Mason City, Illinois climate: Normal weather by month, frost dates and records
Mason City's normal year runs from 16°F January nights to 85°F July afternoons, with 39.3 inches of precipitation and 13 inches of snow a year. (NOAA 1991-2020 normals)
In metric: a yearly mean of 10.9°C, January lows of −9°C, July highs of 30°C, 997 mm of precipitation a year.
- NOAA stationMason City (4 mi SE)
- Station IDUSC00115413
- Location40.16°N 89.65°W
- Elevation600 ft (183 m)
- From the center4 miles
- Population1,982
On October 23, the normal high in Mason City is 62°F and the normal low 40°F, and the chance of measurable rain or snow is 28%.
Normals describe a typical year at the station, not a forecast. The National Weather Service forecast for Mason City.
- Jan 33° 16°
- Feb 38° 19°
- Mar 51° 30°
- Apr 64° 41°
- May 74° 53°
- Jun 82° 62°
- Jul 85° 65°
- Aug 84° 62°
- Sep 79° 54°
- Oct 66° 43°
- Nov 51° 31°
- Dec 38° 22°
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 Mason City each month?
July is the warmest month, with normal highs of 85°F; January the coldest, with normal lows of 16°F. The most precipitation falls in July (5.14 in), the least in December (1.70 in).
| Month | High | Low | Mean | Precip. | Snow | Wet days | 90°F days | Freezing nights | Daylight |
|---|---|---|---|---|---|---|---|---|---|
| January | 33°F | 16°F | 24.6°F | 2.34 | 5.5 | 8 | <1 | 28 | 9 h 40 min |
| February | 38°F | 19°F | 28.6°F | 1.96 | 2.8 | 7 | <1 | 24 | 10 h 40 min |
| March | 51°F | 30°F | 40.1°F | 2.49 | 1.7 | 8 | <1 | 19 | 11 h 50 min |
| April | 64°F | 41°F | 52.8°F | 3.73 | 0.0 | 11 | <1 | 5 | 13 h 10 min |
| May | 74°F | 53°F | 63.3°F | 4.47 | 0.0 | 11 | 1 | <1 | 14 h 20 min |
| June | 82°F | 62°F | 72.1°F | 4.20 | 0.0 | 10 | 6 | <1 | 15 h |
| July | 85°F | 65°F | 74.9°F | 5.14 | 0.0 | 9 | 10 | <1 | 14 h 50 min |
| August | 84°F | 62°F | 72.8°F | 3.57 | 0.0 | 8 | 7 | <1 | 13 h 50 min |
| September | 79°F | 54°F | 66.3°F | 3.55 | 0.0 | 7 | 3 | <1 | 12 h 30 min |
| October | 66°F | 43°F | 54.3°F | 3.33 | 0.0 | 9 | <1 | 4 | 11 h 10 min |
| November | 51°F | 31°F | 40.7°F | 2.78 | 0.3 | 7 | <1 | 17 | 10 h |
| December | 38°F | 22°F | 29.8°F | 1.70 | 2.6 | 6 | <1 | 26 | 9 h 20 min |
| Year (annual) | 62°F | 41°F | 51.7°F | 39.26 | 12.9 | 99 | 27 | 122 |
The same normals in °C and millimetres
| Month | High | Low | Mean | Precip. | Snow |
|---|---|---|---|---|---|
| January | 1°C | −9°C | −4.1°C | 59 mm | 14 cm |
| February | 3°C | −7°C | −1.9°C | 50 mm | 7 cm |
| March | 10°C | −1°C | 4.5°C | 63 mm | 4 cm |
| April | 18°C | 5°C | 11.6°C | 95 mm | 0 cm |
| May | 23°C | 12°C | 17.4°C | 114 mm | 0 cm |
| June | 28°C | 17°C | 22.3°C | 107 mm | 0 cm |
| July | 30°C | 18°C | 23.8°C | 131 mm | 0 cm |
| August | 29°C | 17°C | 22.7°C | 91 mm | 0 cm |
| September | 26°C | 12°C | 19.1°C | 90 mm | 0 cm |
| October | 19°C | 6°C | 12.4°C | 85 mm | 0 cm |
| November | 11°C | −1°C | 4.8°C | 71 mm | 1 cm |
| December | 3°C | −6°C | −1.2°C | 43 mm | 7 cm |
Source: NOAA NCEI 1991-2020 monthly normals, station USC00115413; 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 Mason City?
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 (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 USC00115413.
When is the last frost in Mason City?
