Simpson, Illinois has a Humid Subtropical Climate climate (Köppen classification: Cfa). Average annual temperatures range from 8.3 °C (low) to 19.6 °C (high). Annual precipitation averages 1287 Millimeters. Simpson is located in USDA Plant Hardiness Zone 7a. The growing season typically runs from the last frost around Apr. 1 - Apr. 10 through the first frost around Oct. 21 - Oct. 31.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | -3.7 °C | 5.7 °C | 97.79 mm |
| February | -2.1 °C | 8.1 °C | 87.63 mm |
| March | 2.5 °C | 13.6 °C | 118.36 mm |
| April | 7.8 °C | 20.2 °C | 137.92 mm |
| May | 13.6 °C | 24.9 °C | 134.37 mm |
| June | 18.2 °C | 29.5 °C | 116.33 mm |
| July | 20.3 °C | 31.2 °C | 107.7 mm |
| August | 19.1 °C | 30.7 °C | 79.76 mm |
| September | 14.8 °C | 27.7 °C | 89.66 mm |
| October | 8.3 °C | 21.6 °C | 95.25 mm |
| November | 2.5 °C | 13.8 °C | 112.52 mm |
| December | -1.6 °C | 7.8 °C | 109.73 mm |
| Annual | 8.3 °C | 19.6 °C | 1287.02 mm |
| Average First Frost Date | Oct. 21 - Oct. 31 |
| Average Last Frost Date | Apr. 1 - Apr. 10 |
| Average Days With Frost | 75-100 Days |
| Plant Hardiness Zone | Zone 7a : -17.8°C to -15°C |
| Plant Hardiness Zone - 2012 | Zone 6b : -20.6°C to -17.8°C |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Cfa - Humid Subtropical Climate |