Mount Vernon, Illinois has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 7.4 °C (low) to 18.5 °C (high). Annual precipitation averages 1155.5 Millimeters. Mount Vernon 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 | -5.2 °C | 4.1 °C | 74.42 mm |
| February | -3.5 °C | 6.8 °C | 69.6 mm |
| March | 1.3 °C | 12.4 °C | 96.52 mm |
| April | 7 °C | 18.8 °C | 126.24 mm |
| May | 12.9 °C | 23.9 °C | 130.56 mm |
| June | 17.8 °C | 28.9 °C | 118.11 mm |
| July | 19.7 °C | 30.6 °C | 109.47 mm |
| August | 18.3 °C | 30.2 °C | 85.6 mm |
| September | 13.8 °C | 26.8 °C | 83.31 mm |
| October | 7.3 °C | 20.3 °C | 85.09 mm |
| November | 1.7 °C | 12.6 °C | 101.35 mm |
| December | -2.8 °C | 6.3 °C | 75.18 mm |
| Annual | 7.4 °C | 18.5 °C | 1155.45 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 |