Somonauk, Illinois has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 39.6 °F (low) to 59.3 °F (high). Annual precipitation averages 37.9 Inches. Somonauk is located in USDA Plant Hardiness Zone 5b. 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 | 14.2 °F | 29.9 °F | 1.81 in. |
| February | 17.6 °F | 34.3 °F | 1.77 in. |
| March | 27.4 °F | 46.6 °F | 2.33 in. |
| April | 38.0 °F | 60.0 °F | 3.67 in. |
| May | 49.5 °F | 71.3 °F | 4.73 in. |
| June | 59.4 °F | 80.6 °F | 4.6 in. |
| July | 63.1 °F | 83.7 °F | 3.92 in. |
| August | 60.9 °F | 81.7 °F | 3.87 in. |
| September | 52.9 °F | 76.2 °F | 3.44 in. |
| October | 41.1 °F | 63.3 °F | 3.2 in. |
| November | 30.4 °F | 48.2 °F | 2.5 in. |
| December | 20.2 °F | 35.4 °F | 2.07 in. |
| Annual | 39.6 °F | 59.3 °F | 37.91 in. |
| Average First Frost Date | Oct. 11 - Oct. 20 |
| Average Last Frost Date | Apr. 11 - Apr. 20 |
| Average Days With Frost | 125-150 Days |
| Plant Hardiness Zone | Zone 5b : -15°F to -10°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 |