Irwin, Iowa has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 38 °F (low) to 59.3 °F (high). Annual precipitation averages 33.9 Inches. Irwin is located in USDA Plant Hardiness Zone 5a. The growing season typically runs from the last frost around Apr. 21 - Apr. 30 through the first frost around Oct. 1 - Oct. 10.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | 10.9 °F | 29.2 °F | 0.8 in. |
| February | 15.0 °F | 34.1 °F | 1.03 in. |
| March | 25.8 °F | 47.4 °F | 1.87 in. |
| April | 36.7 °F | 60.8 °F | 3.53 in. |
| May | 49.2 °F | 71.4 °F | 4.87 in. |
| June | 59.7 °F | 81.0 °F | 4.99 in. |
| July | 63.4 °F | 84.2 °F | 3.85 in. |
| August | 60.6 °F | 82.2 °F | 4.21 in. |
| September | 51.4 °F | 76.3 °F | 3.41 in. |
| October | 39.5 °F | 63.7 °F | 2.62 in. |
| November | 26.7 °F | 47.2 °F | 1.47 in. |
| December | 16.6 °F | 34.3 °F | 1.27 in. |
| Annual | 38 °F | 59.3 °F | 33.92 in. |
| Average First Frost Date | Oct. 1 - Oct. 10 |
| Average Last Frost Date | Apr. 21 - Apr. 30 |
| Average Days With Frost | 150-180 Days |
| Plant Hardiness Zone | Zone 5a : -20°F to -15°F |
| Plant Hardiness Zone - 2012 | Zone 5a : -20°F to -15°F |
| Current Drought Conditions | Abnormally Dry |
| Köppen Climate Classification | Dfa - Humid Continental Hot Summers With Year Around Precipitation |