Sullivan, Illinois climate: Normal weather by month, frost dates and records
Sullivan's normal year runs from 19°F January nights to 86°F July afternoons, with 41.6 inches of precipitation and 13 inches of snow a year. (NOAA 1991-2020 normals)
In metric: a yearly mean of 11.7°C, January lows of −7°C, July highs of 30°C, 1,057 mm of precipitation a year.
- NOAA stationSullivan (3 mi S)
- Station IDUSC00118389
- Location39.56°N 88.61°W
- Elevation640 ft (195 m)
- From the center2 miles
- Population4,298
On October 23, the normal high in Sullivan is 64°F and the normal low 41°F, and the chance of measurable rain or snow is 29%.
Normals describe a typical year at the station, not a forecast. The National Weather Service forecast for Sullivan.
- Jan 35° 19°
- Feb 40° 22°
- Mar 51° 31°
- Apr 65° 42°
- May 75° 53°
- Jun 83° 63°
- Jul 86° 65°
- Aug 85° 62°
- Sep 80° 55°
- Oct 67° 44°
- Nov 52° 33°
- Dec 40° 25°
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 Sullivan each month?
July is the warmest month, with normal highs of 86°F; January the coldest, with normal lows of 19°F. The most precipitation falls in May (4.98 in), the least in February (2.23 in).
| Month | High | Low | Mean | Precip. | Snow | Wet days | 90°F days | Freezing nights | Daylight |
|---|---|---|---|---|---|---|---|---|---|
| January | 35°F | 19°F | 26.9°F | 2.41 | 5.2 | 8 | <1 | 28 | 9 h 40 min |
| February | 40°F | 22°F | 31.0°F | 2.23 | 2.4 | 8 | <1 | 24 | 10 h 40 min |
| March | 51°F | 31°F | 41.2°F | 2.83 | 1.0 | 9 | <1 | 19 | 11 h 50 min |
| April | 65°F | 42°F | 53.3°F | 4.48 | 0.2 | 10 | <1 | 6 | 13 h 10 min |
| May | 75°F | 53°F | 64.0°F | 4.98 | 0.0 | 12 | 1 | <1 | 14 h 20 min |
| June | 83°F | 63°F | 72.9°F | 4.95 | 0.0 | 10 | 4 | <1 | 15 h |
| July | 86°F | 65°F | 75.6°F | 4.35 | 0.0 | 8 | 8 | <1 | 14 h 40 min |
| August | 85°F | 62°F | 73.7°F | 3.16 | 0.0 | 7 | 6 | <1 | 13 h 50 min |
| September | 80°F | 55°F | 67.3°F | 2.71 | 0.0 | 7 | 4 | <1 | 12 h 30 min |
| October | 67°F | 44°F | 55.6°F | 3.30 | 0.0 | 8 | <1 | 4 | 11 h 10 min |
| November | 52°F | 33°F | 42.6°F | 3.45 | 0.4 | 9 | <1 | 15 | 10 h |
| December | 40°F | 25°F | 32.4°F | 2.75 | 3.7 | 8 | <1 | 25 | 9 h 20 min |
| Year (annual) | 63°F | 43°F | 53.0°F | 41.60 | 12.9 | 103 | 22 | 122 |
The same normals in °C and millimetres
| Month | High | Low | Mean | Precip. | Snow |
|---|---|---|---|---|---|
| January | 2°C | −7°C | −2.8°C | 61 mm | 13 cm |
| February | 5°C | −6°C | −0.6°C | 57 mm | 6 cm |
| March | 11°C | 0°C | 5.1°C | 72 mm | 3 cm |
| April | 18°C | 5°C | 11.8°C | 114 mm | 1 cm |
| May | 24°C | 12°C | 17.8°C | 126 mm | 0 cm |
| June | 28°C | 17°C | 22.7°C | 126 mm | 0 cm |
| July | 30°C | 18°C | 24.2°C | 110 mm | 0 cm |
| August | 30°C | 17°C | 23.2°C | 80 mm | 0 cm |
| September | 27°C | 13°C | 19.6°C | 69 mm | 0 cm |
| October | 20°C | 7°C | 13.1°C | 84 mm | 0 cm |
| November | 11°C | 1°C | 5.9°C | 88 mm | 1 cm |
| December | 5°C | −4°C | 0.2°C | 70 mm | 9 cm |
Source: NOAA NCEI 1991-2020 monthly normals, station USC00118389; 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 Sullivan?
Normal highs sit between 65°F and 85°F in May, June, September and October. Of those, September has the fewest days with rain (7).
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 USC00118389.
When is the last frost in Sullivan?
In Sullivan, the last spring frost (32°F) comes after April 15 in half of years and after April 30 in one year in ten; the first autumn frost comes before October 22 in half of years and before October 6 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 151 year in 10 after April 30; 9 in 10 after April 2
- First autumn frost, half of years before
- October 221 year in 10 before October 6; 9 in 10 before November 2
NOAA puts the median frost-free season at 187 days, and the season without a hard freeze (28°F) at 210 days.
