Oswego, Illinois has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 40.3 °F (low) to 59.8 °F (high). Annual precipitation averages 38.8 Inches. Oswego 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 | 15.4 °F | 31.1 °F | 1.95 in. |
| February | 18.2 °F | 35.1 °F | 1.88 in. |
| March | 28.0 °F | 47.2 °F | 2.4 in. |
| April | 38.6 °F | 60.2 °F | 3.93 in. |
| May | 49.6 °F | 71.2 °F | 4.76 in. |
| June | 59.7 °F | 80.8 °F | 4.51 in. |
| July | 63.6 °F | 84.2 °F | 3.96 in. |
| August | 61.9 °F | 82.4 °F | 3.93 in. |
| September | 54.1 °F | 76.6 °F | 3.31 in. |
| October | 42.0 °F | 63.6 °F | 3.42 in. |
| November | 31.2 °F | 48.9 °F | 2.53 in. |
| December | 21.3 °F | 36.4 °F | 2.17 in. |
| Annual | 40.3 °F | 59.8 °F | 38.75 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 |