Illinois City, Illinois climate: Normal weather by month, frost dates and records
Illinois City's normal year runs from 13°F January nights to 85°F July afternoons, with 36.4 inches of precipitation a year. (NOAA 1991-2020 normals)
In metric: a yearly mean of 10.2°C, January lows of −10°C, July highs of 29°C, 926 mm of precipitation a year.
- NOAA stationIllinois City Dam 16
- Station IDUSC00114355
- Location41.43°N 91.01°W
- Elevation550 ft (168 m)
- From the center6 miles
- Population (census place)18
On October 14, the normal high in Illinois City is 65°F and the normal low 44°F, and the chance of measurable rain or snow is 32%.
Normals describe a typical year at the station, not a forecast. The National Weather Service forecast for Illinois City.
- Jan 31° 13°
- Feb 35° 17°
- Mar 48° 30°
- Apr 61° 39°
- May 72° 51°
- Jun 81° 62°
- Jul 85° 65°
- Aug 83° 63°
- Sep 77° 55°
- Oct 64° 43°
- Nov 49° 31°
- Dec 36° 20°
Each month: normal high (large) and low, °F. Fill: the month's normal mean, colder blue to hotter red.
What is the weather like in Illinois City each month?
July is the warmest month, with normal highs of 85°F; January the coldest, with normal lows of 13°F. The most precipitation falls in June (5.00 in), the least in January (1.18 in).
| Month | High | Low | Mean | Precip. | Wet days | 90°F days | Freezing nights | Daylight |
|---|---|---|---|---|---|---|---|---|
| January | 31°F | 13°F | 22.0°F | 1.18 | 11 | <1 | 30 | 9 h 30 min |
| February | 35°F | 17°F | 26.4°F | 1.55 | 10 | <1 | 26 | 10 h 40 min |
| March | 48°F | 30°F | 38.8°F | 2.29 | 12 | <1 | 20 | 11 h 50 min |
| April | 61°F | 39°F | 50.1°F | 3.47 | 12 | <1 | 6 | 13 h 20 min |
| May | 72°F | 51°F | 61.7°F | 4.82 | 14 | 1 | <1 | 14 h 30 min |
| June | 81°F | 62°F | 71.4°F | 5.00 | 13 | 4 | <1 | 15 h 10 min |
| July | 85°F | 65°F | 74.9°F | 4.03 | 10 | 8 | <1 | 14 h 50 min |
| August | 83°F | 63°F | 73.0°F | 3.94 | 10 | 5 | <1 | 13 h 50 min |
| September | 77°F | 55°F | 65.7°F | 3.57 | 9 | 2 | <1 | 12 h 30 min |
| October | 64°F | 43°F | 53.6°F | 2.78 | 10 | <1 | 4 | 11 h 10 min |
| November | 49°F | 31°F | 39.6°F | 2.13 | 9 | <1 | 18 | 9 h 50 min |
| December | 36°F | 20°F | 28.1°F | 1.68 | 11 | <1 | 27 | 9 h 10 min |
| Year (annual) | 60°F | 41°F | 50.4°F | 36.44 | 131 | 21 | 131 |
The same normals in °C and millimetres
| Month | High | Low | Mean | Precip. |
|---|---|---|---|---|
| January | −1°C | −10°C | −5.6°C | 30 mm |
| February | 2°C | −8°C | −3.1°C | 39 mm |
| March | 9°C | −1°C | 3.8°C | 58 mm |
| April | 16°C | 4°C | 10.1°C | 88 mm |
| May | 22°C | 11°C | 16.5°C | 122 mm |
| June | 27°C | 16°C | 21.9°C | 127 mm |
| July | 29°C | 18°C | 23.8°C | 102 mm |
| August | 28°C | 17°C | 22.8°C | 100 mm |
| September | 25°C | 13°C | 18.7°C | 91 mm |
| October | 18°C | 6°C | 12.0°C | 71 mm |
| November | 9°C | −1°C | 4.2°C | 54 mm |
| December | 2°C | −7°C | −2.2°C | 43 mm |
NOAA publishes no 1991-2020 snowfall normal for this station. The nearest record that keeps one, Muscatine (4 miles away), gets 24 inches of snow in a normal year; elevation and distance can change that.
