Glen Carbon, Illinois has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 8.3 °C (low) to 18.8 °C (high). Annual precipitation averages 1135.7 Millimeters. Glen Carbon is located in USDA Plant Hardiness Zone 7a. The growing season typically runs from the last frost around Apr. 1 - Apr. 10 through the first frost around Oct. 21 - Oct. 31.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | -4.8 °C | 4.2 °C | 67.82 mm |
| February | -2.9 °C | 7.2 °C | 61.72 mm |
| March | 1.9 °C | 12.8 °C | 88.9 mm |
| April | 7.9 °C | 19.4 °C | 125.48 mm |
| May | 13.9 °C | 24.4 °C | 133.35 mm |
| June | 18.8 °C | 29.3 °C | 121.67 mm |
| July | 20.9 °C | 31.1 °C | 111.51 mm |
| August | 19.8 °C | 30.5 °C | 100.08 mm |
| September | 15.2 °C | 27 °C | 80.52 mm |
| October | 8.8 °C | 20.6 °C | 84.07 mm |
| November | 2.6 °C | 12.8 °C | 91.19 mm |
| December | -2.3 °C | 6.7 °C | 69.34 mm |
| Annual | 8.3 °C | 18.8 °C | 1135.65 mm |
| Average First Frost Date | Oct. 21 - Oct. 31 |
| Average Last Frost Date | Apr. 1 - Apr. 10 |
| Average Days With Frost | 75-100 Days |
| Plant Hardiness Zone | Zone 7a : -17.8°C to -15°C |
| Plant Hardiness Zone - 2012 | Zone 6b : -20.6°C to -17.8°C |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Dfa - Humid Continental Hot Summers With Year Around Precipitation |