Ottawa, Illinois has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 40.7 °F (low) to 60.6 °F (high). Annual precipitation averages 38.2 Inches. Ottawa 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 | 15.6 °F | 31.7 °F | 1.98 in. |
| February | 19.0 °F | 36.1 °F | 1.89 in. |
| March | 28.4 °F | 48.4 °F | 2.5 in. |
| April | 39.1 °F | 61.6 °F | 3.51 in. |
| May | 50.4 °F | 72.6 °F | 4.71 in. |
| June | 60.1 °F | 81.4 °F | 4.6 in. |
| July | 63.8 °F | 84.4 °F | 3.85 in. |
| August | 61.8 °F | 82.6 °F | 3.9 in. |
| September | 54.0 °F | 77.2 °F | 3.32 in. |
| October | 42.5 °F | 64.7 °F | 3.1 in. |
| November | 31.7 °F | 49.8 °F | 2.7 in. |
| December | 21.5 °F | 36.9 °F | 2.12 in. |
| Annual | 40.7 °F | 60.6 °F | 38.18 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 |