Seymour, Illinois has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 5.4 °C (low) to 16.6 °C (high). Annual precipitation averages 1007.1 Millimeters. Seymour is located in USDA Plant Hardiness Zone 6a. The growing season typically runs from the last frost around Apr. 11 - Apr. 20 through the first frost around Oct. 11 - Oct. 20.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | -7.9 °C | 0.9 °C | 57.66 mm |
| February | -6.3 °C | 3.3 °C | 50.55 mm |
| March | -1.2 °C | 9.9 °C | 69.6 mm |
| April | 4.7 °C | 17.4 °C | 103.63 mm |
| May | 11.3 °C | 23.2 °C | 114.55 mm |
| June | 16.2 °C | 27.9 °C | 118.11 mm |
| July | 17.8 °C | 29 °C | 104.39 mm |
| August | 16.4 °C | 28.2 °C | 81.53 mm |
| September | 12.4 °C | 25.9 °C | 80.52 mm |
| October | 5.8 °C | 18.7 °C | 87.12 mm |
| November | -0.1 °C | 10.6 °C | 78.23 mm |
| December | -4.8 °C | 3.6 °C | 61.21 mm |
| Annual | 5.4 °C | 16.6 °C | 1007.1 mm |
| Average First Frost Date | Oct. 11 - Oct. 20 |
| Average Last Frost Date | Apr. 11 - Apr. 20 |
| Average Days With Frost | 100-125 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 | Normal |
| Köppen Climate Classification | Dfa - Humid Continental Hot Summers With Year Around Precipitation |