Grafton, Illinois has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 45.7 °F (low) to 65.5 °F (high). Annual precipitation averages 41.2 Inches. 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 | 21.9 °F | 38.6 °F | 2.44 in. |
| February | 25.4 °F | 44.1 °F | 2.23 in. |
| March | 34.1 °F | 54.6 °F | 3.24 in. |
| April | 45.0 °F | 66.6 °F | 4.66 in. |
| May | 56.0 °F | 75.7 °F | 4.77 in. |
| June | 64.9 °F | 84.3 °F | 4.23 in. |
| July | 68.6 °F | 87.8 °F | 3.77 in. |
| August | 66.4 °F | 86.7 °F | 3.63 in. |
| September | 57.9 °F | 80.4 °F | 3.27 in. |
| October | 46.2 °F | 68.5 °F | 3.06 in. |
| November | 35.3 °F | 54.8 °F | 3.33 in. |
| December | 26.5 °F | 43.3 °F | 2.6 in. |
| Annual | 45.7 °F | 65.5 °F | 41.23 in. |
| 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 : -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 |