Irwin, Missouri has a Humid Subtropical Climate climate (Köppen classification: Cfa). Average annual temperatures range from 7.9 °C (low) to 19.7 °C (high). Annual precipitation averages 1179.1 Millimeters. Irwin 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 | -4.9 °C | 6 °C | 46.23 mm |
| February | -2.8 °C | 8.9 °C | 53.09 mm |
| March | 1.8 °C | 14.4 °C | 82.3 mm |
| April | 7.1 °C | 19.7 °C | 123.44 mm |
| May | 12.9 °C | 24.3 °C | 170.18 mm |
| June | 18.2 °C | 29.2 °C | 147.57 mm |
| July | 20.5 °C | 31.8 °C | 111.76 mm |
| August | 19.4 °C | 31.6 °C | 89.66 mm |
| September | 14.7 °C | 27.3 °C | 116.84 mm |
| October | 8.2 °C | 21.2 °C | 91.95 mm |
| November | 2.1 °C | 13.9 °C | 81.28 mm |
| December | -2.8 °C | 7.9 °C | 64.77 mm |
| Annual | 7.9 °C | 19.7 °C | 1179.07 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 |