Monticello, Iowa has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 37.7 °F (low) to 58.1 °F (high). Annual precipitation averages 38.9 Inches. Monticello is located in USDA Plant Hardiness Zone 5a. The growing season typically runs from the last frost around May 1 - May 10 through the first frost around Oct. 1 - Oct. 10.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | 10.7 °F | 27.7 °F | 1.26 in. |
| February | 14.3 °F | 32.1 °F | 1.46 in. |
| March | 25.8 °F | 45.4 °F | 2.23 in. |
| April | 36.8 °F | 59.6 °F | 3.84 in. |
| May | 48.6 °F | 70.8 °F | 4.51 in. |
| June | 59.0 °F | 80.2 °F | 5.72 in. |
| July | 62.3 °F | 83.2 °F | 4.72 in. |
| August | 59.9 °F | 81.6 °F | 4.44 in. |
| September | 51.0 °F | 75.0 °F | 3.67 in. |
| October | 39.2 °F | 62.1 °F | 3.16 in. |
| November | 27.7 °F | 46.5 °F | 2.23 in. |
| December | 17.3 °F | 33.4 °F | 1.7 in. |
| Annual | 37.7 °F | 58.1 °F | 38.94 in. |
| Average First Frost Date | Oct. 1 - Oct. 10 |
| Average Last Frost Date | May 1 - May 10 |
| Average Days With Frost | 125-150 Days |
| Plant Hardiness Zone | Zone 5a : -20°F to -15°F |
| Plant Hardiness Zone - 2012 | Zone 5a : -20°F to -15°F |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Dfa - Humid Continental Hot Summers With Year Around Precipitation |