Firth, Idaho has a Cold Semi-Arid Climate climate (Köppen classification: BSk). Average annual temperatures range from -0.1 °C (low) to 14.5 °C (high). Annual precipitation averages 272.3 Millimeters. Firth is located in USDA Plant Hardiness Zone 5b. The growing season typically runs from the last frost around May 21 - May 31 through the first frost around Sep.11 - Sep. 20.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | -10.2 °C | -0.8 °C | 22.1 mm |
| February | -8.4 °C | 1.9 °C | 15.75 mm |
| March | -3.4 °C | 8.8 °C | 24.13 mm |
| April | -0.5 °C | 14.2 °C | 27.18 mm |
| May | 3.8 °C | 19.4 °C | 39.88 mm |
| June | 7.3 °C | 24.3 °C | 27.69 mm |
| July | 10.4 °C | 30.1 °C | 12.19 mm |
| August | 9.6 °C | 29.8 °C | 14.99 mm |
| September | 5.3 °C | 24.1 °C | 23.88 mm |
| October | -0.1 °C | 15.8 °C | 26.16 mm |
| November | -5.2 °C | 6.8 °C | 18.03 mm |
| December | -9.7 °C | -0.2 °C | 20.32 mm |
| Annual | -0.1 °C | 14.5 °C | 272.3 mm |
| Average First Frost Date | Sep.11 - Sep. 20 |
| Average Last Frost Date | May 21 - May 31 |
| Average Days With Frost | 150-180 Days |
| Plant Hardiness Zone | Zone 5b : -26.1°C to -23.3°C |
| Plant Hardiness Zone - 2012 | Zone 5a : -28.9°C to -26.1°C |
| Current Drought Conditions | Extreme Drought |
| Köppen Climate Classification | BSk - Cold Semi-Arid Climate |