Irwin, Idaho has a Humid Continental Mild Summer, Wet All Year climate (Köppen classification: Dfb). Average annual temperatures range from -0.3 °C (low) to 13.3 °C (high). Annual precipitation averages 500.9 Millimeters. Irwin is located in USDA Plant Hardiness Zone 5a. The growing season typically runs from the last frost around Jun. 1 - Jun. 10 through the first frost around Sep.1 - Sep. 10.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | -10.2 °C | -1.2 °C | 46.74 mm |
| February | -9.2 °C | 0.9 °C | 32.26 mm |
| March | -4.9 °C | 6.7 °C | 39.12 mm |
| April | -1.1 °C | 12.3 °C | 46.99 mm |
| May | 3.2 °C | 18.1 °C | 61.98 mm |
| June | 6.9 °C | 23.1 °C | 53.09 mm |
| July | 10.3 °C | 28.8 °C | 28.7 mm |
| August | 9.6 °C | 28.4 °C | 29.72 mm |
| September | 5.3 °C | 23 °C | 40.39 mm |
| October | 0.4 °C | 14.8 °C | 43.43 mm |
| November | -4.8 °C | 5.7 °C | 41.15 mm |
| December | -9.3 °C | -0.9 °C | 37.34 mm |
| Annual | -0.3 °C | 13.3 °C | 500.91 mm |
| Average First Frost Date | Sep.1 - Sep. 10 |
| Average Last Frost Date | Jun. 1 - Jun. 10 |
| Average Days With Frost | Over 180 Days |
| Plant Hardiness Zone | Zone 5a : -28.9°C to -26.1°C |
| Plant Hardiness Zone - 2012 | Zone 5a : -28.9°C to -26.1°C |
| Current Drought Conditions | Extreme Drought |
| Köppen Climate Classification | Dfb - Humid Continental Mild Summer, Wet All Year |