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Monthly climate in O'Fallon, Illinois, United States

Last updated: May 6, 2024

Under the Köppen–Geiger climate classification O'Fallon features a humid subtropical climate (Cfa).Temperatures typically range between 1 °C (33 °F) and 27 °C (80 °F) through the year, but rarely can drop to -20 °C (-5 °F) or can rise to as high as 39 °C (102 °F).The average annual precipitation amounts to about 1239 mm (48.8 inches) and receives 119 rainy days on the 1 mm (0.04 inches) threshold annually.O'Fallon enjoys an average of 2963 hours of sunshine throughout the year, and daylight varies from 9 hours 30 minutes to 14 hours 48 minutes per day.

Monthly average temperatures in O'Fallon, Illinois, United States §

The warmest months in O'Fallon are June, July and August, with daily mean temperatures ranging from 25 to 27 °C (77 - 80 °F) throughout the day. The coldest temperatures usually occur in January, February and December, when daily mean temperatures range from 1 to 4 °C (33 - 40 °F) throughout the day. On average each year, O'Fallon experiences 123 days above 25 °C (77.0 °F) and 72 days below 0 °C (32.0 °F).

Monthly average temperatures with daily means, minimums and maximums for period, average high and low temperatures in O'Fallon, Illinois, United States.Generated using Copernicus Climate Change Service information.Data for period from January 2017 to June 2022.
  Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Maximum 20°C
(68°F)		 25°C
(77°F)		 27°C
(82°F)		 31°C
(88°F)		 35°C
(95°F)		 36°C
(97°F)		 39°C
(102°F)		 36°C
(97°F)		 34°C
(94°F)		 34°C
(94°F)		 25°C
(78°F)		 23°C
(73°F)
Mean maximum 17°C
(62°F)		 21°C
(69°F)		 25°C
(76°F)		 29°C
(84°F)		 33°C
(91°F)		 34°C
(94°F)		 36°C
(97°F)		 35°C
(95°F)		 34°C
(93°F)		 32°C
(90°F)		 23°C
(73°F)		 20°C
(69°F)
Average high 5°C
(41°F)		 7°C
(46°F)		 14°C
(57°F)		 19°C
(66°F)		 25°C
(77°F)		 30°C
(86°F)		 31°C
(89°F)		 30°C
(86°F)		 28°C
(83°F)		 21°C
(70°F)		 12°C
(54°F)		 9°C
(49°F)
Daily mean 1°C
(33°F)		 2°C
(36°F)		 9°C
(48°F)		 13°C
(56°F)		 20°C
(68°F)		 25°C
(77°F)		 27°C
(80°F)		 25°C
(77°F)		 23°C
(73°F)		 15°C
(60°F)		 7°C
(45°F)		 4°C
(40°F)
Average low -3°C
(26°F)		 -2°C
(28°F)		 4°C
(39°F)		 8°C
(46°F)		 15°C
(60°F)		 20°C
(69°F)		 22°C
(72°F)		 20°C
(68°F)		 17°C
(63°F)		 10°C
(51°F)		 2°C
(36°F)		 -0°C
(32°F)
Mean minimum -15°C
(4°F)		 -14°C
(6°F)		 -8°C
(19°F)		 -0°C
(32°F)		 7°C
(45°F)		 15°C
(60°F)		 17°C
(64°F)		 14°C
(58°F)		 9°C
(49°F)		 0°C
(33°F)		 -6°C
(21°F)		 -10°C
(14°F)
Minimum -20°C
(-5°F)		 -18°C
(-1°F)		 -16°C
(3°F)		 -4°C
(25°F)		 4°C
(39°F)		 12°C
(54°F)		 16°C
(62°F)		 13°C
(55°F)		 8°C
(46°F)		 -3°C
(28°F)		 -12°C
(11°F)		 -16°C
(3°F)

Compare climate in O'Fallon with other places

Compare temperature, precipitation, sunshine, UV index in O'Fallon, Illinois, United States with other places around the world.

Monthly average precipitation in O'Fallon, Illinois, United States §

O'Fallon usually has the most precipitation in March, April and May, with an average of 12 rainy days and 148 mm (5.8 inches) of precipitation per month.The driest months in O'Fallon are September, November and December. On average, 68 mm (2.7 inches) of precipitation falls during these months.

Monthly average precipitation and precipitation days in O'Fallon, Illinois, United States.Generated using Copernicus Climate Change Service information.Data for period from January 2017 to January 2022.
  Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Average precipitation mm (inches) 80 (3.2) 83 (3.3) 139 (5.5) 148 (5.8) 158 (6.2) 100 (4.0) 91 (3.6) 129 (5.1) 59 (2.3) 107 (4.2) 77 (3.0) 67 (2.7)
Average precipitation days (≥ 1 mm) 8 10 13 11 14 11 10 11 6 10 8 7

Monthly sunshine hours in O'Fallon, Illinois, United States §

The sunniest months in O'Fallon are June, July and August, when the sun shines an average of 10 hours 52 minutes a day.Least sunny months in O'Fallon: January, February and December receive an average of 5 hours 24 minutes of sunshine daily.

Mean monthly sunshine duration, average sunshine hours per day and mean daily daylight hours in O'Fallon, Illinois, United States.Generated using Copernicus Atmosphere Monitoring Service information.Data for period from January 2017 to January 2022.
  Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Mean monthly sunshine hours 125 174 203 239 286 325 348 326 289 238 214 191
Mean daily daylight hours 9h 48m 10h 45m 11h 58m 13h 13m 14h 17m 14h 48m 14h 31m 13h 35m 12h 22m 11h 8m 10h 3m 9h 30m

Solar UV radiation in O'Fallon, Illinois, United States §

O'Fallon experiences the highest level of ultraviolet (UV) radiation in May, June and August, when the maximum UV index can reach values of 10 - 11, which corresponds to the Very High / Extreme category of sun exposure.January, February and December are in the Moderate exposure category. In these months, the maximum values of the UV index do not exceed 5. For more detailed information, check out our hourly O'Fallon UV Index data.

Average daily maximum UV levels by month for O'Fallon, Illinois, United States.Generated using Copernicus Atmosphere Monitoring Service information.Data for period from January 2022 to January 2023.
  Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
UV Index (month maximum) 3 5 7 9 10 11 10 10 9 7 5 3