O'Fallon, Illinois Climate by Month
O'Fallon, Illinois, United States
Hot summers and cold winters, with rain spread through the year.
O'Fallon has hot summers and cold winters, with rain spread through the year. July is the warmest month, with highs around 88°F (31°C); January is the coolest at 39°F (4°C), and nights dip to 23°F (-5°C). About 40 in (1027 mm) of rain falls in a typical year, with no clear rainy season.
- Warmest month
- 88°F(31°C)
- Average high, July
- Coldest nights
- 23°F(-5°C)
- Average low, January
- Rain per year
- 40 in(1027 mm)
- No clear rainy season
- Humidity
- 70%
- Sunniest: July
Best and worst months in O'Fallon
Best for weatherMay, September and October are the best time for weather in O'Fallon: highs near 76°F (24°C) and about 3.8 in (97 mm) of rain a month.
Least pleasantJanuary is the least pleasant month: it's the coldest, with highs of only 39°F (4°C).
Comfort combines how close the afternoon high is to 75°F (24°C) with how much rain falls and how humid it is. See where the weather is best in May.
Temperature and rain through the year
Highs and lows (°F)
Average highAverage low
Rain (in per month)
O'Fallon weather by month
| Month | High | Low | Rain | Humidity | Sun |
|---|---|---|---|---|---|
| JanJanuary | 39°F(4°C) | 23°F(-5°C) | 2.4 in(60 mm) | 85% | 1.8 |
| FebFebruary | 45°F(7°C) | 26°F(-3°C) | 2.4 in(61 mm) | 72% | 2.7 |
| MarMarch | 56°F(13°C) | 35°F(2°C) | 3.3 in(84 mm) | 68% | 3.8 |
| AprApril | 67°F(20°C) | 45°F(7°C) | 4.0 in(101 mm) | 65% | 4.8 |
| MayMayBest | 77°F(25°C) | 56°F(13°C) | 4.9 in(124 mm) | 68% | 5.4 |
| JunJune | 85°F(30°C) | 65°F(18°C) | 4.2 in(106 mm) | 68% | 6.1 |
| JulJuly | 88°F(31°C) | 68°F(20°C) | 3.8 in(96 mm) | 70% | 6.2 |
| AugAugust | 87°F(31°C) | 66°F(19°C) | 3.3 in(83 mm) | 70% | 5.6 |
| SepSeptemberBest | 81°F(27°C) | 58°F(14°C) | 3.7 in(94 mm) | 65% | 4.7 |
| OctOctoberBest | 69°F(21°C) | 46°F(8°C) | 2.8 in(72 mm) | 67% | 3.3 |
| NovNovember | 55°F(13°C) | 36°F(2°C) | 3.0 in(77 mm) | 68% | 2.1 |
| DecDecember | 44°F(7°C) | 27°F(-3°C) | 2.7 in(69 mm) | 71% | 1.6 |
1991–2020 averages. Sun is solar radiation in kWh/m² per day. Highlighted: the most comfortable months.
Places with a similar climate
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