Alton, Iowa has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 2.5 °C (low) to 14.4 °C (high). Annual precipitation averages 729.7 Millimeters. Alton is located in USDA Plant Hardiness Zone 5a. The growing season typically runs from the last frost around Apr. 21 - Apr. 30 through the first frost around Oct. 1 - Oct. 10.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | -13.1 °C | -2.9 °C | 18.54 mm |
| February | -10.7 °C | -0.3 °C | 19.81 mm |
| March | -4.4 °C | 6.8 °C | 43.94 mm |
| April | 1.7 °C | 15.1 °C | 76.45 mm |
| May | 8.9 °C | 21.8 °C | 101.09 mm |
| June | 15 °C | 27.3 °C | 101.85 mm |
| July | 16.8 °C | 28.9 °C | 86.11 mm |
| August | 15.4 °C | 27.8 °C | 85.85 mm |
| September | 10.3 °C | 24.3 °C | 78.23 mm |
| October | 3.2 °C | 16.9 °C | 59.44 mm |
| November | -3.9 °C | 7.4 °C | 34.29 mm |
| December | -9.8 °C | -0.2 °C | 24.13 mm |
| Annual | 2.5 °C | 14.4 °C | 729.73 mm |
| Average First Frost Date | Oct. 1 - Oct. 10 |
| Average Last Frost Date | Apr. 21 - Apr. 30 |
| Average Days With Frost | 150-180 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 | Dfa - Humid Continental Hot Summers With Year Around Precipitation |