Hamilton, Montana has a Humid Continental Mild Summer, Wet All Year climate (Köppen classification: Dfb). Average annual temperatures range from 0.2 °C (low) to 14.9 °C (high). Annual precipitation averages 350.3 Millimeters. Hamilton is located in USDA Plant Hardiness Zone 5b. The growing season typically runs from the last frost around May 11 - May 20 through the first frost around Sep.11 - Sep. 20.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | -7.4 °C | 2.2 °C | 25.4 mm |
| February | -6.4 °C | 4.8 °C | 27.18 mm |
| March | -3.6 °C | 9.9 °C | 24.64 mm |
| April | -0.4 °C | 14.4 °C | 25.65 mm |
| May | 3.3 °C | 19.6 °C | 39.62 mm |
| June | 6.9 °C | 23.6 °C | 43.94 mm |
| July | 9.6 °C | 29.8 °C | 25.4 mm |
| August | 8.6 °C | 29.1 °C | 21.84 mm |
| September | 4.6 °C | 23.3 °C | 21.08 mm |
| October | -0.2 °C | 14.9 °C | 25.15 mm |
| November | -4.5 °C | 6.3 °C | 36.83 mm |
| December | -7.6 °C | 1.3 °C | 33.53 mm |
| Annual | 0.2 °C | 14.9 °C | 350.26 mm |
| Average First Frost Date | Sep.11 - Sep. 20 |
| Average Last Frost Date | May 11 - May 20 |
| Average Days With Frost | Over 180 Days |
| Plant Hardiness Zone | Zone 5b : -26.1°C to -23.3°C |
| Plant Hardiness Zone - 2012 | Zone 5b : -26.1°C to -23.3°C |
| Current Drought Conditions | Abnormally Dry |
| Köppen Climate Classification | Dfb - Humid Continental Mild Summer, Wet All Year |