McHenry, Illinois has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 38.3 °F (low) to 57.9 °F (high). Annual precipitation averages 38.1 Inches. McHenry is located in USDA Plant Hardiness Zone 5b. The growing season typically runs from the last frost around Apr. 21 - Apr. 30 through the first frost around Oct. 11 - Oct. 20.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | 13.3 °F | 29.2 °F | 1.78 in. |
| February | 16.1 °F | 33.1 °F | 1.84 in. |
| March | 25.7 °F | 44.6 °F | 2.21 in. |
| April | 36.2 °F | 57.7 °F | 3.75 in. |
| May | 47.1 °F | 69.3 °F | 4.68 in. |
| June | 57.2 °F | 79.0 °F | 4.92 in. |
| July | 61.9 °F | 83.0 °F | 3.88 in. |
| August | 60.3 °F | 81.2 °F | 3.87 in. |
| September | 52.2 °F | 74.5 °F | 3.58 in. |
| October | 40.5 °F | 61.4 °F | 3.14 in. |
| November | 29.8 °F | 46.9 °F | 2.41 in. |
| December | 19.5 °F | 34.6 °F | 2.0 in. |
| Annual | 38.3 °F | 57.9 °F | 38.06 in. |
| Average First Frost Date | Oct. 11 - Oct. 20 |
| Average Last Frost Date | Apr. 21 - Apr. 30 |
| 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 |