Byrnes Mill, Missouri has a Humid Subtropical Climate climate (Köppen classification: Cfa). Average annual temperatures range from 7.9 °C (low) to 19.3 °C (high). Annual precipitation averages 1117.9 Millimeters. Byrnes Mill 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 | -5.3 °C | 4.6 °C | 70.36 mm |
| February | -3.3 °C | 7.7 °C | 61.47 mm |
| March | 1.6 °C | 13.3 °C | 94.23 mm |
| April | 7.5 °C | 20 °C | 121.67 mm |
| May | 13.3 °C | 24.9 °C | 131.83 mm |
| June | 18.3 °C | 29.6 °C | 113.03 mm |
| July | 20.6 °C | 31.7 °C | 101.85 mm |
| August | 19.4 °C | 30.9 °C | 92.46 mm |
| September | 14.7 °C | 27.3 °C | 87.88 mm |
| October | 8.1 °C | 20.8 °C | 83.82 mm |
| November | 2.1 °C | 13.3 °C | 89.92 mm |
| December | -2.8 °C | 7.1 °C | 69.34 mm |
| Annual | 7.9 °C | 19.3 °C | 1117.86 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 6b : -20.6°C to -17.8°C |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Cfa - Humid Subtropical Climate |