Flat Rock, Illinois has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 44.4 °F (low) to 65.2 °F (high). Annual precipitation averages 45.8 Inches. 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 | 22.0 °F | 38.1 °F | 3.2 in. |
| February | 24.9 °F | 43.0 °F | 2.49 in. |
| March | 33.4 °F | 53.7 °F | 3.63 in. |
| April | 43.8 °F | 66.1 °F | 5.08 in. |
| May | 54.7 °F | 75.9 °F | 4.93 in. |
| June | 63.4 °F | 84.5 °F | 4.87 in. |
| July | 66.0 °F | 87.4 °F | 4.3 in. |
| August | 63.7 °F | 86.5 °F | 3.12 in. |
| September | 55.9 °F | 81.1 °F | 3.47 in. |
| October | 44.5 °F | 68.9 °F | 3.65 in. |
| November | 34.3 °F | 54.3 °F | 3.82 in. |
| December | 26.4 °F | 42.6 °F | 3.22 in. |
| Annual | 44.4 °F | 65.2 °F | 45.78 in. |
| 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 : -5°F to 0°F |
| Plant Hardiness Zone - 2012 | Zone 6a : -10°F to -5°F |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Dfa - Humid Continental Hot Summers With Year Around Precipitation |