Climate averages 1991–2020
O'Fallon weather
in August.
A hot, moderately wet month: the 2nd warmest and the 4th driest of the year.
Long-term climate patterns, not a forecast for a particular trip.
August is a hot month in O'Fallon: daytime highs average 30.1°C and nights cool to about 20.9°C. By daytime temperature it is the 2nd warmest of the 12 months.
Expect about 76 mm of rain on roughly 9 days, which makes August a moderately wet month (the 4th driest of the 12 months). Daylight, from sunrise to sunset, lasts about 13 h 41 min on an average August day.
The nearest other city in this atlas is St. Louis (48 km away). In August its highs average 30.4°C, about as warm as O'Fallon, with 74 mm of rain.
Among the 498 cities listed in the United States, O'Fallon ranks joint 232nd of 498 for the warmest August highs and joint 246th of 498 for the driest August (rainfall, lowest first). The mean August high of those 498 cities is 29.9°C; O'Fallon is 0.2°C warmer than that mean.
From August to September, average highs fall by 4.1°C to 26.0°C, and monthly rainfall rises from 76 mm to 81 mm.
The annual picture
August 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
O'Fallon climate table
| Month | Average highHigh / low | Average low | Rainfall mm/month | Rain days ≥1 mm/day | Daylight hours/dayDaylight h/day |
|---|---|---|---|---|---|
| January | 4.0°C-4.4°C | -4.4°C | 65 | 7.7 | 9.810 h |
| February | 6.8°C-2.6°C | -2.6°C | 61 | 7.7 | 10.711 h |
| March | 12.6°C2.6°C | 2.6°C | 92 | 10.6 | 11.912 h |
| April | 18.7°C8.5°C | 8.5°C | 114 | 11.3 | 13.213 h |
| May | 23.8°C14.3°C | 14.3°C | 123 | 13.3 | 14.314 h |
| June | 28.6°C19.3°C | 19.3°C | 101 | 11.0 | 14.815 h |
| July | 30.8°C21.6°C | 21.6°C | 82 | 9.9 | 14.615 h |
| August, selected | 30.1°C20.9°C | 20.9°C | 76 | 9.1 | 13.714 h |
| September | 26.0°C16.7°C | 16.7°C | 81 | 7.6 | 12.512 h |
| October | 19.5°C10.4°C | 10.4°C | 84 | 8.4 | 11.211 h |
| November | 12.0°C3.7°C | 3.7°C | 89 | 8.3 | 10.110 h |
| December | 6.1°C-1.7°C | -1.7°C | 62 | 7.8 | 9.510 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
August in nearby cities
- St. Louis in AugustUnited States · 48 kmHigh 30.4°C · Low 20.9°C · 74 mm rain
- Columbia (Missouri), United States in AugustUnited States · 142 kmHigh 30.4°C · Low 20.8°C · 76 mm rain
- Springfield (Illinois), United States in AugustUnited States · 143 kmHigh 28.6°C · Low 19.5°C · 78 mm rain
- Decatur in AugustUnited States · 189 kmHigh 28.3°C · Low 19.3°C · 77 mm rain
- Peoria (Illinois), United States in AugustUnited States · 230 kmHigh 28.0°C · Low 19.4°C · 79 mm rain
- Bloomington (Illinois), United States in AugustUnited States · 237 kmHigh 27.6°C · Low 18.7°C · 82 mm rain
- Champaign in AugustUnited States · 256 kmHigh 27.9°C · Low 18.7°C · 74 mm rain
- Springfield (Missouri), United States in AugustUnited States · 289 kmHigh 30.6°C · Low 20.6°C · 83 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 O'Fallon) is explained once on the O'Fallon year overview and on the Sources, method and limits page.