Thornton, Iowa has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 2.2 °C (low) to 13.3 °C (high). Annual precipitation averages 914.9 Millimeters. Thornton is located in USDA Plant Hardiness Zone 5a. The growing season typically runs from the last frost around May 1 - May 10 through the first frost around Oct. 1 - Oct. 10.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | -13.7 °C | -4.2 °C | 23.11 mm |
| February | -11.3 °C | -1.6 °C | 26.92 mm |
| March | -4.7 °C | 5.4 °C | 51.31 mm |
| April | 1.7 °C | 13.9 °C | 98.81 mm |
| May | 8.7 °C | 20.7 °C | 129.54 mm |
| June | 14.7 °C | 26.2 °C | 143 mm |
| July | 16.4 °C | 27.8 °C | 113.79 mm |
| August | 14.9 °C | 26.6 °C | 103.63 mm |
| September | 9.9 °C | 23.3 °C | 83.31 mm |
| October | 3.1 °C | 15.8 °C | 63.5 mm |
| November | -3.7 °C | 6.6 °C | 43.69 mm |
| December | -9.9 °C | -1 °C | 34.29 mm |
| Annual | 2.2 °C | 13.3 °C | 914.9 mm |
| Average First Frost Date | Oct. 1 - Oct. 10 |
| Average Last Frost Date | May 1 - May 10 |
| 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 |