Johnston City, Illinois has a Humid Subtropical Climate climate (Köppen classification: Cfa). Average annual temperatures range from 7.8 °C (low) to 19.1 °C (high). Annual precipitation averages 1208.8 Millimeters. Johnston City 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. 11 - Oct. 20.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | -4.5 °C | 4.9 °C | 90.17 mm |
| February | -2.8 °C | 7.7 °C | 76.96 mm |
| March | 1.9 °C | 13.3 °C | 112.52 mm |
| April | 7.5 °C | 19.6 °C | 137.67 mm |
| May | 13.3 °C | 24.6 °C | 129.54 mm |
| June | 18.1 °C | 29.2 °C | 113.54 mm |
| July | 19.9 °C | 30.9 °C | 101.6 mm |
| August | 18.6 °C | 30.5 °C | 78.23 mm |
| September | 14.1 °C | 27.3 °C | 80.01 mm |
| October | 7.6 °C | 20.9 °C | 88.9 mm |
| November | 2.1 °C | 13.4 °C | 109.73 mm |
| December | -2.2 °C | 7.3 °C | 89.92 mm |
| Annual | 7.8 °C | 19.1 °C | 1208.79 mm |
| Average First Frost Date | Oct. 11 - Oct. 20 |
| 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 | Abnormally Dry |
| Köppen Climate Classification | Cfa - Humid Subtropical Climate |