In Mason City, the last spring frost (32°F) comes after April 17 in half of years and after May 4 in one year in ten; the first autumn frost comes before October 18 in half of years and before October 4 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 171 year in 10 after May 4; 9 in 10 after April 4
- First autumn frost, half of years before
- October 181 year in 10 before October 4; 9 in 10 before October 30
NOAA puts the median frost-free season at 182 days, and the season without a hard freeze (28°F) at 203 days.
A hard freeze (28°F) comes after April 6 in half of springs and before October 29 in half of autumns.
The USDA's 2023 Plant Hardiness Zone Map puts Mason City in zone 6a in ZIP code 62664, 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 USC00115413; 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 Mason City?
In a normal year in Mason City, 27 days a year reach 90°F, 122 nights fall to 32°F or colder and 99 days bring measurable precipitation.
- Days at 90°F or more
- 27a 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
- 34high of 32°F or less
- Days with 0.01 inch of precipitation
- 99a normal year
- Days with 1 inch of snow or more
- 5a normal year
The highest temperature in this station's record (since 1962) is 103°F, on June 25, 1988; the lowest is −24°F, on January 17, 1977.
The wettest calendar year on that record is 1993, with 55.36 in; the snowiest season is 1977–78, with 45.0 inches.
Source: NOAA NCEI 1991-2020 annual normals, station USC00115413; records: ACIS (NOAA Regional Climate Centers), station USC00115413. Records use complete months only; a month missing more than three days is left out.
How sunny is Mason City?
Over a year 74% of the sunlight that would reach the ground under clear skies gets through the clouds in the area around Mason City, 4.1 kWh per square metre a day. July is the sunniest month (83%) and December the cloudiest (62%).
| Month | Share of clear-sky sunshine | Sunshine, kWh/m² a day |
|---|---|---|
| January | 64% | 1.9 |
| February | 67% | 2.8 |
| March | 68% | 3.7 |
| April | 71% | 4.9 |
| May | 72% | 5.6 |
| June | 79% | 6.3 |
| July | 83% | 6.4 |
| August | 81% | 5.6 |
| September | 81% | 4.7 |
| October | 73% | 3.2 |
| November | 70% | 2.2 |
| December | 62% | 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 Mason City need?
A normal year in Mason City brings 5,821 heating degree days, 8% more than the median U.S. place (5,373), and 1,006 cooling degree days, 11% 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 3,481 growing degree days a year above 50°F.
| Month | Heating | Cooling |
|---|---|---|
| January | 1,251 | 0 |
| February | 1,018 | 0 |
| March | 774 | 1 |
| April | 378 | 10 |
| May | 135 | 81 |
| June | 16 | 228 |
| July | 1 | 308 |
| August | 6 | 248 |
| September | 73 | 112 |
| October | 348 | 18 |
| November | 729 | 1 |
| December | 1,093 | 0 |
| Year (annual) | 5,821 | 1,006 |
Source: NOAA NCEI 1991-2020 monthly normals, station USC00115413. 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.
How does Mason City's climate compare?
Mason City is warmer than 69 of the other 143 places with their own NOAA record in Illinois, and wetter than 57. Across the 7,247 U.S. stations with normals, its annual mean of 51.7°F is warmer than 48%.
In the Köppen classification computed from these normals, Mason 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 Mason City share this climate record?
6 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.
- Delavan, IL1,545 people · 15 mi N
- Green Valley, IL599 people · 17 mi N
- San Jose, IL469 people · 10 mi N
- Middletown, IL323 people · 6 mi SE
- Easton, IL302 people · 11 mi NW
- New Holland, IL267 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 Mason City's climate
What is the coldest month in Mason City?
January, with a normal high of 33°F and a normal low of 16°F. The warmest is July (85°F by day, 65°F at night).
Does it snow in Mason City?
Yes: 13 inches in a normal year, falling on 11 days with 0.1 inch or more, the most in January (5.5 inches).
What is the rainiest month in Mason City?
July, with 5.14 in of precipitation in a normal year; the driest is December with 1.70 in.
What plant hardiness zone is Mason City in?
The USDA's 2023 map puts Mason City in zone 6a in ZIP code 62664, the nearest ZIP area.
How late can frost come in Mason City?
In half of years the last 32°F night comes after April 17; in one year in ten it comes after May 4. Waiting until May 4 leaves about a one-in-ten chance of a later frost.
Nearby and next
- Illinois climateThe statewide normals and every place in the state
- Petersburg, IL14 miles from this station, 52.3°F a year
- Lincoln, IL17 miles from this station, 52.2°F a year
- Athens, IL18 miles from this station, 52.0°F a year
- Springfield, IL22 miles from this station, 54.0°F a year
- Compare two placesMonth by month, side by side
Download these normals (CSV) · JSON · How these figures are made