Harristown, Illinois has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 42.7 °F (low) to 63.2 °F (high). Annual precipitation averages 39.4 Inches. Harristown 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 | 19.1 °F | 35.1 °F | 2.27 in. |
| February | 21.5 °F | 40.1 °F | 2.01 in. |
| March | 30.7 °F | 51.2 °F | 2.73 in. |
| April | 42.3 °F | 64.5 °F | 4.15 in. |
| May | 53.6 °F | 74.4 °F | 4.61 in. |
| June | 61.9 °F | 83.4 °F | 4.67 in. |
| July | 65.2 °F | 86.3 °F | 3.79 in. |
| August | 63.6 °F | 84.7 °F | 3.29 in. |
| September | 54.8 °F | 79.6 °F | 3.15 in. |
| October | 43.1 °F | 66.9 °F | 3.28 in. |
| November | 32.4 °F | 51.8 °F | 3.08 in. |
| December | 24.7 °F | 39.9 °F | 2.37 in. |
| Annual | 42.7 °F | 63.2 °F | 39.4 in. |
| 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 : -10°F to -5°F |
| Plant Hardiness Zone - 2012 | Zone 6a : -10°F to -5°F |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Dfa - Humid Continental Hot Summers With Year Around Precipitation |