Ottawa, Illinois climate: Normal weather by month, frost dates and records
Ottawa's normal year runs from 15°F January nights to 83°F July afternoons, with 36.2 inches of precipitation and 24 inches of snow a year. (NOAA 1991-2020 normals)
In metric: a yearly mean of 10.1°C, January lows of −9°C, July highs of 28°C, 919 mm of precipitation a year.
- NOAA stationOttawa (5 mi SW)
- Station IDUSC00116526
- Location41.33°N 88.91°W
- Elevation525 ft (160 m)
- Population18,775
On October 23, the normal high in Ottawa is 61°F and the normal low 40°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 Ottawa.
- Jan 31° 15°
- Feb 35° 19°
- Mar 48° 29°
- Apr 61° 39°
- May 72° 51°
- Jun 80° 61°
- Jul 83° 64°
- Aug 82° 62°
- Sep 76° 54°
- Oct 64° 43°
- Nov 49° 32°
- Dec 36° 21°
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 Ottawa each month?
July is the warmest month, with normal highs of 83°F; January the coldest, with normal lows of 15°F. The most precipitation falls in May (4.48 in), the least in February (1.54 in).
| Month | High | Low | Mean | Precip. | Snow | Wet days | 90°F days | Freezing nights | Daylight |
|---|---|---|---|---|---|---|---|---|---|
| January | 31°F | 15°F | 22.9°F | 1.78 | 9.2 | 9 | <1 | 29 | 9 h 30 min |
| February | 35°F | 19°F | 27.0°F | 1.54 | 5.2 | 8 | <1 | 25 | 10 h 40 min |
| March | 48°F | 29°F | 38.3°F | 2.29 | 3.3 | 9 | <1 | 21 | 11 h 50 min |
| April | 61°F | 39°F | 50.3°F | 3.53 | 0.4 | 12 | <1 | 6 | 13 h 20 min |
| May | 72°F | 51°F | 61.3°F | 4.48 | 0.0 | 12 | 1 | <1 | 14 h 30 min |
| June | 80°F | 61°F | 70.2°F | 4.40 | 0.0 | 11 | 4 | <1 | 15 h 10 min |
| July | 83°F | 64°F | 73.7°F | 3.63 | 0.0 | 8 | 6 | <1 | 14 h 50 min |
| August | 82°F | 62°F | 71.9°F | 3.72 | 0.0 | 9 | 5 | <1 | 13 h 50 min |
| September | 76°F | 54°F | 65.3°F | 3.23 | 0.0 | 8 | 3 | <1 | 12 h 30 min |
| October | 64°F | 43°F | 53.3°F | 3.10 | 0.0 | 9 | <1 | 4 | 11 h 10 min |
| November | 49°F | 32°F | 40.0°F | 2.50 | 0.8 | 9 | <1 | 17 | 9 h 50 min |
| December | 36°F | 21°F | 28.5°F | 1.99 | 4.8 | 9 | <1 | 27 | 9 h 10 min |
| Year (annual) | 60°F | 41°F | 50.2°F | 36.19 | 23.7 | 112 | 17 | 129 |
The same normals in °C and millimetres
| Month | High | Low | Mean | Precip. | Snow |
|---|---|---|---|---|---|
| January | −1°C | −9°C | −5.1°C | 45 mm | 23 cm |
| February | 2°C | −7°C | −2.8°C | 39 mm | 13 cm |
| March | 9°C | −2°C | 3.5°C | 58 mm | 8 cm |
| April | 16°C | 4°C | 10.2°C | 90 mm | 1 cm |
| May | 22°C | 11°C | 16.3°C | 114 mm | 0 cm |
| June | 27°C | 16°C | 21.2°C | 112 mm | 0 cm |
| July | 28°C | 18°C | 23.2°C | 92 mm | 0 cm |
| August | 28°C | 17°C | 22.2°C | 94 mm | 0 cm |
| September | 25°C | 12°C | 18.5°C | 82 mm | 0 cm |
| October | 18°C | 6°C | 11.8°C | 79 mm | 0 cm |
| November | 9°C | 0°C | 4.4°C | 64 mm | 2 cm |
| December | 2°C | −6°C | −1.9°C | 51 mm | 12 cm |
Source: NOAA NCEI 1991-2020 monthly normals, station USC00116526; 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 Ottawa?
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 (8).
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 USC00116526.
When is the last frost in Ottawa?
In Ottawa, the last spring frost (32°F) comes after April 20 in half of years and after May 5 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 201 year in 10 after May 5; 9 in 10 after April 6
- First autumn frost, half of years before
- October 221 year in 10 before October 7; 9 in 10 before November 4
NOAA puts the median frost-free season at 183 days, and the season without a hard freeze (28°F) at 206 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 Ottawa in zone 5b in ZIP code 61350, the nearest ZIP area. By the map's own method, the average of each year's coldest night over 1991-2020, Ottawa's NOAA station is in zone 6a: the coldest night of a typical year reaches −10°F (29 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 USC00116526; 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.
How hot and how cold does it get in Ottawa?