Source: NOAA NCEI 1991-2020 monthly normals, station USC00114355; annual normals. Wet days: 0.01 inch or more; 90°F days and freezing nights: NOAA's day counts. Daylight: sunrise to sunset on the 15th, computed from the latitude.
When is the best weather to visit Illinois City?
Normal highs sit between 65°F and 85°F in May, June, July, August and September. Of those, September has the fewest days with rain (9).
That is a measurement, not a recommendation: it counts the months whose normal afternoon high sits between 65°F and 85°F, where a light layer is enough by day.
Source: NOAA NCEI 1991-2020 monthly normals, station USC00114355.
When is the last frost in Illinois City?
In Illinois City, the last spring frost (32°F) comes after April 18 in half of years and after May 2 in one year in ten; the first autumn frost comes before October 24 in half of years and before October 11 in one year in ten.
Frost-free in half of yearsFrost in 1 to 9 years out of 10Frost most years
- Last spring frost, half of years after
- April 181 year in 10 after May 2; 9 in 10 after April 5
- First autumn frost, half of years before
- October 241 year in 10 before October 11; 9 in 10 before November 6
NOAA puts the median frost-free season at 188 days, and the season without a hard freeze (28°F) at 208 days.
A hard freeze (28°F) comes after April 8 in half of springs and before November 1 in half of autumns.
The USDA's 2023 Plant Hardiness Zone Map puts Illinois City in zone 5b in ZIP code 61259, the nearest ZIP area.
Frost dates for every NOAA station in Illinois · Frost dates by ZIP code
Source: NOAA NCEI 1991-2020 annual normals (frost probabilities), station USC00114355; USDA 2023 Plant Hardiness Zone Map, ZIP listings: PRISM Group, Oregon State University, https://prism.oregonstate.edu, accessed October 8, 2026. A 10% date means one year in ten has a frost after it in spring or before it in autumn.
How hot and how cold does it get in Illinois City?
In a normal year in Illinois City, 21 days a year reach 90°F (1 of them 100°F or more), 131 nights fall to 32°F or colder and 131 days bring measurable precipitation.
- Days at 90°F or more
- 21a normal year
- Days at 100°F or more
- 1a normal year
- Nights at 32°F or colder
- 131a normal year
- Days that stay below freezing
- 43high of 32°F or less
- Days with 0.01 inch of precipitation
- 131a normal year
Source: NOAA NCEI 1991-2020 annual normals, station USC00114355.
How sunny is Illinois City?
Over a year 74% of the sunlight that would reach the ground under clear skies gets through the clouds in the area around Illinois City, 4.0 kWh per square metre a day. July is the sunniest month (81%) and December the cloudiest (63%).
| Month | Share of clear-sky sunshine | Sunshine, kWh/m² a day |
|---|---|---|
| January | 65% | 1.9 |
| February | 71% | 2.9 |
| March | 69% | 3.8 |
| April | 70% | 4.8 |
| May | 72% | 5.6 |
| June | 77% | 6.2 |
| July | 81% | 6.3 |
| August | 80% | 5.5 |
| September | 81% | 4.6 |
| October | 72% | 3.0 |
| November | 70% | 2.1 |
| December | 63% | 1.5 |
Source: NASA POWER solar climatology, 2001-2020 (CERES SYN1deg). Satellite-based, for the 1-degree cell (about 100 km across) around the station, so it describes the area rather than the airport.
How much heating and cooling does Illinois City need?
A normal year in Illinois City brings 6,221 heating degree days, 16% more than the median U.S. place (5,373), and 951 cooling degree days, 5% more than the median (908). Seven months need 300 or more heating degree days, a monthly mean of about 55°F or colder. For gardens and crops, NOAA counts 3,309 growing degree days a year above 50°F.
| Month | Heating | Cooling |
|---|---|---|
| January | 1,332 | 0 |
| February | 1,082 | 0 |
| March | 814 | 2 |
| April | 451 | 4 |
| May | 165 | 62 |
| June | 14 | 205 |
| July | 3 | 308 |
| August | 6 | 254 |
| September | 80 | 101 |
| October | 370 | 15 |
| November | 762 | 0 |
| December | 1,142 | 0 |
| Year (annual) | 6,221 | 951 |
Source: NOAA NCEI 1991-2020 monthly normals, station USC00114355. A degree day is one degree of a day's mean temperature below 65°F (heating) or above it (cooling); utilities use them to estimate energy use.
