St. Clair County Climate
St. Clair County, Illinois
Avg Annual Temp
56.8°F
Annual Precip
43.0"
Annual Snowfall
12.2"
Extreme Heat Days
46.1
days >= 90°F/yr
Monthly Temperature
Precipitation Calendar
Seasonal Overview
National Comparison
Climate Profile
About St. Clair County Weather
One of the Nation's Warmer Zones
St. Clair County averages a warm 56.8°F annually, far exceeding the national median of 52.7°F. This makes it one of the more temperate regions in the Midwest.
Topping the State Temperature Charts
The county is significantly warmer than the Illinois average of 52.8°F. It also receives 43.0 inches of rain, making it wetter than the state average of 41.7 inches.
The Heat of the Metro East
St. Clair is much hotter than Sangamon County, recording 46 extreme heat days compared to Sangamon’s 26. It represents a southern shift in climate for Illinois residents.
Sultry Summers and Light Snow
July averages a sweltering 78.9°F, leading to 46 days above 90°F annually. Winters remain mild by Illinois standards, with only 12.2 inches of annual snowfall.
Invest in Heavy-Duty Cooling
The frequency of extreme heat requires robust HVAC systems and heat-safety planning for outdoor workers. Minimal snow means residents can focus more on storm drainage than heavy plows.
Frequently Asked Questions
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Track your own weather in St. Clair County
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Data source: NOAA National Centers for Environmental Information (NCEI), U.S. Climate Normals 1991-2020.
Climate normals are 30-year averages of weather variables computed from 1991 to 2020. Station data is aggregated to county level using spatial averaging. Data is informational only — not a substitute for official National Weather Service forecasts or emergency weather warnings.