A hard freeze (28°F) comes after April 4 in half of springs and before November 1 in half of autumns.
The USDA's 2023 Plant Hardiness Zone Map puts Sullivan in zone 6a in ZIP code 61951, 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 USC00118389; 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 Sullivan?
In a normal year in Sullivan, 22 days a year reach 90°F, 122 nights fall to 32°F or colder and 103 days bring measurable precipitation.
- Days at 90°F or more
- 22a normal year
- Days at 100°F or more
- fewer than 1a normal year
- Nights at 32°F or colder
- 122a normal year
- Days that stay below freezing
- 33high of 32°F or less
- Days with 0.01 inch of precipitation
- 103a normal year
- Days with 1 inch of snow or more
- 6a normal year
The highest temperature in this station's record (since 1899) is 111°F, on July 22, 1901; the lowest is −24°F, on February 13, 1905.
The wettest calendar year on that record is 2009, with 56.16 in; the snowiest season is 2005–06, with 23.4 inches.
Source: NOAA NCEI 1991-2020 annual normals, station USC00118389; records: ACIS (NOAA Regional Climate Centers), station USC00118389. Records use complete months only; a month missing more than three days is left out.
How sunny is Sullivan?
Over a year 74% of the sunlight that would reach the ground under clear skies gets through the clouds in the area around Sullivan, 4.1 kWh per square metre a day. July is the sunniest month (83%) and December the cloudiest (61%).
| Month | Share of clear-sky sunshine | Sunshine, kWh/m² a day |
|---|---|---|
| January | 62% | 1.9 |
| February | 65% | 2.7 |
| March | 67% | 3.7 |
| April | 72% | 4.9 |
| May | 73% | 5.5 |
| June | 81% | 6.4 |
| July | 83% | 6.3 |
| August | 82% | 5.6 |
| September | 81% | 4.7 |
| October | 74% | 3.3 |
| November | 70% | 2.3 |
| December | 61% | 1.6 |
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 Sullivan need?
A normal year in Sullivan brings 5,449 heating degree days, 1% more than the median U.S. place (5,373), and 1,128 cooling degree days, 24% 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 3,674 growing degree days a year above 50°F.
| Month | Heating | Cooling |
|---|---|---|
| January | 1,181 | 0 |
| February | 951 | 0 |
| March | 742 | 2 |
| April | 365 | 14 |
| May | 127 | 96 |
| June | 11 | 249 |
| July | 2 | 330 |
| August | 5 | 274 |
| September | 65 | 135 |
| October | 318 | 26 |
| November | 674 | 1 |
| December | 1,011 | 0 |
| Year (annual) | 5,449 | 1,128 |
Source: NOAA NCEI 1991-2020 monthly normals, station USC00118389. 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 Sullivan been warmer than normal lately?
2022 averaged 52.3°F, 0.7°F below the 1991-2020 normal of 53.0°F.
| Year | Mean | Against normal | Precip. | Snow |
|---|---|---|---|---|
| 2022 | 52.3°F | −0.7°F | 43.40 | 8.6 |
Source: observed years: ACIS, station USC00118389. A year counts only with all twelve months complete.
How does Sullivan's climate compare?
Sullivan is warmer than 94 of the other 143 places with their own NOAA record in Illinois, and wetter than 97. Across the 7,247 U.S. stations with normals, its annual mean of 53.0°F is warmer than 54%.
In the Köppen classification computed from these normals, Sullivan 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 Sullivan share this climate record?
3 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.
- Bethany, IL1,198 people · 9 mi NW
- Lovington, IL1,020 people · 11 mi N
- Allenville, IL130 people · 4 mi E
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 Sullivan's climate
What is the coldest month in Sullivan?
January, with a normal high of 35°F and a normal low of 19°F. The warmest is July (86°F by day, 65°F at night).
Does it snow in Sullivan?
Yes: 13 inches in a normal year, falling on 9 days with 0.1 inch or more, the most in January (5.2 inches).
What is the rainiest month in Sullivan?
May, with 4.98 in of precipitation in a normal year; the driest is February with 2.23 in.
What plant hardiness zone is Sullivan in?
The USDA's 2023 map puts Sullivan in zone 6a in ZIP code 61951, the nearest ZIP area.
How late can frost come in Sullivan?
In half of years the last 32°F night comes after April 15; in one year in ten it comes after April 30. Waiting until April 30 leaves about a one-in-ten chance of a later frost.
Nearby and next
- Illinois climateThe statewide normals and every place in the state
- Windsor, IL8 miles from this station, 52.6°F a year
- Shelbyville, IL14 miles from this station, 53.6°F a year
- Neoga, IL16 miles from this station, 53.6°F a year
- Mattoon, IL19 miles from this station, 53.4°F a year
- Compare two placesMonth by month, side by side
Download these normals (CSV) · JSON · How these figures are made