Letts, Iowa has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 40.6 °F (low) to 60.5 °F (high). Annual precipitation averages 37.7 Inches. 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 | 14.3 °F | 30.8 °F | 1.36 in. |
| February | 17.8 °F | 35.3 °F | 1.68 in. |
| March | 28.5 °F | 48.2 °F | 2.42 in. |
| April | 39.5 °F | 61.9 °F | 3.69 in. |
| May | 51.4 °F | 72.5 °F | 5.0 in. |
| June | 61.4 °F | 81.8 °F | 5.0 in. |
| July | 64.9 °F | 85.3 °F | 4.04 in. |
| August | 62.4 °F | 83.4 °F | 4.1 in. |
| September | 53.9 °F | 77.5 °F | 3.5 in. |
| October | 42.0 °F | 64.2 °F | 2.84 in. |
| November | 30.5 °F | 49.0 °F | 2.23 in. |
| December | 20.1 °F | 36.0 °F | 1.79 in. |
| Annual | 40.6 °F | 60.5 °F | 37.65 in. |
| 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 : -15°F to -10°F |
| Plant Hardiness Zone - 2012 | Zone 5b : -15°F to -10°F |
| Current Drought Conditions | Abnormally Dry |
| Köppen Climate Classification | Dfa - Humid Continental Hot Summers With Year Around Precipitation |