Ontonagon, Michigan has a Humid Continental Mild Summer, Wet All Year climate (Köppen classification: Dfb). Average annual temperatures range from 32.8 °F (low) to 51.8 °F (high). Annual precipitation averages 31.3 Inches. 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 | 10.3 °F | 24.6 °F | 2.19 in. |
| February | 9.2 °F | 27.2 °F | 1.53 in. |
| March | 17.2 °F | 36.9 °F | 1.38 in. |
| April | 28.6 °F | 49.2 °F | 2.45 in. |
| May | 39.8 °F | 63.1 °F | 3.03 in. |
| June | 49.0 °F | 72.1 °F | 3.24 in. |
| July | 55.1 °F | 77.0 °F | 3.36 in. |
| August | 55.2 °F | 76.3 °F | 2.69 in. |
| September | 48.5 °F | 69.0 °F | 2.88 in. |
| October | 37.5 °F | 55.5 °F | 3.23 in. |
| November | 27.1 °F | 40.8 °F | 2.6 in. |
| December | 16.3 °F | 29.6 °F | 2.75 in. |
| Annual | 32.8 °F | 51.8 °F | 31.33 in. |
| 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 : -15°F to -10°F |
| Plant Hardiness Zone - 2012 | Zone 5a : -20°F to -15°F |
| Current Drought Conditions | Abnormally Dry |
| Köppen Climate Classification | Dfb - Humid Continental Mild Summer, Wet All Year |