Sullivan, Ohio has a Humid Continental Mild Summer, Wet All Year climate (Köppen classification: Dfb). Average annual temperatures range from 4.4 °C (low) to 15.3 °C (high). Annual precipitation averages 1021.6 Millimeters. Sullivan is located in USDA Plant Hardiness Zone 6a. The growing season typically runs from the last frost around Apr. 21 - Apr. 30 through the first frost around Oct. 11 - Oct. 20.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | -7.9 °C | 0.8 °C | 71.88 mm |
| February | -7.2 °C | 2.3 °C | 54.36 mm |
| March | -2.7 °C | 7.7 °C | 74.68 mm |
| April | 3.3 °C | 15.4 °C | 96.27 mm |
| May | 9.3 °C | 21.3 °C | 102.87 mm |
| June | 14.4 °C | 26.2 °C | 109.47 mm |
| July | 16.3 °C | 28.1 °C | 106.43 mm |
| August | 15.3 °C | 27.3 °C | 97.28 mm |
| September | 11.3 °C | 24 °C | 85.85 mm |
| October | 5.3 °C | 17.1 °C | 80.01 mm |
| November | 0 °C | 9.7 °C | 73.91 mm |
| December | -4.6 °C | 3.4 °C | 68.58 mm |
| Annual | 4.4 °C | 15.3 °C | 1021.59 mm |
| Average First Frost Date | Oct. 11 - Oct. 20 |
| Average Last Frost Date | Apr. 21 - Apr. 30 |
| Average Days With Frost | 125-150 Days |
| Plant Hardiness Zone | Zone 6a : -23.3°C to -20.6°C |
| Plant Hardiness Zone - 2012 | Zone 6a : -23.3°C to -20.6°C |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Dfb - Humid Continental Mild Summer, Wet All Year |