O'Fallon (IL) Weather & Climate: Best Time to Visit
Average temperatures, rainfall, sunshine and the best time to visit O'Fallon.
Climate overview of O'Fallon
The climate in O'Fallon, Illinois, United States of America, is marked by large temperature swings across the seasons, ranging from 31°C (88°F) in July to 5°C (41°F) in January.
The city has a wet climate, receiving around 1171 mm (46 in) of rain/snowfall per year. Despite the high annual total, it has very warm summers and cold winters. June is the wettest month. Sunshine peaks in July, when the city sees around 10.4 hours of sunshine per day.
O'Fallon Weather by Month
Pick a month to see the average temperature, rainfall and sunshine in O'Fallon - and find the best time to visit.
Monthly Temperature in O'Fallon
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
Rainfall in O'Fallon
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
Sunshine Hours in O'Fallon
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
Humidity in O'Fallon by Month
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
Best Time to Visit O'Fallon
Each month is rated for comfortable sightseeing weather — mild temperatures and low rainfall. Tap a month for its full averages; your ideal time may differ for a beach or winter trip (see below).
- Best overall: October
- Warmest weather: June, July and August
- Most sunshine: July
- Fewer rainy days: January, February, August, October and November
- Climate seasons: very warm summers and cold winters
- Best for winter weather: January, with freezing nights around -5°C (23°F).
Frequently asked questions about the climate in O'Fallon
What is the best time to visit O'Fallon?
October typically offers the most optimal weather in O'Fallon. In contrast, January, February and December tend to have less optimal conditions. O'Fallon has very warm summers and cold winters.
What temperatures can I expect in O'Fallon?
Daytime highs range from 5°C (41°F) in January to 31°C (88°F) in July. Nighttime lows range from -5°C (23°F) to 20°C (68°F). Temperatures vary considerably through the year.
How much rain does O'Fallon get?
Annual rainfall is around 1171 mm (46 in). June is the wettest month with 142 mm (5.6 in), while January is the driest with 56 mm (2.2 in).
How sunny is O'Fallon?
O'Fallon receives around 2,594 hours of sunshine per year. July is the sunniest month with 311 hours, while December is the cloudiest with just 128 hours. Overall, O'Fallon enjoys abundant sunshine.
O'Fallon 3-Day Forecast
O'Fallon (IL) 3-Day Forecast
O'Fallon on the map
O'Fallon is highlighted - select any city to see its climate.
Popular this month
Where to go in July - average daytime high across popular destinations.
Bucket list
Need travel inspiration? Check out our bucket list articles and be inspired.
Yosemite National Park is definitely worth a visit
Vast forests, blue lakes, towering sequoias, and majestic waterfalls hint at the beauty of Yosemite National Park—a true natural masterpiece.
The 10 most alternative cities in the United States
Discover the top 10 fun and alternative U.S. cities rich in art, culture, and adventure. From Portland's hippy charm to Sedona's stunning landscapes.
Sedona: Explore Majestic Red Rock Landscapes
A picturesque artists' town surrounded by red rocks and known for its spirituality, vortex energy, and stunning hikes like Cathedral Rock.
10 most idyllic villages to visit in the United States
Discover 10 idyllic villages in the USA, from California to Vermont. Explore their unique charm, rich history, stunning nature, and vibrant arts scenes.