| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 13°F | 27°F | 19°F | 0.5" |
| February | 14°F | 29°F | 21°F | 0.4" |
| March | 29°F | 45°F | 36°F | 1.5" |
| April | 40°F | 59°F | 48°F | 3.4" |
| May | 50°F | 70°F | 60°F | 2.9" |
| June | 61°F | 79°F | 69°F | 4.3" |
| July | 65°F | 82°F | 73°F | 3.9" |
| August | 62°F | 79°F | 70°F | 4.0" |
| September | 54°F | 73°F | 63°F | 3.4" |
| October | 43°F | 61°F | 51°F | 3.0" |
| November | 31°F | 47°F | 38°F | 1.0" |
| December | 18°F | 31°F | 24°F | 0.9" |
| Index | Castalia | Iowa | National |
| Air quality index | 35 | 39 | 42 |
| Pollution index | 530,484 | 1,154,535 | 2,726,193 |
| Measurement | Total |
| Days measured | 365 |
| Days with good air quality | 310 |
| Days with moderate air quality | 55 |
| Days w/ poor A.Q. for sensitive groups | 0 |
| Days with unhealthy air quality | 0 |
| Pollutant | Total |
| Arsenic | 0.000% |
| Benzene | 0.010% |
| Carbon Tetrachloride | 0.001% |
| Lead | 0.002% |
| Mercury | 0.000% |
| City | Population | Air Quality Index | National Avg. |
| Monona, IA | 1,639 | 29 | 42.0% |
| St. Olaf, IA | 128 | 29 | 42.0% |
| Waukon, IA | 3,663 | 35 | 30.0% |
| Luana, IA | 238 | 35 | 30.0% |
| Decorah, IA | 7,615 | 35 | 30.0% |
| Castalia, IA | 169 | 35 | 30.0% |
| City | Population | Air Quality Index | National Avg. |
| Vernon, AZ | 169 | 44 | 12.0% |
| Farmington, CA | 169 | 45 | 10.0% |
| Sisquoc, CA | 169 | 46 | 8.0% |
| Admire, KS | 169 | 22 | 56.0% |
| Westphalia, KS | 169 | 22 | 56.0% |
| Castalia, IA | 169 | 35 | 30.0% |
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