Alton, 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 18.6 °C (high). Annual precipitation averages 1085.9 Millimeters. Alton 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 Nov. 1 - Nov. 10.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | -4.9 °C | 3.9 °C | 65.79 mm |
| February | -3.1 °C | 6.9 °C | 58.67 mm |
| March | 1.8 °C | 12.6 °C | 84.33 mm |
| April | 7.8 °C | 19.1 °C | 119.38 mm |
| May | 13.8 °C | 24.2 °C | 131.32 mm |
| June | 18.9 °C | 29.1 °C | 109.22 mm |
| July | 20.9 °C | 30.8 °C | 100.33 mm |
| August | 19.8 °C | 30.3 °C | 91.95 mm |
| September | 15.2 °C | 26.8 °C | 84.07 mm |
| October | 8.7 °C | 20.3 °C | 83.82 mm |
| November | 2.5 °C | 12.7 °C | 89.66 mm |
| December | -2.4 °C | 6.5 °C | 67.31 mm |
| Annual | 8.3 °C | 18.6 °C | 1085.85 mm |
| Average First Frost Date | Nov. 1 - Nov. 10 |
| 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 |