Castalia, Iowa has a Humid Continental Mild Summer, Wet All Year climate (Köppen classification: Dfb). Average annual temperatures range from 35.6 °F (low) to 55.6 °F (high). Annual precipitation averages 38.5 Inches. Castalia 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 | 7.5 °F | 24.5 °F | 1.08 in. |
| February | 10.8 °F | 29.0 °F | 1.16 in. |
| March | 23.0 °F | 42.0 °F | 2.14 in. |
| April | 34.6 °F | 56.5 °F | 4.09 in. |
| May | 46.6 °F | 68.5 °F | 4.92 in. |
| June | 57.6 °F | 78.3 °F | 5.95 in. |
| July | 60.9 °F | 81.5 °F | 4.65 in. |
| August | 58.2 °F | 79.6 °F | 4.27 in. |
| September | 49.9 °F | 73.0 °F | 3.88 in. |
| October | 37.5 °F | 60.0 °F | 2.89 in. |
| November | 25.8 °F | 43.4 °F | 1.97 in. |
| December | 14.4 °F | 30.3 °F | 1.47 in. |
| Annual | 35.6 °F | 55.6 °F | 38.47 in. |
| 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 : -20°F to -15°F |
| Plant Hardiness Zone - 2012 | Zone 4b : -25°F to -20°F |
| Current Drought Conditions | Normal |
| Köppen Climate Classification | Dfb - Humid Continental Mild Summer, Wet All Year |