Photo: Robert Lawton · CC BY-SA 2.5Neither notably warm, cold, wet nor dry, O'Fallon sits near the middle of U.S. towns
January mornings start at about 23°. Temperatures cover 65° across the year, low to high. Its July heat outdoes that of 69% of U.S. towns. Seedlings are safe outdoors from about April 12, with the first autumn frost due around October 20.
A year in O'Fallon at a glance
Each month’s typical high and low, and what it brings.
What stands out about O'Fallon’s climate
Compared with towns across the U.S., with O'Fallon’s place in Illinois beside it.
What a year in O'Fallon feels like
Typical highs and lows by season and by month, from NOAA’s 30-year normals.
About 189 frost-free days a year
In half of years the last 32° night comes by April 12 and the first by October 20. Frost and day counts come from NOAA’s station at Belleville Siu Rsch, IL, 7 miles from the town’s center, at about the same height.
The 2023 USDA map puts O'Fallon’s center in zone 7a: the coldest night of a typical winter, averaged over 1991–2020, reaches about 1°. Plant labels use the zone; frost dates tell you when to plant.
Freezing nights by month
Average nights at or below 32°
90° days by month
Average days at 90° or more
About 11 inches of snow a winter
Snow, measured at Belleville Siu Rsch, IL, is heaviest from January through February, about 3 inches a month. Precipitation (rain plus melted snow) adds up to 43 inches a year; May is the wettest month and February the driest. Rain or snow falls on about 100 days a year.
Rain + melted snow by month
Inches of water
Snowfall by month
Inches of snow
Heating and cooling a home in O'Fallon
73% of the work is heating. Months averaging under 55° run from November through March.
Degree days add up how far each day’s average falls below (heating) or rises above (cooling) 65° over a year. NOAA normals at Belleville Siu Rsch, IL.
Living with O'Fallon’s climate
What the numbers mean for a garden, a free afternoon and a bad-weather day.
189 frost-free days in zone 7a. The last frost comes by Apr 12 in half of years, so many gardeners wait a week or two past that to set out tomatoes. The first fall frost comes by Oct 20.
Months with highs of 62–84° and lows of 40° or more: April, May, September and October. 3.6 miles of trails are waiting.
Parks & trails →Of the yearly building losses FEMA expects in O'Fallon from natural hazards, river flooding is the largest share (32%); heat waves, cold waves, winter storms and ice storms together are 26%. Normals are averages; the hazards page covers the extremes.
Natural hazards →O'Fallon and its neighbors
Every town next door is within a degree of O'Fallon in January and July, closer than the 5 km grid can tell apart. Tap a town to see its climate.
Climates most like O'Fallon’s, farther away
TownRatings’ own comparison of NOAA 1991–2020 normals, not a NOAA product: places of 5,000+ at least 150 miles away, one per state, closest on January low, July high, precipitation and snow.
| Town | Jan. low | July high | Precip. | Snow |
|---|---|---|---|---|
| O'Fallon, IL | 23° | 88° | 43" | 11" |
| Lake of the Woods, VA | 25° | 88° | 44" | 11" |
| Springfield, MO | 23° | 88° | 45" | 14" |
| Louisburg, KS | 21° | 88° | 41" | 14" |
| Hurricane, WV | 25° | 87° | 44" | 11" |
Temperatures and rain come from NOAA’s 5 km grid, so each town gets its own value. Snow and frost-free days come from the nearest weather station, so neighbors often share them.
Warmer than it used to be
The last 30 years in O'Fallon averaged 1.1° warmer than the 1900s. The biggest change is in December (+2.2°).
A smaller shift than in 69% of U.S. towns.
Change by month
The two periods overlap (1991–2000 is in both), which makes the gap a little smaller than it would be between separate periods.
How O'Fallon compares
Against 12,954 U.S. towns of 2,500 or more (one per 5 km climate cell), with O'Fallon’s place among Illinois’.
