Cass County, Illinois Climate
The warmest month is July, with an average high of 87°F; the coldest is January, with an average low of 18°F. A normal year brings 39.2 in of precipitation and 16.2 in of snow.
Temperature through the year
The normal daily high and low and the record for every date at Jacksonville (IJX), since 1895. The line marks today.
- Normal low
- Normal high
- Record high
- Record low
Normals for Sep 24
- Normal high
- 78°F
- Normal low
- 50°F
- Record high
- 95°Fin 2017
- Record low
- 29°Fin 1989
- Chance of a wet day
- 28%
- Daylight
- 12 h 04 min
Month by month
Averages for 1991-2020. Wet days have at least 0.01 inch of precipitation. Freeze nights have a low of 32°F or colder; degree days are on a 65°F base. Each month links to its own page.
| Month | Avg high | Avg low | Record high | Record low | Precipitation | Wet days | Snowfall | Freeze nights | Heating DD | Cooling DD |
|---|---|---|---|---|---|---|---|---|---|---|
| January | 36°F | 18°F | 75°F (1950) | -24°F (1912) | 1.93 in | 7.7 | 5.1 in | 28.2 | 1,176 | 0 |
| February | 41°F | 21°F | 81°F (2024) | -28°F (1934) | 1.72 in | 7.2 | 4.3 in | 23.9 | 945 | 0 |
| March | 52°F | 30°F | 93°F (2026) | -14°F (1948) | 2.81 in | 10.8 | 2.0 in | 19.8 | 739 | 2 |
| April | 65°F | 40°F | 94°F (1930) | 10°F (1920) | 4.27 in | 12.3 | 0.3 in | 6.4 | 385 | 13 |
| May | 75°F | 52°F | 103°F (1934) | 26°F (1966) | 4.82 in | 13.3 | 0.0 in | 0.4 | 139 | 85 |
| June | 83°F | 61°F | 107°F (1931) | 36°F (1993) | 4.89 in | 10.5 | 0.0 in | 0.0 | 17 | 228 |
| July | 87°F | 64°F | 114°F (1954) | 43°F (1930) | 3.96 in | 8.7 | 0.0 in | 0.0 | 2 | 320 |
| August | 85°F | 62°F | 112°F (1934) | 39°F (1986) | 3.39 in | 8.9 | 0.0 in | 0.0 | 6 | 267 |
| September | 80°F | 54°F | 106°F (1899) | 23°F (1899) | 3.50 in | 8.2 | 0.0 in | 0.2 | 75 | 127 |
| October | 68°F | 42°F | 98°F (1897) | 9°F (1925) | 2.97 in | 9.9 | 0.0 in | 5.6 | 333 | 23 |
| November | 53°F | 32°F | 84°F (1937) | -5°F (1929) | 3.01 in | 9.8 | 1.0 in | 17.6 | 677 | 1 |
| December | 41°F | 23°F | 74°F (2012) | -21°F (1914) | 1.98 in | 8.1 | 3.5 in | 26.1 | 1,015 | 0 |
| Year | 64°F | 42°F | 39.25 in | 115 | 16.2 in | 128 | 5,510 | 1,068 |
Precipitation
Rain and melted snow together. The chance of a wet day is the share of days with at least 0.01 inch of precipitation.
39.2 in in a normal year, most in June.
Chance of a wet day
- Chance of a wet day
- Chance of snowfall
Snow
NOAA 1991-2020 snowfall normals for Jacksonville 2E.
16.2 in of snow in a normal year.
Frost and the growing season
NOAA's 1991-2020 freeze probabilities for Jacksonville 2E. A median date has an even chance of a later freeze in spring, or an earlier one in fall.
The last spring freeze typically comes around April 19 and the first fall freeze around October 17, a growing season of about 179 days.
- Median at 36°F
- last April 30, first October 6
- Median at 32°F
- last April 19, first October 17
- Median at 28°F
- last April 7, first October 27
Humidity and comfort
NASA POWER reanalysis, 1991-2020, for the grid cell around Cass County.
