Illinois has an average annual temperature of
49.8°F
(across all 38 cities) with approximately
949 mm of annual precipitation
and average humidity of 78%.
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.
This page covers 45 years of NASA POWER climate data (1981–2025)
including temperature, rainfall, humidity, wind speed, and solar radiation.
Aug is typically the warmest month across Illinois with average highs around
91.4°F, while
Jan is the coldest with average lows near
-6.7°F.
May is the wettest month with approximately
109 mm of rainfall on average.
Month
Max Temp
Min Temp
Avg Temp
Rain (mm)
Humidity
Wind (m/s)
Solar (kWh/m²)
Jan
46.9°F
-6.7°F
20.1°F
47
88%
4.3
1.70
Feb
53.4°F
-0.8°F
26.4°F
48
85%
4.2
2.60
Mar
66.7°F
13.3°F
40.1°F
65
79%
4.1
3.60
Apr
75°F
25.2°F
50°F
93
77%
4.0
4.60
May
81.7°F
35.8°F
58.8°F
109
77%
3.3
5.50
Jun
87.8°F
46°F
66.9°F
105
76%
2.8
6.10
Jul
90.7°F
55°F
72.9°F
102
75%
2.5
6.00
Aug
91.4°F
54.3°F
72.9°F
99
73%
2.5
5.30
Sep
87.8°F
42.6°F
65.3°F
81
72%
2.9
4.30
Oct
78.8°F
29.7°F
54.3°F
84
74%
3.5
3.00
Nov
65.8°F
18.1°F
42.1°F
72
80%
3.9
1.90
Dec
52.3°F
1.9°F
27.1°F
59
86%
4.0
1.40
📈 Illinois Yearly Climate Trends
🌡️ Annual Temperature Trend
State-wide avg annual high & low · Last 10 years
📈 Illinois Long-Term Climate Trends (1981–2025)
This chart shows how Illinois's average temperatures have shifted across 45 years.
Use the range buttons to focus on recent decades or view the full historical record.
Data is averaged across all 38 cities in the state.
🌧️ Annual Precipitation Trend
State-wide avg annual precipitation · Last 10 years · mm/year
🔗 Explore Cities in Illinois
Browse detailed weather history, monthly averages, and 45-year climate trends for every city in Illinois.
Belleville is the wettest city in Illinois with approximately 1070 mm of annual precipitation on average. View Belleville weather history →
What is the warmest month in Illinois?▼
Aug is the warmest month across Illinois, with state-wide average highs of 91.4°F and average lows of 54.3°F.
How many cities in Illinois have weather data?▼
TownPillar has historical weather data for 38 cities in Illinois, covering 45 years of NASA POWER climate records from 1981 to 2025.
Which month has the most sunshine in Illinois?▼
Jun is the sunniest month across Illinois with an average solar radiation of 6.10 kWh/m²/day state-wide. This solar radiation data from NASA POWER is useful for solar energy planning, agriculture, and understanding seasonal sunshine availability.
Where does this climate data come from?▼
All data is sourced from NASA POWER (Prediction Of Worldwide Energy Resources), based on MERRA-2 satellite reanalysis data. It includes monthly temperature, precipitation, humidity, wind speed, and solar radiation at 0.5° × 0.625° spatial resolution.
📚 Data Source
All weather data is sourced from NASA POWER (Prediction of Worldwide Energy Resources). Data covers 45 years (1981–2025) across 38 cities in Illinois at monthly temporal resolution.