O'Fallon (IL) Weather & Climate: Best Time to Visit

In O'Fallon, temperatures differ significantly between summer and winter months. On average, daytime temperatures range from a very warm 31°C (88°F) in July to a chilly 5°C (41°F) in January.

Nighttime temperatures range from 20°C (68°F) in July to -5°C (23°F) in January.

Check out our detailed temperature page for more information.

Temperature ranges by month

O'Fallon experiences significant rain/snowfall throughout the year, averaging 1171 mm (46 in) of precipitation annually. The seasons in O'Fallon bring significant changes in precipitation. The wettest month, June, receives high rainfall, with an average of 142 mm (5.6 in) of precipitation. This rainfall is distributed across 12 rainy days.

In contrast, the driest month, January, experiences much less snow/rainfall, totaling 56 mm (2.2 in) over 7 snowy/rainy days. These distinct seasonal differences provide diverse experiences throughout the year. For more details, please visit our O'Fallon Precipitation page.

The mean monthly precipitation over the year, including rain, hail, and snow

The average number of days each month with precipitation

In O'Fallon, summer days are longer and more sunny, with daily sunshine hours peaking at 10.4 hours in July. As the darker season arrives, the brightness of the sun becomes less. December sees a soft sun for only 4.3 hours per average day.

Visit our detailed sunshine hours page for more information.

Monthly hours of sunshine

Daily hours of sunshine

Daily percentage of sunshine

The relative humidity is high throughout the year in O'Fallon.

The city experiences its highest humidity in December, reaching 74%. In April, the humidity drops to its lowest level at 63%. What does this mean? Read our detailed page on humidity levels for further details.

Relative humidity over the year