Shannon, Illinois has a Humid Continental Mild Summer, Wet All Year climate (Köppen classification: Dfb). Average annual temperatures range from 3.2 °C (low) to 14.4 °C (high). Annual precipitation averages 970.3 Millimeters. Shannon is located in USDA Plant Hardiness Zone 5a. The growing season typically runs from the last frost around May 1 - May 10 through the first frost around Oct. 1 - Oct. 10.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | -11.2 °C | -2.1 °C | 40.39 mm |
| February | -9.2 °C | 0.3 °C | 43.94 mm |
| March | -3.4 °C | 7.2 °C | 56.64 mm |
| April | 2.4 °C | 14.8 °C | 95.5 mm |
| May | 9 °C | 21.3 °C | 115.06 mm |
| June | 14.5 °C | 26.4 °C | 132.84 mm |
| July | 16.3 °C | 27.9 °C | 116.33 mm |
| August | 15.2 °C | 27.2 °C | 97.79 mm |
| September | 10.4 °C | 23.7 °C | 93.73 mm |
| October | 4.3 °C | 16.6 °C | 71.88 mm |
| November | -1.8 °C | 8 °C | 56.9 mm |
| December | -7.7 °C | 0.9 °C | 49.28 mm |
| Annual | 3.2 °C | 14.4 °C | 970.28 mm |
| Average First Frost Date | Oct. 1 - Oct. 10 |
| Average Last Frost Date | May 1 - May 10 |
| Average Days With Frost | 125-150 Days |
| Plant Hardiness Zone | Zone 5a : -28.9°C to -26.1°C |
| Plant Hardiness Zone - 2012 | Zone 5a : -28.9°C to -26.1°C |
| Current Drought Conditions | Abnormally Dry |
| Köppen Climate Classification | Dfb - Humid Continental Mild Summer, Wet All Year |