Romeoville, Illinois has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 41 °F (low) to 60.1 °F (high). Annual precipitation averages 40.3 Inches. Romeoville is located in USDA Plant Hardiness Zone 6a. 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 | 16.3 °F | 31.6 °F | 2.15 in. |
| February | 19.0 °F | 35.5 °F | 2.0 in. |
| March | 28.6 °F | 47.4 °F | 2.55 in. |
| April | 39.1 °F | 60.4 °F | 4.0 in. |
| May | 50.0 °F | 71.3 °F | 4.86 in. |
| June | 60.1 °F | 80.8 °F | 4.71 in. |
| July | 64.2 °F | 84.5 °F | 4.24 in. |
| August | 62.8 °F | 82.6 °F | 4.24 in. |
| September | 55.0 °F | 76.9 °F | 3.32 in. |
| October | 42.8 °F | 63.7 °F | 3.52 in. |
| November | 32.0 °F | 49.2 °F | 2.5 in. |
| December | 22.0 °F | 36.8 °F | 2.22 in. |
| Annual | 41 °F | 60.1 °F | 40.31 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 6a : -10°F to -5°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 |