Monticello, Illinois has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 42.1 °F (low) to 62.2 °F (high). Annual precipitation averages 39.4 Inches. Monticello is located in USDA Plant Hardiness Zone 6a. 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 | 18.0 °F | 34.0 °F | 2.33 in. |
| February | 21.0 °F | 38.6 °F | 1.97 in. |
| March | 30.2 °F | 50.1 °F | 2.76 in. |
| April | 40.9 °F | 63.7 °F | 4.09 in. |
| May | 52.7 °F | 74.0 °F | 4.67 in. |
| June | 61.7 °F | 82.7 °F | 4.61 in. |
| July | 64.6 °F | 85.0 °F | 3.98 in. |
| August | 62.1 °F | 83.2 °F | 3.24 in. |
| September | 54.8 °F | 79.1 °F | 3.08 in. |
| October | 42.8 °F | 66.1 °F | 3.34 in. |
| November | 32.3 °F | 51.3 °F | 2.99 in. |
| December | 23.8 °F | 38.8 °F | 2.34 in. |
| Annual | 42.1 °F | 62.2 °F | 39.4 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 6a : -10°F to -5°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 |