Colchester, Illinois has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 41.7 °F (low) to 61.7 °F (high). Annual precipitation averages 39.1 Inches. Colchester 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 | 16.1 °F | 33.0 °F | 1.82 in. |
| February | 19.8 °F | 37.7 °F | 1.94 in. |
| March | 29.8 °F | 50.0 °F | 2.42 in. |
| April | 40.6 °F | 62.9 °F | 3.94 in. |
| May | 52.4 °F | 73.3 °F | 5.15 in. |
| June | 62.0 °F | 82.1 °F | 4.91 in. |
| July | 65.4 °F | 85.3 °F | 3.89 in. |
| August | 63.0 °F | 83.7 °F | 3.63 in. |
| September | 54.6 °F | 78.2 °F | 3.69 in. |
| October | 42.9 °F | 65.6 °F | 3.11 in. |
| November | 31.6 °F | 50.7 °F | 2.68 in. |
| December | 21.9 °F | 38.1 °F | 1.95 in. |
| Annual | 41.7 °F | 61.7 °F | 39.13 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 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 |