Petersburg has chilly winters and warm summers
A typical July afternoon reaches 86°, a January night drops to 19°, and about 18 inches of snow falls in a normal winter.
A year in Petersburg at a glance
Each month’s typical high and low, and what it brings.
What stands out about Petersburg’s climate
Read from where Petersburg sits among Illinois’s towns. The page puts what stands out first.
Warmer than it used to be
The last 30 years in Petersburg averaged 0.8° warmer than the 1900s. The biggest change is in December (+2.1°).
A smaller shift than in 87% of U.S. towns.
Change by month
What a year in Petersburg feels like
Typical highs and lows by season and by month, from NOAA’s 30-year normals.
18 inches of snow, 38 of rain and melt
May is the wettest month and February the driest. Rain or snow falls on about 90 days a year.
Rain + melted snow by month
Inches of water
Snowfall by month
Inches of snow
About 189 frost-free days a year
In half of years the last 32° night comes by April 13 and the first by October 21.
Freezing nights by month
Average nights at or below 32°
90° days by month
Average days at 90° or more
Living with Petersburg’s climate
What the numbers mean for a garden, a driveway, a furnace and a free afternoon.
189 frost-free days. The last frost comes by Apr 13 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 21.
About 18" a winter over 7 days with an inch or more, most in January.
Months averaging under 55° run from November through April. Heating is 84% of the year’s heating-and-cooling load.
About 19 days a year reach 90°, and July afternoons average 86°.
The nicest months are April, May, June, September and October. 0.1 miles of trails are waiting.
Parks & trails →Normals are averages. Ice storms, nor’easters, floods and heat waves are on the natural hazards page.
Natural hazards →How Petersburg compares
Against every U.S. town on TownTapestry, each counted once, with its place in Illinois.
Petersburg weather by month
Average highs, lows, rain and snow for every month, from NOAA’s 1991–2020 normals.
| Month | High | Low | Rain + melt | Snow | Wet days | 30-yr range |
|---|---|---|---|---|---|---|
| January | 35° | 19° | 2.1" | 6.9" | 7 | 20° – 39° |
| February | 40° | 23° | 1.9" | 4.2" | 5 | 19° – 41° |
| March | 52° | 32° | 2.6" | 1.9" | 7 | 34° – 56° |
| April | 65° | 42° | 4.1" | 0 | 9 | 46° – 60° |
| May | 75° | 53° | 4.7" | 0 | 10 | 58° – 73° |
| June | 83° | 62° | 4.3" | 0 | 9 | 68° – 76° |
| July | 86° | 65° | 3.8" | 0 | 7 | 70° – 82° |
| August | 85° | 63° | 3.6" | 0 | 7 | 69° – 79° |
| September | 79° | 55° | 3.1" | 0 | 7 | 62° – 72° |
| October | 67° | 43° | 3.2" | 0 | 8 | 50° – 60° |
| November | 52° | 33° | 2.8" | 0.5" | 6 | 35° – 50° |
| December | 40° | 24° | 2.1" | 4.9" | 7 | 18° – 41° |
| Year | 63° | 43° | 38.3" | 18.4" | 90 |
Heating and cooling a home in Petersburg
84% of the work is heating, so the furnace matters far more than the air conditioner.
Degree days add up how far each day’s average falls below (heating) or rises above (cooling) 65° over a year. Source: NOAA station normals.
How much a month can swing
A normal is an average. December swings the most: from 1991 to 2020 its average ranged from 18° to 41°. June is the most reliable, staying within 7°.
Common questions about Petersburg’s climate
What is the coldest month in Petersburg?
January, with a typical high of 35° and a low of 19°.
What is the hottest month in Petersburg?
July, with a typical high of 86° and a low of 65°. About 19 days a year reach 90°.
How much snow does Petersburg get?
About 18 inches in a normal winter, most of it in January.
When is the first frost in Petersburg?
In half of years the first 32° night comes by October 21, and the last spring frost by April 13.
What is the rainiest month in Petersburg?
May, with 4.7 inches of rain and melted snow. The year totals 38 inches.
Neighbors talk weather
Gardens, snow, and the best days of the year in Petersburg.
Your answer becomes a post in Town Square.
Temperatures and rainfall come from NOAA’s gridded climate normals for Petersburg itself (about a 3-mile square). Snow, frost dates and day counts come from NOAA’s weather station at Petersburg 3ssw, IL (4 mi, snow) and Petersburg 1e, IL (1 mi, frost, heat and degree days). Normals are 30-year averages, not a forecast. Source: NOAA NCEI U.S. Climate Normals, 1991–2020 and 1901–2000. Today’s forecast: National Weather Service.