Union, West Virginia has a Humid Continental Mild Summer, Wet All Year climate (Köppen classification: Dfb). Average annual temperatures range from 4.6 °C (low) to 17.6 °C (high). Annual precipitation averages 995.4 Millimeters. Union is located in USDA Plant Hardiness Zone 6b. The growing season typically runs from the last frost around May 1 - May 10 through the first frost around Oct. 11 - Oct. 20.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | -5.6 °C | 5.4 °C | 67.31 mm |
| February | -4.6 °C | 7.2 °C | 58.17 mm |
| March | -1.3 °C | 12.1 °C | 85.09 mm |
| April | 3.7 °C | 18.4 °C | 89.66 mm |
| May | 8.9 °C | 22.7 °C | 113.28 mm |
| June | 13.4 °C | 26.3 °C | 107.44 mm |
| July | 15.7 °C | 28.3 °C | 94.49 mm |
| August | 15 °C | 27.7 °C | 92.2 mm |
| September | 11.1 °C | 24.6 °C | 85.85 mm |
| October | 4.3 °C | 19 °C | 66.04 mm |
| November | -1 °C | 12.4 °C | 62.23 mm |
| December | -3.9 °C | 7.1 °C | 73.66 mm |
| Annual | 4.6 °C | 17.6 °C | 995.42 mm |
| Average First Frost Date | Oct. 11 - Oct. 20 |
| Average Last Frost Date | May 1 - May 10 |
| Average Days With Frost | 125-150 Days |
| Plant Hardiness Zone | Zone 6b : -20.6°C to -17.8°C |
| Plant Hardiness Zone - 2012 | Zone 6b : -20.6°C to -17.8°C |
| Current Drought Conditions | Moderate Drought |
| Köppen Climate Classification | Dfb - Humid Continental Mild Summer, Wet All Year |