Denison, Iowa has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 37.7 °F (low) to 59.1 °F (high). Annual precipitation averages 33.4 Inches. Denison is located in USDA Plant Hardiness Zone 5a. The growing season typically runs from the last frost around Apr. 11 - Apr. 20 through the first frost around Oct. 11 - Oct. 20.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | 10.7 °F | 28.8 °F | 0.73 in. |
| February | 14.8 °F | 33.7 °F | 0.97 in. |
| March | 25.7 °F | 46.9 °F | 1.85 in. |
| April | 36.3 °F | 60.3 °F | 3.39 in. |
| May | 48.8 °F | 71.1 °F | 4.64 in. |
| June | 59.5 °F | 81.1 °F | 4.91 in. |
| July | 63.3 °F | 84.1 °F | 4.08 in. |
| August | 60.3 °F | 82.1 °F | 4.39 in. |
| September | 51.2 °F | 76.2 °F | 3.24 in. |
| October | 38.9 °F | 63.3 °F | 2.56 in. |
| November | 26.2 °F | 47.1 °F | 1.43 in. |
| December | 16.4 °F | 34.0 °F | 1.22 in. |
| Annual | 37.7 °F | 59.1 °F | 33.41 in. |
| Average First Frost Date | Oct. 11 - Oct. 20 |
| Average Last Frost Date | Apr. 11 - Apr. 20 |
| Average Days With Frost | 125-150 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 | Dfa - Humid Continental Hot Summers With Year Around Precipitation |