Johnston, Iowa has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 4.4 °C (low) to 15.7 °C (high). Annual precipitation averages 922.8 Millimeters. Johnston is located in USDA Plant Hardiness Zone 5b. 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 °C | -1 °C | 25.65 mm |
| February | -8.6 °C | 1.6 °C | 30.99 mm |
| March | -2.4 °C | 8.8 °C | 54.61 mm |
| April | 3.9 °C | 16.6 °C | 97.54 mm |
| May | 10.6 °C | 22.3 °C | 127 mm |
| June | 16.3 °C | 27.6 °C | 134.87 mm |
| July | 18.4 °C | 29.8 °C | 107.7 mm |
| August | 17 °C | 28.8 °C | 106.93 mm |
| September | 12.2 °C | 25.3 °C | 86.36 mm |
| October | 5.3 °C | 17.8 °C | 67.82 mm |
| November | -1.5 °C | 9.3 °C | 46.99 mm |
| December | -7.5 °C | 1.9 °C | 36.32 mm |
| Annual | 4.4 °C | 15.7 °C | 922.78 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 5b : -26.1°C to -23.3°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 |