Decatur, Illinois climate: Normal weather by month, frost dates and records
Decatur's normal year runs from 20°F January nights to 86°F July afternoons, with 36.4 inches of precipitation 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, 924 mm of precipitation a year.
- NOAA stationDecatur Airport
- Station IDUSW00003887
- Location39.83°N 88.87°W
- Elevation675 ft (206 m)
- From the center4 miles
- Population68,763
On October 23, the normal high in Decatur is 63°F and the normal low 42°F, and the chance of measurable rain or snow is 31%.
Normals describe a typical year at the station, not a forecast. The National Weather Service forecast for Decatur.
- Jan 35° 20°
- Feb 40° 23°
- Mar 52° 32°
- Apr 64° 42°
- May 74° 53°
- Jun 83° 62°
- Jul 86° 66°
- Aug 84° 63°
- Sep 79° 55°
- Oct 66° 44°
- Nov 51° 34°
- 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 Decatur each month?
July is the warmest month, with normal highs of 86°F; January the coldest, with normal lows of 20°F. The most precipitation falls in May (4.38 in), the least in February (1.84 in).
Each month has its own page with the day-by-day normals, the month's records and its recent years:
| Month | High | Low | Mean | Precip. | Wet days | 90°F days | Freezing nights | Daylight |
|---|---|---|---|---|---|---|---|---|
| January | 35°F | 20°F | 27.4°F | 1.99 | 8 | <1 | 27 | 9 h 40 min |
| February | 40°F | 23°F | 31.6°F | 1.84 | 7 | <1 | 24 | 10 h 40 min |
| March | 52°F | 32°F | 42.1°F | 2.51 | 11 | <1 | 17 | 11 h 50 min |
| April | 64°F | 42°F | 53.1°F | 3.66 | 11 | <1 | 5 | 13 h 10 min |
| May | 74°F | 53°F | 63.9°F | 4.38 | 13 | 1 | <1 | 14 h 20 min |
| June | 83°F | 62°F | 72.7°F | 4.38 | 11 | 6 | <1 | 15 h |
| July | 86°F | 66°F | 75.7°F | 3.32 | 10 | 9 | <1 | 14 h 40 min |
| August | 84°F | 63°F | 73.7°F | 3.03 | 9 | 7 | <1 | 13 h 50 min |
| September | 79°F | 55°F | 67.0°F | 2.98 | 9 | 4 | <1 | 12 h 30 min |
| October | 66°F | 44°F | 55.2°F | 3.18 | 10 | <1 | 3 | 11 h 10 min |
| November | 51°F | 34°F | 42.7°F | 2.97 | 9 | <1 | 14 | 10 h |
| December | 40°F | 25°F | 32.4°F | 2.12 | 9 | <1 | 24 | 9 h 20 min |
| Year (annual) | 63°F | 43°F | 53.1°F | 36.36 | 118 | 27 | 113 |
The same normals in °C and millimetres
| Month | High | Low | Mean | Precip. |
|---|---|---|---|---|
| January | 2°C | −7°C | −2.6°C | 51 mm |
| February | 4°C | −5°C | −0.2°C | 47 mm |
| March | 11°C | 0°C | 5.6°C | 64 mm |
| April | 18°C | 6°C | 11.7°C | 93 mm |
| May | 24°C | 12°C | 17.7°C | 111 mm |
| June | 28°C | 17°C | 22.6°C | 111 mm |
| July | 30°C | 19°C | 24.3°C | 84 mm |
| August | 29°C | 17°C | 23.2°C | 77 mm |
| September | 26°C | 13°C | 19.4°C | 76 mm |
| October | 19°C | 7°C | 12.9°C | 81 mm |
| November | 11°C | 1°C | 5.9°C | 75 mm |
| December | 4°C | −4°C | 0.2°C | 54 mm |
NOAA publishes no 1991-2020 snowfall normal for this station. Decatur's own co-op station Decatur Water Treatment Plant does: 14 inches of snow in a normal year (see the other stations below).
Source: NOAA NCEI 1991-2020 monthly normals, station USW00003887; 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 Decatur?
Normal highs sit between 65°F and 85°F in May, June, August, September and October. 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 USW00003887.
When is the last frost in Decatur?
In Decatur, the last spring frost (32°F) comes after April 16 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 7 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 161 year in 10 after April 30; 9 in 10 after April 3
- First autumn frost, half of years before
- October 221 year in 10 before October 7; 9 in 10 before November 3
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 5 in half of springs and before November 2 in half of autumns.
The USDA's 2023 Plant Hardiness Zone Map puts Decatur in zone 6a across its 4 ZIP code areas (6a in 62526, the largest). By the map's own method, the average of each year's coldest night over 1991-2020, Decatur's NOAA station is in zone 6b: the coldest night of a typical year reaches −4°F (20 complete years); the map interpolates between stations and terrain, so the station and the ZIP area can differ by half a zone.
