Royal, Illinois has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 41.9 °F (low) to 61.8 °F (high). Annual precipitation averages 40.3 Inches. Royal 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 | 17.8 °F | 33.6 °F | 2.35 in. |
| February | 21.0 °F | 38.2 °F | 2.09 in. |
| March | 30.1 °F | 49.8 °F | 2.81 in. |
| April | 40.7 °F | 63.5 °F | 4.03 in. |
| May | 52.5 °F | 73.7 °F | 4.51 in. |
| June | 61.5 °F | 82.0 °F | 4.76 in. |
| July | 64.2 °F | 84.3 °F | 4.2 in. |
| August | 61.9 °F | 82.9 °F | 3.26 in. |
| September | 54.6 °F | 78.3 °F | 3.27 in. |
| October | 43.0 °F | 65.7 °F | 3.38 in. |
| November | 32.4 °F | 51.1 °F | 3.19 in. |
| December | 23.5 °F | 38.5 °F | 2.4 in. |
| Annual | 41.9 °F | 61.8 °F | 40.25 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 |