| Month | High | Low | Rain | Wet days | Score |
|---|---|---|---|---|---|
| January | 4.2° | -6° | 63.4mm | 8 | 21 |
| February | 6° | -5° | 53.9mm | 8.5 | 28 |
| March | 13.5° | 0.5° | 92.2mm | 10.9 | 55 |
| April | 20.4° | 7.1° | 119mm | 10.8 | 78 |
| May | 24.6° | 12.2° | 127.9mm | 12.3 | 75 |
| June | 29.4° | 17.6° | 122.5mm | 10.4 | 70 |
| July | 31.1° | 19.4° | 124.1mm | 9.2 | 65 |
| August | 30.5° | 18.3° | 92.3mm | 8.6 | 69 |
| September | 26.8° | 13.8° | 79.5mm | 7.6 | 85 |
| October | 20.4° | 7.3° | 69mm | 7.9 | 84 |
| November | 13.4° | 1.9° | 86.5mm | 7.7 | 63 |
| December | 6.4° | -2.9° | 79.2mm | 9.1 | 31 |
January in O'Fallon averages a daily high of 4.2°C with nights around -6°C. Expect roughly 8 wet days totalling 63.4mm. A hard month to visit for comfort alone.
February in O'Fallon averages a daily high of 6°C with nights around -5°C. Expect roughly 8.5 wet days totalling 53.9mm. A hard month to visit for comfort alone.
March in O'Fallon averages a daily high of 13.5°C with nights around 0.5°C. Expect roughly 10.9 wet days totalling 92.2mm. Generally workable, with some weather to plan around.
April in O'Fallon averages a daily high of 20.4°C with nights around 7.1°C. Expect roughly 10.8 wet days totalling 119mm. Comfortable conditions for being outdoors all day.
May in O'Fallon averages a daily high of 24.6°C with nights around 12.2°C. Expect roughly 12.3 wet days totalling 127.9mm. Comfortable conditions for being outdoors all day.
June in O'Fallon averages a daily high of 29.4°C with nights around 17.6°C. Expect roughly 10.4 wet days totalling 122.5mm. Generally workable, with some weather to plan around.
July in O'Fallon averages a daily high of 31.1°C with nights around 19.4°C. Expect roughly 9.2 wet days totalling 124.1mm. Generally workable, with some weather to plan around.
August in O'Fallon averages a daily high of 30.5°C with nights around 18.3°C. Expect roughly 8.6 wet days totalling 92.3mm. Generally workable, with some weather to plan around.
September in O'Fallon averages a daily high of 26.8°C with nights around 13.8°C. Expect roughly 7.6 wet days totalling 79.5mm. Comfortable conditions for being outdoors all day.
October in O'Fallon averages a daily high of 20.4°C with nights around 7.3°C. Expect roughly 7.9 wet days totalling 69mm. Comfortable conditions for being outdoors all day.
November in O'Fallon averages a daily high of 13.4°C with nights around 1.9°C. Expect roughly 7.7 wet days totalling 86.5mm. Generally workable, with some weather to plan around.
December in O'Fallon averages a daily high of 6.4°C with nights around -2.9°C. Expect roughly 9.1 wet days totalling 79.2mm. A hard month to visit for comfort alone.
Each month scores out of 100. It starts at 100 and loses points for daytime highs outside a 20–27°C band, for wet days, and for nights below 2°C. Nothing else is weighted in — no crowd data, no prices, no opinion. O'Fallon peaks at 85.
Based on 30 years of records, September–October scores highest for comfort in O'Fallon, balancing daytime temperature against rainfall.
July is the warmest, averaging a daily high of 31.1°C.
May is the wettest, averaging 127.9mm of rain across roughly 12.3 wet days.
January is the coolest month, with average daily highs around 4.2°C.