Ontonagon, Michigan has a Humid Continental Mild Summer, Wet All Year climate (Köppen classification: Dfb). Average annual temperatures range from 0.5 °C (low) to 11 °C (high). Annual precipitation averages 795.8 Millimeters. Ontonagon 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. 21 - Sep. 30.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | -12.1 °C | -4.1 °C | 55.63 mm |
| February | -12.7 °C | -2.7 °C | 38.86 mm |
| March | -8.2 °C | 2.7 °C | 35.05 mm |
| April | -1.9 °C | 9.6 °C | 62.23 mm |
| May | 4.3 °C | 17.3 °C | 76.96 mm |
| June | 9.4 °C | 22.3 °C | 82.3 mm |
| July | 12.8 °C | 25 °C | 85.34 mm |
| August | 12.9 °C | 24.6 °C | 68.33 mm |
| September | 9.2 °C | 20.6 °C | 73.15 mm |
| October | 3.1 °C | 13.1 °C | 82.04 mm |
| November | -2.7 °C | 4.9 °C | 66.04 mm |
| December | -8.7 °C | -1.3 °C | 69.85 mm |
| Annual | 0.5 °C | 11 °C | 795.78 mm |
| Average First Frost Date | Sep. 21 - Sep. 30 |
| 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 5a : -28.9°C to -26.1°C |
| Current Drought Conditions | Moderate Drought |
| Köppen Climate Classification | Dfb - Humid Continental Mild Summer, Wet All Year |