Keystone, Iowa has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 3.4 °C (low) to 14.7 °C (high). Annual precipitation averages 935.5 Millimeters. Keystone 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. 11 - Oct. 20.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | -11.7 °C | -2.4 °C | 28.45 mm |
| February | -9.7 °C | 0.2 °C | 31.75 mm |
| March | -3.4 °C | 7.6 °C | 53.34 mm |
| April | 2.7 °C | 15.4 °C | 95.25 mm |
| May | 9.6 °C | 21.7 °C | 120.4 mm |
| June | 15.5 °C | 26.9 °C | 137.41 mm |
| July | 17.3 °C | 28.8 °C | 105.92 mm |
| August | 15.8 °C | 27.7 °C | 105.92 mm |
| September | 10.9 °C | 24.2 °C | 92.71 mm |
| October | 4.3 °C | 16.9 °C | 71.88 mm |
| November | -2.4 °C | 8.1 °C | 53.09 mm |
| December | -8.2 °C | 0.8 °C | 39.37 mm |
| Annual | 3.4 °C | 14.7 °C | 935.49 mm |
| Average First Frost Date | Oct. 11 - Oct. 20 |
| Average Last Frost Date | Apr. 21 - Apr. 30 |
| 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 | Normal |
| Köppen Climate Classification | Dfa - Humid Continental Hot Summers With Year Around Precipitation |