Harlan, Iowa has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 38.3 °F (low) to 59.7 °F (high). Annual precipitation averages 34.3 Inches. Harlan 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 | 11.1 °F | 29.8 °F | 0.87 in. |
| February | 15.3 °F | 34.6 °F | 1.07 in. |
| March | 26.2 °F | 48.1 °F | 1.95 in. |
| April | 37.1 °F | 61.1 °F | 3.59 in. |
| May | 49.6 °F | 71.6 °F | 4.84 in. |
| June | 60.0 °F | 81.2 °F | 4.97 in. |
| July | 63.8 °F | 84.4 °F | 3.74 in. |
| August | 60.9 °F | 82.5 °F | 4.24 in. |
| September | 51.7 °F | 76.6 °F | 3.63 in. |
| October | 39.8 °F | 64.1 °F | 2.66 in. |
| November | 27.0 °F | 47.6 °F | 1.44 in. |
| December | 17.0 °F | 34.8 °F | 1.29 in. |
| Annual | 38.3 °F | 59.7 °F | 34.29 in. |
| 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 : -15°F to -10°F |
| Plant Hardiness Zone - 2012 | Zone 5a : -20°F to -15°F |
| Current Drought Conditions | Abnormally Dry |
| Köppen Climate Classification | Dfa - Humid Continental Hot Summers With Year Around Precipitation |