Has Illinois City been warmer than normal lately?
2025 averaged 51.4°F, 1.0°F above the 1991-2020 normal of 50.4°F. 4 of the 5 complete years since 2021 ran warmer than the normal.
| Year | Mean | Against normal | Precip. |
|---|---|---|---|
| 2021 | 52.4°F | +2.0°F | 31.77 |
| 2022 | 50.2°F | −0.2°F | 30.97 |
| 2023 | 52.4°F | +2.0°F | 26.25 |
| 2024 | 53.3°F | +2.9°F | 37.48 |
| 2025 | 51.4°F | +1.0°F | 37.36 |
Source: observed years: ACIS, station USC00114355. A year counts only with all twelve months complete.
How does Illinois City's climate compare?
Illinois City is warmer than 42 of the other 143 places with their own NOAA record in Illinois, and wetter than 9. Across the 7,247 U.S. stations with normals, its annual mean of 50.4°F is warmer than 42%.
In the Köppen classification computed from these normals, Illinois City is Dfa: continental, no dry season, hot summer. What the classes mean.
Places with the most similar year elsewhere
Measured month by month on the normal high, low and precipitation, among places of 20,000 or more people in other states:
Source: NOAA NCEI 1991-2020 normals for every station PlainClimate covers. Similarity: the root-mean-square difference of the 36 monthly figures, precipitation weighted so an inch counts like 6°F.
Which places near Illinois City share this climate record?
11 places have no NOAA station with 1991-2020 normals of their own, and this station is the nearest full record. A place's own climate can differ from these figures, by several degrees where it sits much higher or lower than the station, nearer the coast, or inside a large city's heat; for the larger places the next nearest records are named with their yearly mean, so the spread around them is visible, and places of 50,000 or more have their own page with NOAA's gridded normals.
- West Liberty, IA3,724 people · 17 mi NW
- Wilton, IA2,914 people · 11 mi N
- Kent Estates, IA1,863 people · 4 mi NW
- Durant, IA1,798 people · 13 mi N
- Fruitland, IA928 people · 8 mi SW
- Moscow, IA494 people · 11 mi N
- Nichols, IA317 people · 16 mi W
- Atalissa, IA287 people · 13 mi NW
- Montpelier, IA228 people · 11 mi E
- Fairport, IA200 people · 5 mi E
- Stockton, IA171 people · 14 mi NE
Source: Census Bureau 2024 Gazetteer (place centers) and Vintage 2024 population estimates. Distance and direction from the station to each place's center.
Questions about Illinois City's climate
What is the coldest month in Illinois City?
January, with a normal high of 31°F and a normal low of 13°F. The warmest is July (85°F by day, 65°F at night).
Does it snow in Illinois City?
Yes. NOAA publishes no 1991-2020 snowfall normal for Illinois City's NOAA station; the nearest record that keeps one, Muscatine (4 miles away), records 24 inches of snow in a normal year.
What is the rainiest month in Illinois City?
June, with 5.00 in of precipitation in a normal year; the driest is January with 1.18 in.
What plant hardiness zone is Illinois City in?
The USDA's 2023 map puts Illinois City in zone 5b in ZIP code 61259, the nearest ZIP area.
How late can frost come in Illinois City?
In half of years the last 32°F night comes after April 18; in one year in ten it comes after May 2. Waiting until May 2 leaves about a one-in-ten chance of a later frost.
Nearby and next
- Illinois climateThe statewide normals and every place in the state
- Muscatine, IA4 miles from this station, 50.5°F a year
- New Boston, IL16 miles from this station, 51.2°F a year
- Columbus Junction, IA21 miles from this station, 48.8°F a year
- Aledo, IL21 miles from this station, 50.0°F a year
- Compare two placesMonth by month, side by side
Download these normals (CSV) · JSON · How these figures are made