Monmouth, Illinois has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 40.6 °F (low) to 60.8 °F (high). Annual precipitation averages 38.5 Inches. Monmouth is located in USDA Plant Hardiness Zone 5b. 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 | 14.8 °F | 31.3 °F | 1.73 in. |
| February | 18.2 °F | 35.9 °F | 1.82 in. |
| March | 28.9 °F | 49.2 °F | 2.53 in. |
| April | 39.4 °F | 62.1 °F | 3.82 in. |
| May | 51.4 °F | 72.8 °F | 5.0 in. |
| June | 61.0 °F | 81.7 °F | 4.74 in. |
| July | 64.2 °F | 84.7 °F | 3.96 in. |
| August | 62.0 °F | 83.2 °F | 3.73 in. |
| September | 53.8 °F | 77.6 °F | 3.84 in. |
| October | 42.1 °F | 65.0 °F | 2.83 in. |
| November | 30.5 °F | 49.3 °F | 2.48 in. |
| December | 20.6 °F | 36.6 °F | 1.98 in. |
| Annual | 40.6 °F | 60.8 °F | 38.46 in. |
| Average First Frost Date | Oct. 11 - Oct. 20 |
| Average Last Frost Date | Apr. 11 - Apr. 20 |
| Average Days With Frost | 100-125 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 | Normal |
| Köppen Climate Classification | Dfa - Humid Continental Hot Summers With Year Around Precipitation |