Irwin, Illinois has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 40.7 °F (low) to 60.9 °F (high). Annual precipitation averages 38.4 Inches. 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 | 16.1 °F | 32.1 °F | 2.08 in. |
| February | 19.3 °F | 36.6 °F | 1.84 in. |
| March | 28.7 °F | 48.6 °F | 2.33 in. |
| April | 39.0 °F | 61.4 °F | 3.63 in. |
| May | 50.4 °F | 72.5 °F | 4.51 in. |
| June | 60.3 °F | 81.6 °F | 4.64 in. |
| July | 63.7 °F | 84.5 °F | 4.5 in. |
| August | 61.6 °F | 83.0 °F | 3.85 in. |
| September | 53.4 °F | 77.8 °F | 3.13 in. |
| October | 42.0 °F | 64.8 °F | 3.28 in. |
| November | 31.5 °F | 50.1 °F | 2.54 in. |
| December | 21.8 °F | 37.4 °F | 2.06 in. |
| Annual | 40.7 °F | 60.9 °F | 38.39 in. |
| 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 : -15°F to -10°F |
| Plant Hardiness Zone - 2012 | Zone 5b : -15°F to -10°F |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Dfa - Humid Continental Hot Summers With Year Around Precipitation |