Flat Rock, Illinois has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 6.9 °C (low) to 18.4 °C (high). Annual precipitation averages 1162.8 Millimeters. Flat Rock is located in USDA Plant Hardiness Zone 6b. 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 | -5.6 °C | 3.4 °C | 81.28 mm |
| February | -3.9 °C | 6.1 °C | 63.25 mm |
| March | 0.8 °C | 12.1 °C | 92.2 mm |
| April | 6.6 °C | 18.9 °C | 129.03 mm |
| May | 12.6 °C | 24.4 °C | 125.22 mm |
| June | 17.4 °C | 29.2 °C | 123.7 mm |
| July | 18.9 °C | 30.8 °C | 109.22 mm |
| August | 17.6 °C | 30.3 °C | 79.25 mm |
| September | 13.3 °C | 27.3 °C | 88.14 mm |
| October | 6.9 °C | 20.5 °C | 92.71 mm |
| November | 1.3 °C | 12.4 °C | 97.03 mm |
| December | -3.1 °C | 5.9 °C | 81.79 mm |
| Annual | 6.9 °C | 18.4 °C | 1162.82 mm |
| Average First Frost Date | Oct. 11 - Oct. 20 |
| Average Last Frost Date | Apr. 11 - Apr. 20 |
| Average Days With Frost | 100-125 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 |