Grinnell, Iowa has a Humid Continental Hot Summers With Year Around Precipitation climate (Köppen classification: Dfa). Average annual temperatures range from 4.1 °C (low) to 14.9 °C (high). Annual precipitation averages 960.1 Millimeters. Grinnell is located in USDA Plant Hardiness Zone 5b. The growing season typically runs from the last frost around Apr. 21 - Apr. 30 through the first frost around Oct. 11 - Oct. 20.
| Month | Avg. Low | Avg. High | Avg. Precip |
|---|---|---|---|
| January | -10.9 °C | -1.9 °C | 26.67 mm |
| February | -8.8 °C | 0.6 °C | 36.58 mm |
| March | -2.9 °C | 7.8 °C | 54.61 mm |
| April | 3.3 °C | 15.6 °C | 100.08 mm |
| May | 10.4 °C | 21.6 °C | 127.51 mm |
| June | 15.9 °C | 26.8 °C | 138.43 mm |
| July | 18.1 °C | 28.7 °C | 106.93 mm |
| August | 16.6 °C | 27.7 °C | 112.01 mm |
| September | 11.7 °C | 24.4 °C | 92.96 mm |
| October | 5 °C | 17.2 °C | 74.17 mm |
| November | -1.7 °C | 8.5 °C | 50.8 mm |
| December | -7.4 °C | 1.2 °C | 39.37 mm |
| Annual | 4.1 °C | 14.9 °C | 960.12 mm |
| Average First Frost Date | Oct. 11 - Oct. 20 |
| Average Last Frost Date | Apr. 21 - Apr. 30 |
| Average Days With Frost | 125-150 Days |
| Plant Hardiness Zone | Zone 5b : -26.1°C to -23.3°C |
| Plant Hardiness Zone - 2012 | Zone 5a : -28.9°C to -26.1°C |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Dfa - Humid Continental Hot Summers With Year Around Precipitation |