Grantfork, Illinois has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 7.5 °C (low) to 18.4 °C (high). Annual precipitation averages 1120.7 Millimeters. 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 | -5.6 °C | 3.4 °C | 68.83 mm |
| February | -3.7 °C | 6.4 °C | 60.2 mm |
| March | 1.3 °C | 12.3 °C | 85.09 mm |
| April | 7.1 °C | 18.9 °C | 124.46 mm |
| May | 13.1 °C | 24.1 °C | 128.02 mm |
| June | 18.1 °C | 29.1 °C | 119.13 mm |
| July | 20.1 °C | 30.8 °C | 108.46 mm |
| August | 18.8 °C | 30.3 °C | 92.96 mm |
| September | 14.2 °C | 26.8 °C | 84.07 mm |
| October | 7.7 °C | 20.3 °C | 84.84 mm |
| November | 1.8 °C | 12.3 °C | 93.73 mm |
| December | -2.9 °C | 6.1 °C | 70.87 mm |
| Annual | 7.5 °C | 18.4 °C | 1120.66 mm |
| 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 : -20.6°C to -17.8°C |
| Plant Hardiness Zone - 2012 | Zone 6a : -23.3°C to -20.6°C |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Dfa - Humid Continental Hot Summers With Year Around Precipitation |