Frost dates for every NOAA station in Illinois · Frost dates by ZIP code
Source: NOAA NCEI 1991-2020 annual normals (frost probabilities), station USW00003887; USDA 2023 Plant Hardiness Zone Map, ZIP listings: PRISM Group, Oregon State University, https://prism.oregonstate.edu, accessed October 8, 2026; the station zone: ACIS daily lows, 1991-2020. A 10% date means one year in ten has a frost after it in spring or before it in autumn.
What hardiness zone is each Decatur ZIP code in?
The USDA's 2023 map gives each ZIP code area one zone. Decatur's 4 ZIP code areas are all in zone 6a. Each ZIP's frost dates are in the frost tool.
Source: USDA 2023 Plant Hardiness Zone Map, ZIP listings: PRISM Group, Oregon State University, https://prism.oregonstate.edu, accessed October 8, 2026; ZIP areas in the place: Census Bureau 2020 ZCTA-to-place relationship file. A ZIP area counts when at least a tenth of its land (and 0.5 km²) lies in the place.
How hot and how cold does it get in Decatur?
In a normal year in Decatur, 27 days a year reach 90°F (1 of them 100°F or more), 113 nights fall to 32°F or colder and 118 days bring measurable precipitation.
- Days at 90°F or more
- 27a normal year
- Days at 100°F or more
- 1a normal year
- Nights at 32°F or colder
- 113a normal year
- Days that stay below freezing
- 31high of 32°F or less
- Days with 0.01 inch of precipitation
- 118a normal year
The highest temperature in this station's record (since 2000) is 104°F, on July 25, 2012; the lowest is −16°F, on January 30, 2019.
The wettest calendar year on that record is 2008, with 54.42 in.
Source: NOAA NCEI 1991-2020 annual normals, station USW00003887; records: ACIS (NOAA Regional Climate Centers), station USW00003887. Records use complete months only; a month missing more than three days is left out.
How humid and windy is Decatur, and how clear are its skies?
The summer (June-August) dew point averages 64°F. The muggiest month is July, with an average dew point of 65°F, above the 65°F at which most people call the air sticky; the driest air comes in January (22°F). March is the windiest month (11.8 mph on average), and August has the most clear or nearly clear hours (72%).
| Month | Dew point | Humidity | Afternoon humidity | Wind | Clear or few clouds | Overcast |
|---|---|---|---|---|---|---|
| January | 22°F | 76% | 65% | 11.2 mph | 51% | 37% |
| February | 27°F | 72% | 59% | 11.7 mph | 48% | 39% |
| March | 37°F | 67% | 51% | 11.8 mph | 54% | 30% |
| April | 47°F | 67% | 52% | 11.0 mph | 55% | 27% |
| May | 57°F | 68% | 51% | 9.2 mph | 61% | 21% |
| June | 65°F | 71% | 53% | 7.7 mph | 69% | 11% |
| July | 65°F | 73% | 55% | 6.5 mph | 71% | 9% |
| August | 63°F | 73% | 51% | 6.9 mph | 72% | 12% |
| September | 50°F | 68% | 45% | 8.3 mph | 71% | 15% |
| October | 39°F | 70% | 54% | 10.4 mph | 56% | 31% |
| November | 30°F | 75% | 64% | 10.9 mph | 52% | 38% |
| December | 23°F | 78% | 70% | 11.1 mph | 47% | 43% |
Source: NOAA NCEI 1991-2020 hourly normals, station USW00003887. Monthly averages of all hours; afternoon humidity at 3 p.m. local standard time; sky as the share of hours NOAA's observations rate clear or few clouds (a dash where NOAA publishes no sky value).
How sunny is Decatur?
Over a year 74% of the sunlight that would reach the ground under clear skies gets through the clouds in the area around Decatur, 4.1 kWh per square metre a day. July is the sunniest month (83%) and December the cloudiest (61%). Among the 459 U.S. places of 50,000 or more, that ranks tied 330th (with 21 others) in sunshine.
| 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 Decatur need?
A normal year in Decatur brings 5,393 heating degree days, 0% more than the median U.S. place (5,373), and 1,105 cooling degree days, 22% 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 3,646 growing degree days a year above 50°F.
| Month | Heating | Cooling |
|---|---|---|
| January | 1,165 | 0 |
| February | 935 | 0 |
| March | 711 | 3 |
| April | 370 | 13 |
| May | 129 | 94 |
| June | 11 | 242 |
| July | 1 | 333 |
| August | 4 | 273 |
| September | 66 | 124 |
| October | 325 | 22 |
| November | 668 | 1 |
| December | 1,009 | 0 |
| Year (annual) | 5,393 | 1,105 |
Source: NOAA NCEI 1991-2020 monthly normals, station USW00003887. 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 Decatur been warmer than normal lately?
