Smithboro, Illinois has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 6.9 °C (low) to 18.2 °C (high). Annual precipitation averages 1088.4 Millimeters. Smithboro 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 | -6 °C | 3.1 °C | 72.64 mm |
| February | -4.1 °C | 5.9 °C | 61.21 mm |
| March | 0.8 °C | 11.9 °C | 81.03 mm |
| April | 6.6 °C | 18.7 °C | 122.94 mm |
| May | 12.6 °C | 24.1 °C | 125.73 mm |
| June | 17.5 °C | 29.1 °C | 115.06 mm |
| July | 19.3 °C | 30.8 °C | 103.89 mm |
| August | 17.9 °C | 30.1 °C | 85.6 mm |
| September | 13.3 °C | 26.8 °C | 81.03 mm |
| October | 6.9 °C | 20.2 °C | 81.28 mm |
| November | 1.3 °C | 12.1 °C | 90.42 mm |
| December | -3.3 °C | 5.7 °C | 67.56 mm |
| Annual | 6.9 °C | 18.2 °C | 1088.39 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 |