Grand Detour, Illinois has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 39 °F (low) to 58.7 °F (high). Annual precipitation averages 38.9 Inches. 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 | 12.7 °F | 29.1 °F | 1.77 in. |
| February | 16.4 °F | 33.7 °F | 1.76 in. |
| March | 27.1 °F | 46.3 °F | 2.44 in. |
| April | 37.6 °F | 59.8 °F | 3.68 in. |
| May | 49.5 °F | 71.0 °F | 4.82 in. |
| June | 59.3 °F | 79.9 °F | 5.16 in. |
| July | 62.6 °F | 82.7 °F | 4.36 in. |
| August | 60.6 °F | 81.2 °F | 4.03 in. |
| September | 52.3 °F | 75.4 °F | 3.64 in. |
| October | 40.6 °F | 62.5 °F | 2.9 in. |
| November | 29.6 °F | 47.4 °F | 2.33 in. |
| December | 19.3 °F | 34.8 °F | 2.04 in. |
| Annual | 39 °F | 58.7 °F | 38.93 in. |
| 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 : -15°F to -10°F |
| Plant Hardiness Zone - 2012 | Zone 5a : -20°F to -15°F |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Dfa - Humid Continental Hot Summers With Year Around Precipitation |