Petersburg, Illinois has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 5.6 °C (low) to 17.1 °C (high). Annual precipitation averages 979.4 Millimeters. Petersburg is located in USDA Plant Hardiness Zone 6a. The growing season typically runs from the last frost around Apr. 11 - Apr. 20 through the first frost around Oct. 11 - Oct. 20.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | -8.1 °C | 1.4 °C | 54.36 mm |
| February | -6.2 °C | 4.1 °C | 47.75 mm |
| March | -0.9 °C | 10.7 °C | 65.28 mm |
| April | 5 °C | 17.9 °C | 100.08 mm |
| May | 11.5 °C | 23.4 °C | 111.76 mm |
| June | 16.7 °C | 28.1 °C | 112.52 mm |
| July | 18.4 °C | 29.8 °C | 97.28 mm |
| August | 17.2 °C | 29.1 °C | 93.73 mm |
| September | 12.5 °C | 26.2 °C | 84.07 mm |
| October | 5.9 °C | 19.2 °C | 81.79 mm |
| November | 0.1 °C | 11.1 °C | 75.69 mm |
| December | -4.9 °C | 4.3 °C | 55.12 mm |
| Annual | 5.6 °C | 17.1 °C | 979.43 mm |
| Average First Frost Date | Oct. 11 - Oct. 20 |
| Average Last Frost Date | Apr. 11 - Apr. 20 |
| Average Days With Frost | 100-125 Days |
| Plant Hardiness Zone | Zone 6a : -23.3°C to -20.6°C |
| Plant Hardiness Zone - 2012 | Zone 5b : -26.1°C to -23.3°C |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Dfa - Humid Continental Hot Summers With Year Around Precipitation |