Ossian, Iowa has a Humid Continental Mild Summer, Wet All Year climate (Köppen classification: Dfb). Average annual temperatures range from 35.6 °F (low) to 55.3 °F (high). Annual precipitation averages 38 Inches. Ossian is located in USDA Plant Hardiness Zone 5a. 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 | 7.4 °F | 24.3 °F | 1.06 in. |
| February | 10.8 °F | 28.7 °F | 1.12 in. |
| March | 22.9 °F | 41.8 °F | 2.02 in. |
| April | 34.6 °F | 56.3 °F | 4.01 in. |
| May | 46.6 °F | 68.2 °F | 4.82 in. |
| June | 57.6 °F | 77.9 °F | 6.08 in. |
| July | 60.9 °F | 81.1 °F | 4.64 in. |
| August | 58.3 °F | 79.1 °F | 4.27 in. |
| September | 50.0 °F | 72.7 °F | 3.79 in. |
| October | 37.6 °F | 59.6 °F | 2.84 in. |
| November | 25.7 °F | 43.3 °F | 1.91 in. |
| December | 14.2 °F | 30.1 °F | 1.44 in. |
| Annual | 35.6 °F | 55.3 °F | 38 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 5a : -20°F to -15°F |
| Plant Hardiness Zone - 2012 | Zone 4b : -25°F to -20°F |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Dfb - Humid Continental Mild Summer, Wet All Year |