Old Agency, Montana has a Humid Continental Mild Summer, Wet All Year climate (Köppen classification: Dfb). Average annual temperatures range from 36.4 °F (low) to 59.3 °F (high). Annual precipitation averages 12.8 Inches. Old Agency is located in USDA Plant Hardiness Zone 6a. 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 | 23.0 °F | 35.5 °F | 0.82 in. |
| February | 23.6 °F | 39.6 °F | 0.81 in. |
| March | 28.9 °F | 49.7 °F | 0.74 in. |
| April | 34.3 °F | 58.5 °F | 1.35 in. |
| May | 41.4 °F | 68.2 °F | 1.41 in. |
| June | 48.0 °F | 75.1 °F | 2.12 in. |
| July | 53.3 °F | 86.5 °F | 0.73 in. |
| August | 52.1 °F | 86.2 °F | 0.64 in. |
| September | 44.9 °F | 74.9 °F | 1.03 in. |
| October | 35.5 °F | 58.8 °F | 1.13 in. |
| November | 28.2 °F | 43.3 °F | 1.16 in. |
| December | 23.2 °F | 35.0 °F | 0.83 in. |
| Annual | 36.4 °F | 59.3 °F | 12.77 in. |
| Average First Frost Date | Oct. 1 - Oct. 10 |
| Average Last Frost Date | May 1 - May 10 |
| Average Days With Frost | 150-180 Days |
| Plant Hardiness Zone | Zone 6a : -10°F to -5°F |
| Plant Hardiness Zone - 2012 | Zone 6a : -10°F to -5°F |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Dfb - Humid Continental Mild Summer, Wet All Year |