Mason City weather by month
What a typical year looks like in Mason City, Illinois: average highs and lows, rain and snow, and the warmest and coldest days on record. Normals are the 1991–2020 averages at Petersburg 1e, 24 km from Mason City; records cover 2014–2026.
The year at a glance
- Precipitation a year
- 37.7 in956 mm
- Snow a year
- 15 in39 cm
- Last spring freeze
- Apr 16
- First fall freeze
- Oct 16
- Heating degree days
- 5,644
- Cooling degree days
- 1,056
Freeze dates are medians (half of years earlier, half later). Degree days use a 65°F base; HVAC design temperatures here are −4°−20° (heating, 99%) and 92°33° (cooling, 1%).
Month by month
Mason City in January
Highs average 34°1° and lows 17°−8°, with 2 in51 mm of precipitation and 5 in14 cm of snow in a typical January.
- Record high
- 67°19° (2023)
- Record low
- −16°−27° (2019)
- Heating degree days
- 1213
- Cooling degree days
- 0
Mason City in February
Highs average 39°4° and lows 21°−6°, with 1.8 in44 mm of precipitation and 2.7 in6.9 cm of snow in a typical February.
- Record high
- 81°27° (2024)
- Record low
- −10°−23° (2015)
- Heating degree days
- 982
- Cooling degree days
- 0
Mason City in March
Highs average 51°10° and lows 30°−1°, with 2.5 in64 mm of precipitation and 1.4 in3.6 cm of snow in a typical March.
- Record high
- 92°33° (2026)
- Record low
- −2°−19° (2019)
- Heating degree days
- 758
- Cooling degree days
- 0
Mason City in April
Highs average 64°18° and lows 41°5°, with 4.1 in104 mm of precipitation and 0.5 in1.3 cm of snow in a typical April.
- Record high
- 88°31° (2026)
- Record low
- 12°−11° (2018)
- Heating degree days
- 385
- Cooling degree days
- 10
Mason City in May
Highs average 74°23° and lows 53°12°, with 4.6 in117 mm of precipitation in a typical May.
- Record high
- 94°34° (2018)
- Record low
- 31°−1° (2020)
- Heating degree days
- 126
- Cooling degree days
- 79
Mason City in June
Highs average 82°28° and lows 62°17°, with 4.1 in105 mm of precipitation in a typical June.
- Record high
- 98°37° (2025)
- Record low
- 48°9° (2015)
- Heating degree days
- 8
- Cooling degree days
- 225
Mason City in July
Highs average 86°30° and lows 65°19°, with 3.9 in98 mm of precipitation in a typical July.
- Record high
- 96°36° (2022)
- Record low
- 46°8° (2015)
- Heating degree days
- 1
- Cooling degree days
- 325
Mason City in August
Highs average 85°29° and lows 63°17°, with 3.7 in93 mm of precipitation in a typical August.
- Record high
- 97°36° (2023)
- Record low
- 44°7° (2015)
- Heating degree days
- 3
- Cooling degree days
- 269
Mason City in September
Highs average 79°26° and lows 55°13°, with 3.1 in79 mm of precipitation in a typical September.
- Record high
- 98°37° (2025)
- Record low
- 36°2° (2022)
- Heating degree days
- 70
- Cooling degree days
- 125
Mason City in OctoberThis month
Highs average 66°19° and lows 43°6°, with 3.2 in81 mm of precipitation in a typical October.
- Record high
- 90°32° (2019)
- Record low
- 25°−4° (2014)
- Heating degree days
- 342
- Cooling degree days
- 21
Mason City in November
Highs average 52°11° and lows 32°0°, with 2.8 in70 mm of precipitation and 1.2 in3 cm of snow in a typical November.
- Record high
- 80°27° (2016)
- Record low
- 5°−15° (2019)
- Heating degree days
- 698
- Cooling degree days
- 2
Mason City in December
Highs average 39°4° and lows 23°−5°, with 1.9 in49 mm of precipitation and 4 in10 cm of snow in a typical December.
- Record high
- 71°22° (2025)
- Record low
- −10°−23° (2022)
- Heating degree days
- 1058
- Cooling degree days
- 0