Union, Iowa has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 37.9 °F (low) to 58 °F (high). Annual precipitation averages 36.6 Inches. Union is located in USDA Plant Hardiness Zone 5a. 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 | 10.2 °F | 27.2 °F | 0.97 in. |
| February | 14.2 °F | 31.9 °F | 1.12 in. |
| March | 25.6 °F | 45.0 °F | 2.08 in. |
| April | 37.0 °F | 59.5 °F | 3.77 in. |
| May | 49.4 °F | 70.7 °F | 5.03 in. |
| June | 60.0 °F | 80.4 °F | 5.64 in. |
| July | 63.2 °F | 83.6 °F | 4.3 in. |
| August | 60.5 °F | 81.5 °F | 4.3 in. |
| September | 51.7 °F | 75.7 °F | 3.41 in. |
| October | 39.4 °F | 62.1 °F | 2.63 in. |
| November | 27.2 °F | 46.1 °F | 1.9 in. |
| December | 16.4 °F | 32.8 °F | 1.47 in. |
| Annual | 37.9 °F | 58 °F | 36.62 in. |
| Average First Frost Date | Oct. 1 - Oct. 10 |
| Average Last Frost Date | Apr. 21 - Apr. 30 |
| 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 | Abnormally Dry |
| Köppen Climate Classification | Dfa - Humid Continental Hot Summers With Year Around Precipitation |