Letts, Iowa has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 4.8 °C (low) to 15.8 °C (high). Annual precipitation averages 956.3 Millimeters. Letts 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 | -9.8 °C | -0.7 °C | 34.54 mm |
| February | -7.9 °C | 1.8 °C | 42.67 mm |
| March | -1.9 °C | 9 °C | 61.47 mm |
| April | 4.2 °C | 16.6 °C | 93.73 mm |
| May | 10.8 °C | 22.5 °C | 127 mm |
| June | 16.3 °C | 27.7 °C | 127 mm |
| July | 18.3 °C | 29.6 °C | 102.62 mm |
| August | 16.9 °C | 28.6 °C | 104.14 mm |
| September | 12.2 °C | 25.3 °C | 88.9 mm |
| October | 5.6 °C | 17.9 °C | 72.14 mm |
| November | -0.8 °C | 9.4 °C | 56.64 mm |
| December | -6.6 °C | 2.2 °C | 45.47 mm |
| Annual | 4.8 °C | 15.8 °C | 956.32 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 |