Cortland, Illinois has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 38.4 °F (low) to 57.9 °F (high). Annual precipitation averages 37.1 Inches. Cortland 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 | 12.7 °F | 28.5 °F | 1.72 in. |
| February | 16.3 °F | 32.8 °F | 1.75 in. |
| March | 26.2 °F | 45.0 °F | 2.22 in. |
| April | 36.6 °F | 58.5 °F | 3.46 in. |
| May | 48.5 °F | 70.0 °F | 4.89 in. |
| June | 58.4 °F | 79.6 °F | 4.5 in. |
| July | 62.2 °F | 82.6 °F | 3.85 in. |
| August | 60.0 °F | 80.7 °F | 3.93 in. |
| September | 51.7 °F | 74.8 °F | 3.38 in. |
| October | 39.9 °F | 62.0 °F | 3.0 in. |
| November | 29.3 °F | 46.8 °F | 2.39 in. |
| December | 18.9 °F | 34.0 °F | 2.0 in. |
| Annual | 38.4 °F | 57.9 °F | 37.09 in. |
| Average First Frost Date | Oct. 11 - Oct. 20 |
| Average Last Frost Date | Apr. 11 - Apr. 20 |
| Average Days With Frost | 125-150 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 | Abnormally Dry |
| Köppen Climate Classification | Dfa - Humid Continental Hot Summers With Year Around Precipitation |