O'Fallon weather by month
Highs, lows and precipitation from NOAA’s grid at O'Fallon’s center; snow, wet days and snow on the ground from Belleville Siu Rsch, IL. 1991–2020 normals.
| Month | High | Low | Rain + melt | Snow | Wet days | Snow on ground | 30-yr range |
|---|---|---|---|---|---|---|---|
| January | 40° | 23° | 2.6" | 3.4" | 7 | 6 | 26° – 42° |
| February | 45° | 27° | 2.3" | 3.3" | 8 | 2 | 26° – 46° |
| March | 56° | 36° | 3.5" | 1.0" | 10 | 0 | 39° – 59° |
| April | 67° | 45° | 4.9" | 0.0" | 10 | 0 | 49° – 62° |
| May | 77° | 56° | 5.1" | 0.0" | 12 | 0 | 61° – 74° |
| June | 85° | 65° | 4.4" | 0.0" | 9 | 0 | 70° – 79° |
| July | 88° | 68° | 3.9" | 0.0" | 7 | 0 | 73° – 85° |
| August | 87° | 66° | 3.4" | 0.0" | 7 | 0 | 71° – 82° |
| September | 81° | 58° | 3.3" | 0.0" | 7 | 0 | 64° – 75° |
| October | 69° | 47° | 3.2" | 0.0" | 8 | 0 | 53° – 64° |
| November | 55° | 36° | 3.5" | 0.6" | 8 | 1 | 38° – 53° |
| December | 44° | 28° | 2.7" | 2.9" | 8 | 3 | 22° – 44° |
| Year | 66° | 46° | 42.7" | 11.2" | 100 | 12 |
How much a month can swing
A normal is an average. December swings the most: from 1991 to 2020 its average ranged from 22° to 44°. June is the most reliable, staying within 9°.
Where O'Fallon’s numbers come from
Normals are 30-year averages, not a forecast. How TownRatings reads climate data →
- Temperatures and precipitation: NOAA’s gridded 1991–2020 normals for the 5 km cell at O'Fallon’s center.
- Snow: Belleville Siu Rsch, IL.
- Frost, freezing nights, 90° and 100° days, heating and cooling: Belleville Siu Rsch, IL, 7 miles from the town’s center, at about the same height; the nearest station that passes our checks for distance, height and agreement with the grid.
- Growing zone: USDA Plant Hardiness Zone Map, 2023. USDA Agricultural Research Service and PRISM Group, Oregon State University, prism.oregonstate.edu/phzm, accessed 8 Oct 2026.
- Then and now: NOAA’s gridded normals for 1901–2000 at the same cell.
- Storms and extremes: FEMA National Risk Index (expected annual losses to buildings).
Common questions about O'Fallon’s climate
What is the weather like in O'Fallon?
A typical July afternoon reaches 88°, a January night drops to 23°, and about 11 inches of snow falls in a normal winter. Milder January nights than 56% of U.S. towns; hotter July afternoons than 69% of U.S. towns.
How cold does it get in O'Fallon in winter?
January nights average 23°. About 93 nights a year reach 32° or below and 2 reach 0° or below. In the coldest January of the last 30 years the month averaged 26°.
How much snow does O'Fallon get?
About 11 inches in a normal winter at Belleville Siu Rsch, IL, heaviest from January through February, about 3 inches a month, with snow on the ground about 12 days a year.
How hot does it get in O'Fallon?
July, with a typical high of 88° and a low of 68°. About 53 days a year reach 90° and 2 reach 100° (Belleville Siu Rsch, IL).
When is the first and last frost in O'Fallon?
In half of years the last spring frost (32°) comes by April 12 and the first fall frost by October 20, about 189 frost-free days, at Belleville Siu Rsch, IL.
What growing zone is O'Fallon in?
Zone 7a on the 2023 USDA Plant Hardiness Zone Map: the coldest night of a typical winter reaches about 1°.
What is the best time of year to visit O'Fallon?
By one simple rule (highs of 62–84° and lows of 40° or more), April, May, September and October. The rule is ours, not NOAA’s; the month table has the numbers to judge by.
Is O'Fallon getting warmer?
Yes: 1991–2020 averaged 1.1° warmer than 1901–2000 on NOAA’s grid. A smaller shift than 69% of U.S. towns.
Neighbors talk weather
Gardens, snow, and the best days of the year in O'Fallon.
Source: NOAA NCEI U.S. Climate Normals, 1991–2020 and 1901–2000; USDA/PRISM 2023 hardiness zones; FEMA National Risk Index. Today’s forecast: National Weather Service.