Vandalia, Illinois has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 7 °C (low) to 18.2 °C (high). Annual precipitation averages 1103.6 Millimeters. Vandalia 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 | -5.9 °C | 3.2 °C | 75.44 mm |
| February | -3.9 °C | 6 °C | 62.74 mm |
| March | 1 °C | 11.9 °C | 84.84 mm |
| April | 6.7 °C | 18.7 °C | 117.35 mm |
| May | 12.7 °C | 24.1 °C | 124.21 mm |
| June | 17.6 °C | 29.1 °C | 117.6 mm |
| July | 19.4 °C | 30.9 °C | 107.44 mm |
| August | 18.1 °C | 30.2 °C | 83.82 mm |
| September | 13.4 °C | 26.7 °C | 81.79 mm |
| October | 6.8 °C | 20.1 °C | 83.31 mm |
| November | 1.4 °C | 12.2 °C | 95.5 mm |
| December | -3.2 °C | 5.7 °C | 69.6 mm |
| Annual | 7 °C | 18.2 °C | 1103.64 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 |