In a normal year in Ottawa, 17 days a year reach 90°F, 129 nights fall to 32°F or colder and 112 days bring measurable precipitation.
- Days at 90°F or more
- 17a normal year
- Days at 100°F or more
- fewer than 1a normal year
- Nights at 32°F or colder
- 129a normal year
- Days that stay below freezing
- 39high of 32°F or less
- Days with 0.01 inch of precipitation
- 112a normal year
- Days with 1 inch of snow or more
- 10a normal year
The highest temperature in this station's record (since 1890) is 112°F, on July 21, 1901; the lowest is −26°F, on January 17, 1893.
The wettest calendar year on that record is 1902, with 55.45 in; the snowiest season is 1925–26, with 72.6 inches.
Source: NOAA NCEI 1991-2020 annual normals, station USC00116526; records: ACIS (NOAA Regional Climate Centers), station USC00116526. Records use complete months only; a month missing more than three days is left out.
How sunny is Ottawa?
Over a year 73% of the sunlight that would reach the ground under clear skies gets through the clouds in the area around Ottawa, 3.9 kWh per square metre a day. July is the sunniest month (81%) and January the cloudiest (61%).
| Month | Share of clear-sky sunshine | Sunshine, kWh/m² a day |
|---|---|---|
| January | 61% | 1.7 |
| February | 65% | 2.6 |
| March | 67% | 3.6 |
| April | 70% | 4.8 |
| May | 71% | 5.5 |
| June | 77% | 6.1 |
| July | 81% | 6.2 |
| August | 80% | 5.4 |
| September | 80% | 4.5 |
| October | 71% | 3.0 |
| November | 67% | 2.0 |
| December | 61% | 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 Ottawa need?
A normal year in Ottawa brings 6,211 heating degree days, 16% more than the median U.S. place (5,373), and 865 cooling degree days, 5% fewer 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 3,202 growing degree days a year above 50°F.
| Month | Heating | Cooling |
|---|---|---|
| January | 1,303 | 0 |
| February | 1,064 | 0 |
| March | 829 | 2 |
| April | 449 | 8 |
| May | 177 | 63 |
| June | 28 | 184 |
| July | 4 | 272 |
| August | 9 | 224 |
| September | 90 | 99 |
| October | 376 | 14 |
| November | 750 | 0 |
| December | 1,131 | 0 |
| Year (annual) | 6,211 | 865 |
Source: NOAA NCEI 1991-2020 monthly normals, station USC00116526. 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 Ottawa been warmer than normal lately?
2025 averaged 50.6°F, 0.4°F above the 1991-2020 normal of 50.2°F. 3 of the 3 complete years since 2023 ran warmer than the normal.
| Year | Mean | Against normal | Precip. | Snow |
|---|---|---|---|---|
| 2023 | 51.5°F | +1.3°F | 31.70 | 11.0 |
| 2024 | 52.4°F | +2.2°F | 38.90 | 19.8 |
| 2025 | 50.6°F | +0.4°F | 28.53 | 24.5 |
Over 106 complete years from 1890 to 2025, the station's annual mean temperature rose at a fitted 1.5°F per century (+0.08°C per decade). A fitted rate summarizes the whole record; single years swing well above and below it.
Source: observed years: ACIS, station USC00116526; trend: NOAA GHCN-Monthly v4. A year counts only with all twelve months complete.
How does Ottawa's climate compare?
Ottawa is warmer than 37 of the other 143 places with their own NOAA record in Illinois, and wetter than 4. Across the 7,247 U.S. stations with normals, its annual mean of 50.2°F is warmer than 41%.
In the Köppen classification computed from these normals, Ottawa 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 Ottawa share this climate record?
2 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.
- Naplate, IL414 people · 2 mi E
- Leonore, IL121 people · 10 mi S
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 Ottawa's climate
What is the coldest month in Ottawa?
January, with a normal high of 31°F and a normal low of 15°F. The warmest is July (83°F by day, 64°F at night).
Does it snow in Ottawa?
Yes: 24 inches in a normal year, falling on 14 days with 0.1 inch or more, the most in January (9.2 inches).
What is the rainiest month in Ottawa?
May, with 4.48 in of precipitation in a normal year; the driest is February with 1.54 in.
What plant hardiness zone is Ottawa in?
The USDA's 2023 map puts Ottawa in zone 5b in ZIP code 61350, the nearest ZIP area. By the same method at Ottawa's NOAA station, the coldest night of a typical year reaches −10°F (29 years, 1991-2020): zone 6a.
How late can frost come in Ottawa?
In half of years the last 32°F night comes after April 20; in one year in ten it comes after May 5. Waiting until May 5 leaves about a one-in-ten chance of a later frost.
Nearby and next
- Illinois climateThe statewide normals and every place in the state
- Marseilles, IL8 miles from this station, 49.7°F a year
- Peru, IL11 miles from this station, 49.6°F a year
- Streator, IL17 miles from this station, 51.6°F a year
- Hennepin, IL23 miles from this station, 51.4°F a year
- Compare two placesMonth by month, side by side
Download these normals (CSV) · JSON · How these figures are made