490 kmHigh 24.0°C · Low 9.4°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 Rapid City) is explained once on the Rapid City year overview and on the Sources, method and limits page.