Decatur, Illinois Climate by Month
Decatur, Illinois, United States
Hot summers and cold winters, with rain spread through the year.
Decatur 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 37°F (3°C), and nights dip to 20°F (-7°C). It gets 41 in (1034 mm) of rain a year, spread across the months.
- Warmest month
- 88°F(31°C)
- Average high, July
- Coldest nights
- 20°F(-7°C)
- Average low, January
- Rain per year
- 41 in(1034 mm)
- No clear rainy season
- Humidity
- 70%
- Sunniest: July
Best and worst months in Decatur
Best for weatherThe most comfortable weather comes in May and September, with highs around 78°F (26°C) and about 4.1 in (104 mm) of rain a month.
Least pleasantJanuary is the least pleasant month: it's the coldest, with highs of only 37°F (3°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)
Decatur weather by month
| Month | High | Low | Rain | Humidity | Sun |
|---|---|---|---|---|---|
| JanJanuary | 37°F(3°C) | 20°F(-7°C) | 2.2 in(55 mm) | 76% | 1.6 |
| FebFebruary | 41°F(5°C) | 23°F(-5°C) | 2.0 in(51 mm) | 82% | 2.6 |
| MarMarch | 53°F(12°C) | 33°F(1°C) | 3.0 in(75 mm) | 64% | 3.7 |
| AprApril | 65°F(19°C) | 43°F(6°C) | 4.1 in(105 mm) | 63% | 4.6 |
| MayMayBest | 76°F(25°C) | 53°F(12°C) | 4.9 in(125 mm) | 67% | 5.3 |
| JunJune | 85°F(29°C) | 62°F(17°C) | 4.6 in(118 mm) | 67% | 5.9 |
| JulJuly | 88°F(31°C) | 66°F(19°C) | 4.3 in(110 mm) | 70% | 6.0 |
| AugAugust | 87°F(30°C) | 64°F(18°C) | 3.4 in(86 mm) | 70% | 5.3 |
| SepSeptemberBest | 81°F(27°C) | 56°F(13°C) | 3.3 in(83 mm) | 64% | 4.6 |
| OctOctober | 68°F(20°C) | 45°F(7°C) | 3.0 in(77 mm) | 64% | 3.1 |
| NovNovember | 53°F(12°C) | 34°F(1°C) | 3.1 in(80 mm) | 73% | 1.9 |
| DecDecember | 41°F(5°C) | 25°F(-4°C) | 2.7 in(69 mm) | 78% | 1.5 |
1991–2020 averages. Sun is solar radiation in kWh/m² per day. Highlighted: the most comfortable months.
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