Millbrook, Illinois has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 40.3 °F (low) to 59.7 °F (high). Annual precipitation averages 38.2 Inches. Millbrook is located in USDA Plant Hardiness Zone 5b. 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 | 15.4 °F | 30.9 °F | 1.84 in. |
| February | 18.3 °F | 34.9 °F | 1.78 in. |
| March | 28.1 °F | 47.0 °F | 2.35 in. |
| April | 38.7 °F | 60.2 °F | 3.78 in. |
| May | 49.9 °F | 71.5 °F | 4.68 in. |
| June | 59.9 °F | 80.8 °F | 4.65 in. |
| July | 63.5 °F | 84.0 °F | 3.93 in. |
| August | 61.7 °F | 82.1 °F | 3.86 in. |
| September | 53.7 °F | 76.7 °F | 3.42 in. |
| October | 41.8 °F | 63.7 °F | 3.28 in. |
| November | 31.2 °F | 48.9 °F | 2.51 in. |
| December | 21.2 °F | 36.2 °F | 2.11 in. |
| Annual | 40.3 °F | 59.7 °F | 38.19 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 5b : -15°F to -10°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 |