Grafton, Wisconsin has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 3.2 °C (low) to 13 °C (high). Annual precipitation averages 962.7 Millimeters. Grafton is located in USDA Plant Hardiness Zone 6a. The growing season typically runs from the last frost around May 1 - May 10 through the first frost around Oct. 11 - Oct. 20.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | -10.4 °C | -2.1 °C | 49.78 mm |
| February | -9 °C | -0.2 °C | 42.67 mm |
| March | -4 °C | 5.5 °C | 58.93 mm |
| April | 1.6 °C | 11.8 °C | 107.95 mm |
| May | 7.4 °C | 18.4 °C | 111 mm |
| June | 13.1 °C | 23.9 °C | 102.62 mm |
| July | 16.2 °C | 26.8 °C | 100.58 mm |
| August | 15.5 °C | 25.8 °C | 105.16 mm |
| September | 11.2 °C | 21.9 °C | 89.92 mm |
| October | 4.8 °C | 14.9 °C | 79.76 mm |
| November | -1.4 °C | 7.6 °C | 56.39 mm |
| December | -6.8 °C | 1.1 °C | 57.91 mm |
| Annual | 3.2 °C | 13 °C | 962.67 mm |
| Average First Frost Date | Oct. 11 - Oct. 20 |
| Average Last Frost Date | May 1 - May 10 |
| Average Days With Frost | 125-150 Days |
| Plant Hardiness Zone | Zone 6a : -23.3°C to -20.6°C |
| Plant Hardiness Zone - 2012 | Zone 5b : -26.1°C to -23.3°C |
| Current Drought Conditions | Abnormally Dry |
| Köppen Climate Classification | Dfa - Humid Continental Hot Summers With Year Around Precipitation |