Hartline, Washington has a Cold Semi-Arid Climate climate (Köppen classification: BSk). Average annual temperatures range from 2.9 °C (low) to 15.6 °C (high). Annual precipitation averages 261.4 Millimeters. Hartline is located in USDA Plant Hardiness Zone 6b. The growing season typically runs from the last frost around May 1 - May 10 through the first frost around Oct. 1 - Oct. 10.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | -5.3 °C | 1.1 °C | 32.77 mm |
| February | -3.8 °C | 4.5 °C | 21.84 mm |
| March | -0.9 °C | 10.2 °C | 25.4 mm |
| April | 1.6 °C | 15.6 °C | 21.59 mm |
| May | 5.8 °C | 20.8 °C | 26.67 mm |
| June | 9 °C | 24.8 °C | 21.08 mm |
| July | 12.8 °C | 30.6 °C | 12.19 mm |
| August | 12.5 °C | 30.3 °C | 7.11 mm |
| September | 8.2 °C | 24.8 °C | 6.86 mm |
| October | 2.7 °C | 16.1 °C | 18.03 mm |
| November | -2.1 °C | 6.8 °C | 31.5 mm |
| December | -5.2 °C | 1.1 °C | 36.32 mm |
| Annual | 2.9 °C | 15.6 °C | 261.36 mm |
| Average First Frost Date | Oct. 1 - Oct. 10 |
| Average Last Frost Date | May 1 - May 10 |
| Average Days With Frost | 125-150 Days |
| Plant Hardiness Zone | Zone 6b : -20.6°C to -17.8°C |
| Plant Hardiness Zone - 2012 | Zone 6b : -20.6°C to -17.8°C |
| Current Drought Conditions | Moderate Drought |
| Köppen Climate Classification | BSk - Cold Semi-Arid Climate |