Grantfork, Illinois has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 45.5 °F (low) to 65.1 °F (high). Annual precipitation averages 44.1 Inches. Grantfork is located in USDA Plant Hardiness Zone 6b. The growing season typically runs from the last frost around Apr. 1 - Apr. 10 through the first frost around Oct. 11 - Oct. 20.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | 21.9 °F | 38.2 °F | 2.71 in. |
| February | 25.4 °F | 43.5 °F | 2.37 in. |
| March | 34.3 °F | 54.1 °F | 3.35 in. |
| April | 44.8 °F | 66.1 °F | 4.9 in. |
| May | 55.6 °F | 75.4 °F | 5.04 in. |
| June | 64.6 °F | 84.3 °F | 4.69 in. |
| July | 68.1 °F | 87.5 °F | 4.27 in. |
| August | 65.9 °F | 86.5 °F | 3.66 in. |
| September | 57.5 °F | 80.2 °F | 3.31 in. |
| October | 45.8 °F | 68.5 °F | 3.34 in. |
| November | 35.2 °F | 54.1 °F | 3.69 in. |
| December | 26.7 °F | 42.9 °F | 2.79 in. |
| Annual | 45.5 °F | 65.1 °F | 44.12 in. |
| Average First Frost Date | Oct. 11 - Oct. 20 |
| Average Last Frost Date | Apr. 1 - Apr. 10 |
| Average Days With Frost | 75-100 Days |
| Plant Hardiness Zone | Zone 6b : -5°F to 0°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 |