30-year climate normals
Jefferson County Climate and Weather Averages
Hottest month — July
85.7°average daily high. The coldest month is January, averaging a low of 21.7°F.
Jefferson County gets 44.97" of precipitation a year and 10.4" of snow, and 33% of it falls in Apr, May, Jun.
Temperature through the year
Each bar spans the average daily low to the average daily high for that month.
| Month | Avg high | Avg low | Precip | Snow |
|---|---|---|---|---|
| January | 38.5° | 21.7° | 2.60" | 3.2" |
| February | 43.6° | 25.0° | 2.62" | 3.1" |
| March | 53.6° | 33.8° | 4.00" | 1.0" |
| April | 64.9° | 44.1° | 4.96" | 0.0" |
| May | 73.8° | 55.0° | 5.06" | 0.0" |
| June | 82.4° | 63.7° | 4.84" | 0.0" |
| July | 85.7° | 67.3° | 3.78" | 0.0" |
| August | 84.8° | 64.8° | 3.33" | 0.0" |
| September | 78.6° | 56.7° | 3.37" | 0.0" |
| October | 67.5° | 44.7° | 3.53" | 0.2" |
| November | 53.7° | 34.2° | 3.99" | 0.4" |
| December | 42.9° | 26.1° | 2.89" | 2.5" |
Rainfall by month
Precipitation is spread fairly evenly through the year here, with 33% falling in the wettest three months (Apr, May, Jun).
What this climate costs you
Degree days measure how hard a building has to work to stay comfortable. They are the reason two identical houses have very different power bills, and they are the link between the weather here and what you pay every month.
These are rough planning figures from degree days and the state electricity price, not a bill estimate. House size, insulation and fuel type move them a lot. See the full cost of owning a home in Jefferson County, where electricity is one line of eight.
Common questions
What is the weather like in Jefferson County?
Jefferson County averages a high of 85.7°F in July and a low of 21.7°F in January. Annual precipitation is 44.97 inches, including 10.4 inches of snow.
When is the rainy season in Jefferson County?
The wettest stretch is Apr, May, Jun, which accounts for 33% of the year's precipitation. Precipitation is relatively evenly distributed across the year.
How hot does it get in Jefferson County?
The average daily high in July, the hottest month, is 85.7°F. That drives 1,191 cooling degree days a year, which is what your air conditioning has to work against.
Does it snow in Jefferson County?
Yes. Jefferson County averages 10.4 inches of snow a year.
Sources
- Temperature, precipitation and snowfall normals — NOAA National Centers for Environmental Information, 1991–2020 Monthly Climate Normals, station MT VERNON 3 NE, IL US (USC00115943), 5.1 miles from the county centroid.
- Heating and cooling degree days — NOAA, same station and period.
- Electricity price — US Energy Information Administration, Electric Power Monthly Table 5.6.A, Illinois residential average.
- Wet-season share is the wettest three consecutive months as a percentage of annual precipitation, calculated from the monthly normals.