Lewistown, Illinois has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 5.2 °C (low) to 16.8 °C (high). Annual precipitation averages 993.9 Millimeters. Lewistown 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 | -8.7 °C | 0.7 °C | 49.28 mm |
| February | -6.8 °C | 3.3 °C | 49.53 mm |
| March | -1.3 °C | 10.2 °C | 63.75 mm |
| April | 4.6 °C | 17.4 °C | 100.84 mm |
| May | 11.2 °C | 23.3 °C | 127.76 mm |
| June | 16.5 °C | 28.2 °C | 121.92 mm |
| July | 18.3 °C | 30 °C | 103.12 mm |
| August | 16.9 °C | 29.2 °C | 91.44 mm |
| September | 12.3 °C | 26.1 °C | 89.41 mm |
| October | 5.7 °C | 19.1 °C | 77.22 mm |
| November | -0.3 °C | 10.6 °C | 64.26 mm |
| December | -5.5 °C | 3.7 °C | 55.37 mm |
| Annual | 5.2 °C | 16.8 °C | 993.9 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 |