Climate almanac
O'Fallon
Illinois, United States · 38.59°, -89.91°
What is the climate like in O'Fallon?
The best time to visit O'Fallon is typically August – October, with trip comfort peaking in September (7.0/10). O'Fallon is warmest in July, when daily highs average 89°F (32°C), and coolest in January, when lows fall to about 27°F (-3°C). The wettest month is typically April and the driest is December; the year sees roughly 47 in (1194 mm) of precipitation.
- Best time to visitAugust – October — comfort peaks in September (7.0/10)
- Warmest monthJuly — average high 89°F (32°C)
- Coolest monthJanuary — average low 27°F (-3°C)
- Wettest monthApril (5.7 in (145 mm))
- Driest monthDecember (2.8 in (70 mm))
- Annual precipitation47 in (1194 mm)
- Based on2016-2025 daily ERA5 reanalysis data (10-year normals)
- Records fromBelleville Siu Rsch, 10 km away (1948-2026)