Denison, Iowa has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 3.2 °C (low) to 15 °C (high). Annual precipitation averages 848.6 Millimeters. 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 | -11.8 °C | -1.8 °C | 18.54 mm |
| February | -9.6 °C | 0.9 °C | 24.64 mm |
| March | -3.5 °C | 8.3 °C | 46.99 mm |
| April | 2.4 °C | 15.7 °C | 86.11 mm |
| May | 9.3 °C | 21.7 °C | 117.86 mm |
| June | 15.3 °C | 27.3 °C | 124.71 mm |
| July | 17.4 °C | 28.9 °C | 103.63 mm |
| August | 15.7 °C | 27.8 °C | 111.51 mm |
| September | 10.7 °C | 24.6 °C | 82.3 mm |
| October | 3.8 °C | 17.4 °C | 65.02 mm |
| November | -3.2 °C | 8.4 °C | 36.32 mm |
| December | -8.7 °C | 1.1 °C | 30.99 mm |
| Annual | 3.2 °C | 15 °C | 848.62 mm |
| 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 : -28.9°C to -26.1°C |
| Plant Hardiness Zone - 2012 | Zone 5a : -28.9°C to -26.1°C |
| Current Drought Conditions | Moderate Drought |
| Köppen Climate Classification | Dfa - Humid Continental Hot Summers With Year Around Precipitation |