2025 averaged 54.8°F, 1.7°F above the 1991-2020 normal of 53.1°F. 4 of the 5 complete years since 2021 ran warmer than the normal.
| Year | Mean | Against normal | Precip. |
|---|---|---|---|
| 2021 | 54.3°F | +1.2°F | 42.29 |
| 2022 | 52.7°F | −0.4°F | 44.01 |
| 2023 | 55.5°F | +2.4°F | 29.14 |
| 2024 | 56.3°F | +3.2°F | 42.25 |
| 2025 | 54.8°F | +1.7°F | – |
Source: observed years: ACIS, station USW00003887. A year counts only with all twelve months complete.
How does Decatur's climate compare?
Decatur is warmer than 96 of the other 143 places with their own NOAA record in Illinois, and wetter than 7. Across the 7,247 U.S. stations with normals, its annual mean of 53.1°F is warmer than 54%.
In the Köppen classification computed from these normals, Decatur 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 Decatur share this climate record?
26 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.
- Mount Zion, IL5,897 people · 4 mi S
- Forsyth, IL3,656 people · 8 mi NW
- Moweaqua, IL1,724 people · 17 mi SW
- Maroa, IL1,533 people · 15 mi N
- Bement, IL1,452 people · 17 mi E
- Cerro Gordo, IL1,295 people · 8 mi NE
- Harristown, IL1,270 people · 10 mi W
- Long Creek, IL1,232 people · 2 mi SE
- Macon, IL1,133 people · 11 mi SW
- Blue Mound, IL1,090 people · 16 mi SW
- Warrensburg, IL1,086 people · 12 mi NW
- Argenta, IL880 people · 11 mi N
- Oreana, IL858 people · 7 mi N
- Illiopolis, IL834 people · 21 mi W
- Niantic, IL588 people · 16 mi W
- Hammond, IL498 people · 15 mi E
- Mount Auburn, IL440 people · 21 mi W
- Dalton City, IL436 people · 9 mi S
- Boody, IL420 people · 11 mi SW
- Weldon, IL359 people · 21 mi N
- Cisco, IL252 people · 14 mi NE
- De Witt, IL165 people · 25 mi N
- Lane, IL145 people · 20 mi N
- LaPlace, IL130 people · 8 mi E
- Lake City, IL103 people · 10 mi SE
- Elwin, IL33 people · 7 mi SW
Source: Census Bureau 2024 Gazetteer (place centers) and Vintage 2024 population estimates. Distance and direction from the station to each place's center.
What do Decatur's other NOAA stations record?
One more NOAA station keeps 1991-2020 normals in or near Decatur. This page uses Decatur Airport; the other differs by site and elevation:
| Station | From the center | Elevation | Year | Jan low | Jul high | Precip. | Snow | Last frost |
|---|---|---|---|---|---|---|---|---|
| Decatur Water Treatment Plant USC00112193 | 2 mi SW | 639 ft | 53.0°F | 20°F | 85°F | 40.4 | 14 | April 14 |
Source: NOAA NCEI 1991-2020 normals for each station. Last frost: the median last spring date of 32°F.
Questions about Decatur's climate
What is the coldest month in Decatur?
January, with a normal high of 35°F and a normal low of 20°F. The warmest is July (86°F by day, 66°F at night).
Does it snow in Decatur?
Yes. NOAA publishes no 1991-2020 snowfall normal for Decatur's NOAA station; Decatur's own co-op station Decatur Water Treatment Plant records 14 inches of snow in a normal year.
What is the rainiest month in Decatur?
May, with 4.38 in of precipitation in a normal year; the driest is February with 1.84 in.
What plant hardiness zone is Decatur in?
The USDA's 2023 map puts Decatur in zone 6a across its 4 ZIP code areas (6a in 62526, the largest). By the same method at Decatur's NOAA station, the coldest night of a typical year reaches −4°F (20 years, 1991-2020): zone 6b.
How late can frost come in Decatur?
In half of years the last 32°F night comes after April 16; 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
- Decatur in OctoberThis month day by day, its records and recent years
- Illinois climateThe statewide normals and every place in the state
- Sullivan, IL24 miles from this station, 53.0°F a year
- Shelbyville, IL30 miles from this station, 53.6°F a year
- Windsor, IL30 miles from this station, 52.6°F a year
- Tuscola, IL30 miles from this station, 52.4°F a year
- Compare two placesMonth by month, side by side
Download these normals (CSV) · JSON · How these figures are made