Irwin, Illinois has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 4.8 °C (low) to 16 °C (high). Annual precipitation averages 975.1 Millimeters. Irwin 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 | -8.8 °C | 0.1 °C | 52.83 mm |
| February | -7.1 °C | 2.6 °C | 46.74 mm |
| March | -1.8 °C | 9.2 °C | 59.18 mm |
| April | 3.9 °C | 16.3 °C | 92.2 mm |
| May | 10.2 °C | 22.5 °C | 114.55 mm |
| June | 15.7 °C | 27.6 °C | 117.86 mm |
| July | 17.6 °C | 29.2 °C | 114.3 mm |
| August | 16.4 °C | 28.3 °C | 97.79 mm |
| September | 11.9 °C | 25.4 °C | 79.5 mm |
| October | 5.6 °C | 18.2 °C | 83.31 mm |
| November | -0.3 °C | 10.1 °C | 64.52 mm |
| December | -5.7 °C | 3 °C | 52.32 mm |
| Annual | 4.8 °C | 16 °C | 975.1 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 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 |