Grand Detour, Illinois has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 3.9 °C (low) to 14.8 °C (high). Annual precipitation averages 988.8 Millimeters. Grand Detour 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 | -10.7 °C | -1.6 °C | 44.96 mm |
| February | -8.7 °C | 0.9 °C | 44.7 mm |
| March | -2.7 °C | 7.9 °C | 61.98 mm |
| April | 3.1 °C | 15.4 °C | 93.47 mm |
| May | 9.7 °C | 21.7 °C | 122.43 mm |
| June | 15.2 °C | 26.6 °C | 131.06 mm |
| July | 17 °C | 28.2 °C | 110.74 mm |
| August | 15.9 °C | 27.3 °C | 102.36 mm |
| September | 11.3 °C | 24.1 °C | 92.46 mm |
| October | 4.8 °C | 16.9 °C | 73.66 mm |
| November | -1.3 °C | 8.6 °C | 59.18 mm |
| December | -7.1 °C | 1.6 °C | 51.82 mm |
| Annual | 3.9 °C | 14.8 °C | 988.82 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 5a : -28.9°C to -26.1°C |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Dfa - Humid Continental Hot Summers With Year Around Precipitation |