Jefferson, 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 15 °C (high). Annual precipitation averages 890 Millimeters. Jefferson 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 | -12 °C | -2.1 °C | 23.11 mm |
| February | -9.6 °C | 0.6 °C | 30.23 mm |
| March | -3.4 °C | 8 °C | 50.55 mm |
| April | 2.8 °C | 16.1 °C | 95.76 mm |
| May | 9.7 °C | 21.9 °C | 125.48 mm |
| June | 15.4 °C | 27.2 °C | 132.33 mm |
| July | 17.5 °C | 29.1 °C | 106.93 mm |
| August | 16 °C | 27.9 °C | 101.85 mm |
| September | 10.9 °C | 24.7 °C | 80.52 mm |
| October | 4.2 °C | 17.5 °C | 64.52 mm |
| November | -2.7 °C | 8.3 °C | 45.21 mm |
| December | -8.6 °C | 0.9 °C | 33.53 mm |
| Annual | 3.4 °C | 15 °C | 890.02 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 5a : -28.9°C to -26.1°C |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Dfa - Humid Continental Hot Summers With Year Around Precipitation |