Dorchester, Illinois has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 7.2 °C (low) to 17.7 °C (high). Annual precipitation averages 1068.1 Millimeters. Dorchester is located in USDA Plant Hardiness Zone 6b. The growing season typically runs from the last frost around Apr. 1 - Apr. 10 through the first frost around Oct. 21 - Oct. 31.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | -6.1 °C | 2.6 °C | 64.26 mm |
| February | -4.1 °C | 5.7 °C | 56.64 mm |
| March | 0.8 °C | 11.6 °C | 80.26 mm |
| April | 6.7 °C | 18.2 °C | 120.14 mm |
| May | 12.8 °C | 23.6 °C | 127.76 mm |
| June | 17.8 °C | 28.4 °C | 109.73 mm |
| July | 19.7 °C | 30.1 °C | 99.57 mm |
| August | 18.5 °C | 29.6 °C | 89.15 mm |
| September | 13.9 °C | 26.4 °C | 84.07 mm |
| October | 7.5 °C | 19.7 °C | 82.8 mm |
| November | 1.6 °C | 11.7 °C | 87.88 mm |
| December | -3.3 °C | 5.3 °C | 65.79 mm |
| Annual | 7.2 °C | 17.7 °C | 1068.05 mm |
| Average First Frost Date | Oct. 21 - Oct. 31 |
| Average Last Frost Date | Apr. 1 - Apr. 10 |
| Average Days With Frost | 75-100 Days |
| Plant Hardiness Zone | Zone 6b : -20.6°C to -17.8°C |
| Plant Hardiness Zone - 2012 | Zone 6a : -23.3°C to -20.6°C |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Dfa - Humid Continental Hot Summers With Year Around Precipitation |