Sumner, Iowa has a Humid Continental Mild Summer, Wet All Year climate (Köppen classification: Dfb). Average annual temperatures range from 36.4 °F (low) to 56.4 °F (high). Annual precipitation averages 38 Inches. Sumner is located in USDA Plant Hardiness Zone 5a. The growing season typically runs from the last frost around Apr. 21 - Apr. 30 through the first frost around Oct. 1 - Oct. 10.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | 8.2 °F | 25.4 °F | 1.1 in. |
| February | 12.1 °F | 29.5 °F | 1.13 in. |
| March | 24.0 °F | 42.9 °F | 1.99 in. |
| April | 35.6 °F | 57.6 °F | 3.9 in. |
| May | 47.8 °F | 69.6 °F | 4.9 in. |
| June | 58.6 °F | 79.3 °F | 6.02 in. |
| July | 61.6 °F | 82.1 °F | 4.77 in. |
| August | 59.1 °F | 80.2 °F | 4.1 in. |
| September | 50.5 °F | 74.2 °F | 3.79 in. |
| October | 38.2 °F | 60.9 °F | 2.82 in. |
| November | 26.0 °F | 44.4 °F | 1.91 in. |
| December | 14.9 °F | 31.0 °F | 1.55 in. |
| Annual | 36.4 °F | 56.4 °F | 37.98 in. |
| Average First Frost Date | Oct. 1 - Oct. 10 |
| Average Last Frost Date | Apr. 21 - Apr. 30 |
| Average Days With Frost | 150-180 Days |
| Plant Hardiness Zone | Zone 5a : -20°F to -15°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 |