Grafton, Illinois has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 7.6 °C (low) to 18.6 °C (high). Annual precipitation averages 1047.2 Millimeters. Grafton 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 | 3.7 °C | 61.98 mm |
| February | -3.7 °C | 6.7 °C | 56.64 mm |
| March | 1.2 °C | 12.6 °C | 82.3 mm |
| April | 7.2 °C | 19.2 °C | 118.36 mm |
| May | 13.3 °C | 24.3 °C | 121.16 mm |
| June | 18.3 °C | 29.1 °C | 107.44 mm |
| July | 20.3 °C | 31 °C | 95.76 mm |
| August | 19.1 °C | 30.4 °C | 92.2 mm |
| September | 14.4 °C | 26.9 °C | 83.06 mm |
| October | 7.9 °C | 20.3 °C | 77.72 mm |
| November | 1.8 °C | 12.7 °C | 84.58 mm |
| December | -3.1 °C | 6.3 °C | 66.04 mm |
| Annual | 7.6 °C | 18.6 °C | 1047.24 mm |
| Average First Frost Date | Oct. 11 - Oct. 20 |
| Average Last Frost Date | Apr. 1 - Apr. 10 |
| Average Days With Frost | 100-125 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 |