Diagonal, Iowa has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 4.2 °C (low) to 15.9 °C (high). Annual precipitation averages 954.3 Millimeters. Diagonal is located in USDA Plant Hardiness Zone 5b. 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 | -10.7 °C | -0.2 °C | 21.59 mm |
| February | -8.3 °C | 2.7 °C | 32.51 mm |
| March | -2.3 °C | 9.7 °C | 55.37 mm |
| April | 3.6 °C | 16.5 °C | 95 mm |
| May | 10.3 °C | 22 °C | 138.94 mm |
| June | 15.9 °C | 27.2 °C | 133.1 mm |
| July | 18.2 °C | 29.6 °C | 112.78 mm |
| August | 16.8 °C | 28.7 °C | 108.2 mm |
| September | 11.7 °C | 24.8 °C | 94.74 mm |
| October | 4.8 °C | 17.9 °C | 74.68 mm |
| November | -1.8 °C | 9.6 °C | 52.07 mm |
| December | -7.6 °C | 2.6 °C | 35.31 mm |
| Annual | 4.2 °C | 15.9 °C | 954.29 mm |
| Average First Frost Date | Oct. 1 - Oct. 10 |
| Average Last Frost Date | Apr. 21 - Apr. 30 |
| Average Days With Frost | 125-150 Days |
| Plant Hardiness Zone | Zone 5b : -26.1°C to -23.3°C |
| Plant Hardiness Zone - 2012 | Zone 5b : -26.1°C to -23.3°C |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Dfa - Humid Continental Hot Summers With Year Around Precipitation |