The muggiest month is July, when 61 percent of days are muggy or worse, with a mean dew point of 65°F or more.
| Month | Dew point | Humidity | Dry | Comfortable | Humid | Muggy | Oppressive | Miserable |
|---|---|---|---|---|---|---|---|---|
| January | 21°F | 84% | 100% | 0% | 0% | 0% | 0% | 0% |
| February | 25°F | 81% | 100% | 0% | 0% | 0% | 0% | 0% |
| March | 34°F | 77% | 95% | 3% | 2% | 0% | 0% | 0% |
| April | 44°F | 74% | 83% | 11% | 6% | 1% | 0% | 0% |
| May | 55°F | 76% | 49% | 15% | 17% | 16% | 3% | 0% |
| June | 64°F | 77% | 13% | 16% | 22% | 24% | 23% | 3% |
| July | 66°F | 74% | 6% | 14% | 19% | 27% | 27% | 7% |
| August | 64°F | 71% | 12% | 19% | 23% | 23% | 19% | 3% |
| September | 55°F | 69% | 46% | 20% | 17% | 12% | 4% | 0% |
| October | 44°F | 71% | 84% | 9% | 5% | 2% | 0% | 0% |
| November | 34°F | 77% | 97% | 3% | 0% | 0% | 0% | 0% |
| December | 26°F | 83% | 99% | 1% | 0% | 0% | 0% | 0% |
Comfort is judged by the day's mean dew point: dry below 55°F, comfortable to 60°F, humid to 65°F, muggy to 70°F, oppressive to 75°F, miserable above.
Clouds
Average share of the sky covered, from satellite (CERES and GEWEX SRB) by way of NASA POWER, 1991-2020.
The clearest month is September, with the sky 47 percent covered on average; the cloudiest is February, at 68 percent.
Wind
The mean wind speed over the whole day, from the MERRA-2 reanalysis by way of NASA POWER. Gusts run well above it.
The windiest month is April, averaging 12.1 mph at 10 meters (33 feet) above the ground.
Daylight and sunshine
The longest day is Jun 18, with 15 h 01 min of daylight; the shortest is Dec 21, with 9 h 19 min.
- Day length
Solar energy
Average daily sunshine reaching the ground on a flat surface, in kilowatt-hours per square meter, clouds included.
How Cass County is running
NOAA NCEI Climate at a Glance, from the nClimGrid analysis of every station, updated early each month. A rank of 1 is the highest or lowest of all years since 1895.
August 2026 in Cass County: 75.0°F (+1.1 °F against 1991-2020), the 50th warmest in 132 years on record.
August 2026 in Cass County: 5.05 in (+1.49 in against 1991-2020), the 24th wettest in 132 years on record.
January to August 2026 in Cass County: 57.0°F (+2.1 °F against 1991-2020), the 9th warmest in 132 years on record.
January to August 2026 in Cass County: 31.66 in (+4.54 in against 1991-2020), the 22nd wettest in 132 years on record.
- Warmest August
- 82.3°F in 1947
- Coolest August
- 67.5°F in 1915
- Wettest August
- 7.87 in in 2018
- Driest August
- 0.28 in in 2013
Every year since 1895
Month pages
The weather in Cass County in each month, day by day.
What this means
A normal is the average of the thirty years 1991 to 2020, the period the World Meteorological Organization and NOAA use for comparison. It is an average, not a forecast: most days depart from it, and some by a long way.
A record is the highest or lowest value on the date in every year the station has reported, back to 1895. It rests on one station, so a neighbor a few miles away can hold different ones.
Cloud, humidity, wind and sunshine come from a reanalysis: a weather model run over the past with every observation fed in, averaged over a grid cell about 50 km across. It is consistent everywhere on Earth; it smooths local detail such as a lake shore or a valley.
More for this place
Nearby
Sources
Normals: NOAA NCEI U.S. Climate Normals, 1991-2020, station USC00114442 (Jacksonville 2E), public domain. Records: NOAA GHCN-Daily, station USC00114442, every year on file, quality-flagged values removed. Cloud, humidity, wind and sunshine: NASA POWER (MERRA-2, CERES and GEWEX SRB), 1991-2020. The data was obtained from the National Aeronautics and Space Administration (NASA) Langley Research Center's Prediction Of Worldwide Energy Resources (POWER) project funded through the NASA Earth Science Division: POWER Daily API v2, accessed 2026-09-24. Sun and moon: computed with the NOAA solar equations and Meeus. Rankings: NOAA NCEI Climate at a Glance, public domain.