Mercer County Climate
Mercer County, Illinois
NOAA NCEI 1991-2020 Climate NormalsAvg Annual Temp
50.6°F
Annual Precip
37.6"
Annual Snowfall
21.1"
Extreme Heat Days
19
days >= 90°F/yr
Monthly Temperature
Precipitation Calendar
Growing Season & Gardening
5b
Minimum winter temperature range for plant survival
3,322
Supports most vegetables & fruits
Seasonal Overview
National Comparison
7-Day Outlook
Forecast
Current Conditions
Air Quality
Safety
Weather Alerts
Climate Advisory: Mercer County
Climate Verdict
Mercer County has a mild, with an average annual temperature of 50.6°F, moderate rainfall of 37.6 inches per year, rare extreme heat with only 18.8 days above 90°F annually, moderate snowfall of 21.1 inches annually. This climate profile shapes everything from energy costs to outdoor recreation opportunities.
Seasonal Breakdown
Winters are very cold — January averages 22.8°F. summers are warm — July averages 74.9°F. Residents experience a substantial 52°F difference between winter and summer.
Outdoor & Gardening Guidance
Growing season and gardening data is currently limited for Mercer County. Check the USDA Plant Hardiness Zone Map and local extension office for specific guidance.
Regional Context
Mercer County closely matches the Illinois statewide average temperature of 52.8°F, it is somewhat drier than average for Illinois.
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 sources: NOAA National Centers for Environmental Information (NCEI), U.S. Climate Normals 1991-2020. Forecast and alert data from National Weather Service API. Air quality data from Open-Meteo.
Climate normals are 30-year averages of weather variables. Station data is aggregated to county level. Forecasts and alerts are fetched live and may change frequently. Data is informational only — not a substitute for official National Weather Service forecasts or emergency weather warnings.