Old Agency, Montana has a Humid Continental Mild Summer, Wet All Year climate (Köppen classification: Dfb). Average annual temperatures range from 2.4 °C (low) to 15.1 °C (high). Annual precipitation averages 324.4 Millimeters. 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 | -5 °C | 1.9 °C | 20.83 mm |
| February | -4.7 °C | 4.2 °C | 20.57 mm |
| March | -1.7 °C | 9.8 °C | 18.8 mm |
| April | 1.3 °C | 14.7 °C | 34.29 mm |
| May | 5.2 °C | 20.1 °C | 35.81 mm |
| June | 8.9 °C | 23.9 °C | 53.85 mm |
| July | 11.8 °C | 30.3 °C | 18.54 mm |
| August | 11.2 °C | 30.1 °C | 16.26 mm |
| September | 7.2 °C | 23.8 °C | 26.16 mm |
| October | 1.9 °C | 14.9 °C | 28.7 mm |
| November | -2.1 °C | 6.3 °C | 29.46 mm |
| December | -4.9 °C | 1.7 °C | 21.08 mm |
| Annual | 2.4 °C | 15.1 °C | 324.35 mm |
| 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 : -23.3°C to -20.6°C |
| Plant Hardiness Zone - 2012 | Zone 6a : -23.3°C to -20.6°C |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Dfb - Humid Continental Mild Summer, Wet All Year |