| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 23°F | 36°F | 29°F | 5.8" |
| February | 17°F | 32°F | 25°F | 2.4" |
| March | 28°F | 47°F | 36°F | 3.5" |
| April | 38°F | 58°F | 48°F | 6.9" |
| May | 47°F | 71°F | 59°F | 2.1" |
| June | 60°F | 83°F | 71°F | 4.1" |
| July | 61°F | 84°F | 72°F | 6.2" |
| August | 62°F | 83°F | 72°F | 13.1" |
| September | 55°F | 78°F | 66°F | 4.6" |
| October | 48°F | 66°F | 56°F | 4.3" |
| November | 33°F | 52°F | 43°F | 4.5" |
| December | 23°F | 36°F | 29°F | 3.6" |
| Index | Castalia | Ohio | National |
| Air quality index | 40 | 40 | 42 |
| Pollution index | 2,402,580 | 4,472,386 | 2,726,193 |
| Measurement | Total |
| Days measured | 365 |
| Days with good air quality | 291 |
| Days with moderate air quality | 73 |
| Days w/ poor A.Q. for sensitive groups | 1 |
| Days with unhealthy air quality | 0 |
| Pollutant | Total |
| Arsenic | 0.001% |
| Benzene | 0.020% |
| Carbon Tetrachloride | 0.005% |
| Lead | 0.020% |
| Mercury | 0.003% |
| City | Population | Air Quality Index | National Avg. |
| Kelleys Island, OH | 283 | 1 | 98.0% |
| Lakeside, OH | 686 | 1 | 98.0% |
| Marblehead, OH | 858 | 1 | 98.0% |
| Sandusky, OH | 24,651 | 1 | 98.0% |
| Green Springs, OH | 1,426 | 40 | 20.0% |
| Castalia, OH | 1,142 | 40 | 20.0% |
| City | Population | Air Quality Index | National Avg. |
| Foreman, AR | 1,142 | 39 | 22.0% |
| Ridgway, CO | 1,142 | 44 | 12.0% |
| Discovery Harbour, HI | 1,142 | 16 | 68.0% |
| Eddyville, IA | 1,142 | 39 | 22.0% |
| Akron, IN | 1,142 | 37 | 26.0% |
| Castalia, OH | 1,142 | 40 | 20.0% |
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