Mercer County Climate
Mercer County, Illinois
Avg Annual Temp
50.6°F
Annual Precip
37.6"
Annual Snowfall
21.1"
Extreme Heat Days
18.8
days >= 90°F/yr
Monthly Temperature
Precipitation Calendar
Seasonal Overview
National Comparison
Climate Profile
About Mercer County Weather
A True Four-Season Midwestern Climate
Mercer County's 50.6°F average annual temperature is cooler than the national median. This northwestern Illinois county experiences a pronounced winter season and warm, humid summers.
Cooler and Drier Than Average Illinois
The county's 50.6°F average and 37.6 inches of rain both fall below the Illinois state averages. It represents the cooler, northern-fringe climate of the state's midsection.
Higher Snowfall Than Central Peers
Mercer averages 21.1 inches of snow, significantly more than neighbors like McLean. It enjoys a milder summer than southern counties, with only 19 extreme heat days.
Cold Winters and Manageable Summers
The January average is a cold 22.8°F, and the county sees over 21 inches of snow each year. Summers are warm but rarely oppressive, with 19 days reaching 90°F or higher.
Gear Up for Significant Winter Snow
Invest in a reliable snow shovel or blower to handle the 21.1 inches of annual snowfall. You will also need robust home heating to stay comfortable during the 22.8°F January lows.
Frequently Asked Questions
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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.