Fairfield, Illinois has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 45.4 °F (low) to 65.9 °F (high). Annual precipitation averages 45.9 Inches. Fairfield is located in USDA Plant Hardiness Zone 6b. The growing season typically runs from the last frost around Apr. 11 - Apr. 20 through the first frost around Oct. 11 - Oct. 20.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | 23.4 °F | 39.5 °F | 3.34 in. |
| February | 26.1 °F | 44.5 °F | 2.75 in. |
| March | 34.5 °F | 54.7 °F | 4.0 in. |
| April | 44.8 °F | 66.8 °F | 4.87 in. |
| May | 55.3 °F | 76.3 °F | 4.92 in. |
| June | 64.0 °F | 85.0 °F | 4.82 in. |
| July | 67.0 °F | 87.8 °F | 3.91 in. |
| August | 64.6 °F | 86.9 °F | 3.38 in. |
| September | 56.7 °F | 81.2 °F | 3.12 in. |
| October | 45.3 °F | 69.6 °F | 3.58 in. |
| November | 35.1 °F | 55.1 °F | 3.9 in. |
| December | 27.5 °F | 43.8 °F | 3.28 in. |
| Annual | 45.4 °F | 65.9 °F | 45.87 in. |
| Average First Frost Date | Oct. 11 - Oct. 20 |
| Average Last Frost Date | Apr. 11 - Apr. 20 |
| Average Days With Frost | 75-100 Days |
| Plant Hardiness Zone | Zone 6b : -5°F to 0°F |
| Plant Hardiness Zone - 2012 | Zone 6a : -10°F to -5°F |
| Current Drought Conditions | Abnormally Dry |
| Köppen Climate Classification | Dfa - Humid Continental Hot Summers With Year Around Precipitation |