Illinois has a temperate climate with warm summers and freezing winters.
Illinois has an average annual temperature of 49.8°F across all 38 cities, with approximately 964 mm of annual precipitation and average humidity of 78%. Temperatures peak in August and drop to their lowest in January, while rainfall is highest during spring and early summer months.
The warmest city is Belleville with average July highs of 96.4°F, while Rockford is the coldest with average January lows of -14.4°F. Humidity remains high throughout the year, especially during winter months, creating cold and damp conditions. These seasonal patterns make summer ideal for outdoor activities, while winters bring freezing conditions and heavy snowfall.
| Month | 🌤️ | High | Low | Avg | Humid % | Rain (mm) | Wind (m/s) | Solar (kWh/m²) |
|---|---|---|---|---|---|---|---|---|
| January | ❄️ | 46.9°F | -6.7°F | 20.1°F | 88% | 47 | 4.3 | 1.70 |
| February | ❄️ | 53.4°F | -0.8°F | 26.4°F | 85% | 46 | 4.2 | 2.60 |
| March | 🌤️ | 66.7°F | 13.3°F | 39.9°F | 79% | 66 | 4.1 | 3.60 |
| April | ⛅ | 75.0°F | 25.2°F | 50.2°F | 77% | 93 | 4.0 | 4.60 |
| May | ⛅ | 81.7°F | 35.8°F | 58.6°F | 77% | 107 | 3.3 | 5.50 |
| June | ⛅ | 87.8°F | 46.0°F | 66.9°F | 76% | 106 | 2.8 | 6.10 |
| July | ⛅ | 90.7°F | 55.0°F | 72.9°F | 75% | 104 | 2.5 | 6.00 |
| August | ⛅ | 91.4°F | 54.3°F | 72.9°F | 73% | 101 | 2.5 | 5.30 |
| September | ⛅ | 87.8°F | 42.6°F | 65.1°F | 72% | 82 | 2.9 | 4.30 |
| October | ⛅ | 78.8°F | 29.7°F | 54.3°F | 74% | 84 | 3.5 | 3.00 |
| November | 🌤️ | 65.8°F | 18.1°F | 41.9°F | 80% | 71 | 3.9 | 1.90 |
| December | ❄️ | 52.3°F | 1.9°F | 27.1°F | 86% | 58 | 4.0 | 1.40 |
Showing average climate conditions across 45 years (1981–2025) · averaged across all 38 cities in Illinois
🧥 Pack heavy layers and prepare for snow — this is a Severe Winter destination.
🌤️ Early mornings and evenings are most comfortable during this Hot Summer season.
☔ Moderate Rainfall expected — carry rain gear and plan outdoor activities around these months.
Region-wide monthly averages across 38 cities · 1981–2025 · °F
Temperatures rise steadily from spring, peak in August, and gradually decline through autumn into winter.
These seasonal patterns make summer ideal for outdoor activities, while winters bring freezing conditions and heavy snowfall.
Region-wide monthly rainfall · 1981–2025 · mm/month
💧 Humidity peaks at 88% in January · annual avg 78%
Monthly averages · 1981–2025
Across 45 years, Illinois's temperature shows a slight warming trend over the past decade. 2012 was the warmest year overall and 1982 the coolest.
The warmest year on record was 2012 (avg: 55.2°F), while the coldest year was 1982 (avg: 45.3°F).
Averaged across all 38 cities in Illinois · Last 30 years · °F
Averaged across all 38 cities in Illinois · Last 30 years · mm/year
🔥 hottest · ❄️ coldest · 🌧️ wettest
ℹ️ Values are averages across all 38 cities in Illinois for each year.
| Year | Min Temp | Avg Temp | Max Temp | Rainfall (mm) | Avg Humidity | Avg Wind |
|---|---|---|---|---|---|---|
2025 | 26.6°F | 51.6°F | 76.5°F | 815 | 74% | 3.5 m/s |
2024 | 29.7°F | 52.5°F | 75.4°F | 979 | 75% | 3.4 m/s |
2023 | 28.6°F | 52.7°F | 76.8°F | 881 | 75% | 3.3 m/s |
2022 | 25.7°F | 50.0°F | 74.1°F | 892 | 76% | 3.7 m/s |
2021 | 29.5°F | 52.2°F | 74.7°F | 844 | 77% | 3.4 m/s |
2020 | 29.1°F | 51.1°F | 73.0°F | 990 | 79% | 3.4 m/s |
2019 | 23.9°F | 47.7°F | 71.4°F | 1206 | 83% | 3.4 m/s |
2018 | 27.7°F | 50.4°F | 73.0°F | 1149 | 80% | 3.3 m/s |
2017 | 28.2°F | 51.8°F | 75.4°F | 1045 | 77% | 3.5 m/s |
2016 | 28.0°F | 51.3°F | 74.3°F | 941 | 81% | 3.4 m/s |
Showing 1 to 10 of 45 years
All weather data is sourced from NASA POWER (Prediction of Worldwide Energy Resources), a NASA project providing solar and meteorological data for renewable energy, building efficiency, and agricultural research.
