Union, Illinois has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 38.5 °F (low) to 58.5 °F (high). Annual precipitation averages 38.4 Inches. Union 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 | 13.4 °F | 29.5 °F | 1.77 in. |
| February | 16.3 °F | 33.7 °F | 1.77 in. |
| March | 26.0 °F | 45.4 °F | 2.24 in. |
| April | 36.4 °F | 58.5 °F | 3.75 in. |
| May | 47.7 °F | 70.3 °F | 4.77 in. |
| June | 57.6 °F | 79.9 °F | 5.03 in. |
| July | 62.1 °F | 83.6 °F | 3.98 in. |
| August | 60.2 °F | 81.7 °F | 4.03 in. |
| September | 52.1 °F | 75.1 °F | 3.59 in. |
| October | 40.6 °F | 62.0 °F | 2.97 in. |
| November | 29.8 °F | 47.2 °F | 2.43 in. |
| December | 19.4 °F | 34.7 °F | 2.02 in. |
| Annual | 38.5 °F | 58.5 °F | 38.35 in. |
| Average First Frost Date | Oct. 11 - Oct. 20 |
| 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 5b : -15°F to -10°F |
| Current Drought Conditions | Abnormally Dry |
| Köppen Climate Classification | Dfa - Humid Continental Hot Summers With Year Around Precipitation |