Promise City, Iowa has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 4.3 °C (low) to 15.8 °C (high). Annual precipitation averages 986.5 Millimeters. Promise City 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. 11 - Oct. 20.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | -10.3 °C | -0.2 °C | 28.19 mm |
| February | -8.1 °C | 2.6 °C | 37.34 mm |
| March | -2.2 °C | 9.7 °C | 57.15 mm |
| April | 3.8 °C | 16.3 °C | 100.58 mm |
| May | 10.2 °C | 21.7 °C | 137.92 mm |
| June | 15.7 °C | 27 °C | 136.91 mm |
| July | 18.1 °C | 29.4 °C | 115.82 mm |
| August | 16.7 °C | 28.6 °C | 109.22 mm |
| September | 11.8 °C | 24.8 °C | 97.54 mm |
| October | 5.1 °C | 17.8 °C | 74.68 mm |
| November | -1.7 °C | 9.6 °C | 52.58 mm |
| December | -7.1 °C | 2.7 °C | 38.61 mm |
| Annual | 4.3 °C | 15.8 °C | 986.54 mm |
| Average First Frost Date | Oct. 11 - Oct. 20 |
| Average Last Frost Date | Apr. 21 - Apr. 30 |
| Average Days With Frost | 100-125 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 |