Harrisburg, Illinois has a Humid Subtropical Climate climate (Köppen classification: Cfa). Average annual temperatures range from 8.1 °C (low) to 19.4 °C (high). Annual precipitation averages 1204.5 Millimeters. Harrisburg is located in USDA Plant Hardiness Zone 7a. 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 | -3.9 °C | 5.2 °C | 97.03 mm |
| February | -2.3 °C | 7.9 °C | 80.01 mm |
| March | 2.2 °C | 13.4 °C | 107.7 mm |
| April | 7.6 °C | 19.9 °C | 127 mm |
| May | 13.5 °C | 24.9 °C | 132.59 mm |
| June | 18.2 °C | 29.3 °C | 120.65 mm |
| July | 20.1 °C | 31.2 °C | 98.55 mm |
| August | 18.8 °C | 30.7 °C | 74.17 mm |
| September | 14.4 °C | 27.8 °C | 80.01 mm |
| October | 7.9 °C | 21.2 °C | 86.87 mm |
| November | 2.2 °C | 13.6 °C | 104.65 mm |
| December | -2.1 °C | 7.5 °C | 95.25 mm |
| Annual | 8.1 °C | 19.4 °C | 1204.48 mm |
| Average First Frost Date | Oct. 11 - Oct. 20 |
| Average Last Frost Date | Apr. 11 - Apr. 20 |
| Average Days With Frost | 75-100 Days |
| Plant Hardiness Zone | Zone 7a : -17.8°C to -15°C |
| Plant Hardiness Zone - 2012 | Zone 6b : -20.6°C to -17.8°C |
| Current Drought Conditions | Abnormally Dry |
| Köppen Climate Classification | Cfa - Humid Subtropical Climate |