Alton, Iowa has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 36.4 °F (low) to 57.9 °F (high). Annual precipitation averages 28.7 Inches. 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 | 8.5 °F | 26.7 °F | 0.73 in. |
| February | 12.8 °F | 31.5 °F | 0.78 in. |
| March | 24.1 °F | 44.3 °F | 1.73 in. |
| April | 35.1 °F | 59.1 °F | 3.01 in. |
| May | 48.0 °F | 71.2 °F | 3.98 in. |
| June | 59.0 °F | 81.1 °F | 4.01 in. |
| July | 62.3 °F | 84.1 °F | 3.39 in. |
| August | 59.8 °F | 82.0 °F | 3.38 in. |
| September | 50.5 °F | 75.8 °F | 3.08 in. |
| October | 37.8 °F | 62.5 °F | 2.34 in. |
| November | 24.9 °F | 45.4 °F | 1.35 in. |
| December | 14.3 °F | 31.6 °F | 0.95 in. |
| Annual | 36.4 °F | 57.9 °F | 28.73 in. |
| 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 : -20°F to -15°F |
| Plant Hardiness Zone - 2012 | Zone 5a : -20°F to -15°F |
| Current Drought Conditions | Abnormally Dry |
| Köppen Climate Classification | Dfa - Humid Continental Hot Summers With Year Around Precipitation |