Climate averages 1991–2020
O'Fallon weather
in May.
A warm, wet month: the 5th warmest and the wettest of the year.
Long-term climate patterns, not a forecast for a particular trip.
May is a warm month in O'Fallon: daytime highs average 23.8°C and nights cool to about 14.3°C. By daytime temperature it is the 5th warmest of the 12 months.
Expect about 123 mm of rain on roughly 13 days, which makes May a wet month (the wettest month of the year). Daylight, from sunrise to sunset, lasts about 14 h 16 min on an average May day.
The nearest other city in this atlas is St. Louis (48 km away). In May its highs average 24.3°C, 0.5°C warmer than O'Fallon, with 128 mm of rain.
Among the 498 cities listed in the United States, O'Fallon ranks joint 239th of 498 for the warmest May highs and joint 452nd of 498 for the driest May (rainfall, lowest first). The mean May high of those 498 cities is 23.8°C; O'Fallon is level with that mean.
From May to June, average highs rise by 4.9°C to 28.6°C, and monthly rainfall falls from 123 mm to 101 mm.
The annual picture
May 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, selected | 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 | 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
May in nearby cities
- St. Louis in MayUnited States · 48 kmHigh 24.3°C · Low 14.3°C · 128 mm rain
- Columbia (Missouri), United States in MayUnited States · 142 kmHigh 23.7°C · Low 14.0°C · 134 mm rain
- Springfield (Illinois), United States in MayUnited States · 143 kmHigh 22.7°C · Low 13.0°C · 117 mm rain
- Decatur in MayUnited States · 189 kmHigh 22.5°C · Low 12.8°C · 122 mm rain
- Peoria (Illinois), United States in MayUnited States · 230 kmHigh 22.1°C · Low 12.5°C · 117 mm rain
- Bloomington (Illinois), United States in MayUnited States · 237 kmHigh 21.9°C · Low 11.9°C · 119 mm rain
- Champaign in MayUnited States · 256 kmHigh 22.2°C · Low 12.2°C · 118 mm rain
- Springfield (Missouri), United States in MayUnited States · 289 kmHigh 23.6°C · Low 14.0°C · 137 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.