Thomson, Illinois has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 4.1 °C (low) to 15 °C (high). Annual precipitation averages 954 Millimeters. Thomson is located in USDA Plant Hardiness Zone 5b. 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 | -10.7 °C | -1.5 °C | 39.37 mm |
| February | -8.7 °C | 1.1 °C | 43.69 mm |
| March | -2.6 °C | 8 °C | 61.72 mm |
| April | 3.4 °C | 15.4 °C | 91.95 mm |
| May | 9.9 °C | 21.9 °C | 113.79 mm |
| June | 15.5 °C | 27.1 °C | 130.3 mm |
| July | 17.4 °C | 28.9 °C | 106.43 mm |
| August | 16.2 °C | 27.8 °C | 98.3 mm |
| September | 11.4 °C | 24.4 °C | 92.71 mm |
| October | 5.1 °C | 17.1 °C | 69.85 mm |
| November | -1.3 °C | 8.7 °C | 57.66 mm |
| December | -7 °C | 1.6 °C | 48.26 mm |
| Annual | 4.1 °C | 15 °C | 954.03 mm |
| Average First Frost Date | Oct. 11 - Oct. 20 |
| Average Last Frost Date | Apr. 11 - Apr. 20 |
| Average Days With Frost | 125-150 Days |
| Plant Hardiness Zone | Zone 5b : -26.1°C to -23.3°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 |