Monticello, Illinois has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 5.6 °C (low) to 16.8 °C (high). Annual precipitation averages 1000.8 Millimeters. 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 | -7.8 °C | 1.1 °C | 59.18 mm |
| February | -6.1 °C | 3.7 °C | 50.04 mm |
| March | -1 °C | 10.1 °C | 70.1 mm |
| April | 4.9 °C | 17.6 °C | 103.89 mm |
| May | 11.5 °C | 23.3 °C | 118.62 mm |
| June | 16.5 °C | 28.2 °C | 117.09 mm |
| July | 18.1 °C | 29.4 °C | 101.09 mm |
| August | 16.7 °C | 28.4 °C | 82.3 mm |
| September | 12.7 °C | 26.2 °C | 78.23 mm |
| October | 6 °C | 18.9 °C | 84.84 mm |
| November | 0.2 °C | 10.7 °C | 75.95 mm |
| December | -4.6 °C | 3.8 °C | 59.44 mm |
| Annual | 5.6 °C | 16.8 °C | 1000.77 mm |
| 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 : -23.3°C to -20.6°C |
| Plant Hardiness Zone - 2012 | Zone 5b : -26.1°C to -23.3°C |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Dfa - Humid Continental Hot Summers With Year Around Precipitation |