Colchester, Illinois has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 5.4 °C (low) to 16.5 °C (high). Annual precipitation averages 993.9 Millimeters. 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 | -8.8 °C | 0.6 °C | 46.23 mm |
| February | -6.8 °C | 3.2 °C | 49.28 mm |
| March | -1.2 °C | 10 °C | 61.47 mm |
| April | 4.8 °C | 17.2 °C | 100.08 mm |
| May | 11.3 °C | 22.9 °C | 130.81 mm |
| June | 16.7 °C | 27.8 °C | 124.71 mm |
| July | 18.6 °C | 29.6 °C | 98.81 mm |
| August | 17.2 °C | 28.7 °C | 92.2 mm |
| September | 12.6 °C | 25.7 °C | 93.73 mm |
| October | 6.1 °C | 18.7 °C | 78.99 mm |
| November | -0.2 °C | 10.4 °C | 68.07 mm |
| December | -5.6 °C | 3.4 °C | 49.53 mm |
| Annual | 5.4 °C | 16.5 °C | 993.91 mm |
| 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 : -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 |