Marengo, Illinois has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 3.6 °C (low) to 14.6 °C (high). Annual precipitation averages 966.5 Millimeters. Marengo is located in USDA Plant Hardiness Zone 5b. The growing season typically runs from the last frost around Apr. 21 - Apr. 30 through the first frost around Oct. 11 - Oct. 20.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | -10.4 °C | -1.6 °C | 43.94 mm |
| February | -8.8 °C | 0.8 °C | 43.18 mm |
| March | -3.3 °C | 7.3 °C | 56.9 mm |
| April | 2.6 °C | 14.7 °C | 95 mm |
| May | 8.8 °C | 21.3 °C | 120.14 mm |
| June | 14.3 °C | 26.6 °C | 129.79 mm |
| July | 16.7 °C | 28.6 °C | 101.09 mm |
| August | 15.7 °C | 27.6 °C | 103.38 mm |
| September | 11.2 °C | 24 °C | 91.19 mm |
| October | 4.7 °C | 16.7 °C | 71.12 mm |
| November | -1.3 °C | 8.3 °C | 60.45 mm |
| December | -7.1 °C | 1.3 °C | 50.29 mm |
| Annual | 3.6 °C | 14.6 °C | 966.47 mm |
| Average First Frost Date | Oct. 11 - Oct. 20 |
| Average Last Frost Date | Apr. 21 - Apr. 30 |
| Average Days With Frost | 125-150 Days |
| Plant Hardiness Zone | Zone 5b : -26.1°C to -23.3°C |
| Plant Hardiness Zone - 2012 | Zone 5b : -26.1°C to -23.3°C |
| Current Drought Conditions | Abnormally Dry |
| Köppen Climate Classification | Dfa - Humid Continental Hot Summers With Year Around Precipitation |