Portland, Iowa has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 36.1 °F (low) to 55.7 °F (high). Annual precipitation averages 36.5 Inches. Portland is located in USDA Plant Hardiness Zone 5a. The growing season typically runs from the last frost around May 1 - May 10 through the first frost around Oct. 1 - Oct. 10.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | 7.5 °F | 24.2 °F | 0.94 in. |
| February | 11.5 °F | 28.8 °F | 1.13 in. |
| March | 23.7 °F | 41.4 °F | 2.01 in. |
| April | 35.3 °F | 56.8 °F | 3.99 in. |
| May | 47.5 °F | 68.9 °F | 5.33 in. |
| June | 58.1 °F | 79.0 °F | 5.54 in. |
| July | 61.8 °F | 82.0 °F | 4.6 in. |
| August | 59.1 °F | 79.7 °F | 3.88 in. |
| September | 50.5 °F | 73.7 °F | 3.35 in. |
| October | 38.0 °F | 60.2 °F | 2.6 in. |
| November | 25.6 °F | 43.7 °F | 1.77 in. |
| December | 14.6 °F | 30.1 °F | 1.32 in. |
| Annual | 36.1 °F | 55.7 °F | 36.46 in. |
| Average First Frost Date | Oct. 1 - Oct. 10 |
| Average Last Frost Date | May 1 - May 10 |
| Average Days With Frost | 150-180 Days |
| Plant Hardiness Zone | Zone 5a : -20°F to -15°F |
| Plant Hardiness Zone - 2012 | Zone 5a : -20°F to -15°F |
| Current Drought Conditions | Moderate Drought |
| Köppen Climate Classification | Dfa - Humid Continental Hot Summers With Year Around Precipitation |