| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 16°F | 33°F | 24°F | 0.5" |
| February | 18°F | 36°F | 26°F | 1.3" |
| March | 31°F | 52°F | 40°F | 7.8" |
| April | 41°F | 65°F | 52°F | 9.4" |
| May | 52°F | 74°F | 62°F | 8.4" |
| June | 62°F | 84°F | 72°F | 5.7" |
| July | 65°F | 87°F | 76°F | 6.6" |
| August | 63°F | 84°F | 73°F | 4.1" |
| September | 55°F | 79°F | 66°F | 7.3" |
| October | 43°F | 66°F | 54°F | 4.3" |
| November | 32°F | 52°F | 41°F | 2.7" |
| December | 20°F | 37°F | 28°F | 3.3" |
| Index | Knoxville | Iowa | National |
| Air quality index | 39 | 39 | 42 |
| Pollution index | 1,478,700 | 1,154,535 | 2,726,193 |
| Measurement | Total |
| Days measured | 365 |
| Days with good air quality | 306 |
| Days with moderate air quality | 59 |
| Days w/ poor A.Q. for sensitive groups | 0 |
| Days with unhealthy air quality | 0 |
| Pollutant | Total |
| Arsenic | 0.010% |
| Benzene | 0.010% |
| Carbon Tetrachloride | 0.002% |
| Lead | 0.010% |
| Mercury | 0.010% |
| City | Population | Air Quality Index | National Avg. |
| Marysville, IA | 50 | 39 | 22.0% |
| Swan, IA | 46 | 39 | 22.0% |
| Hamilton, IA | 61 | 39 | 22.0% |
| Harvey, IA | 176 | 39 | 22.0% |
| Melcher-Dallas, IA | 1,313 | 39 | 22.0% |
| Knoxville, IA | 7,154 | 39 | 22.0% |
| City | Population | Air Quality Index | National Avg. |
| Gridley, CA | 7,154 | 44 | 12.0% |
| Walled Lake, MI | 7,153 | 56 | 12.0% |
| Seminole, OK | 7,155 | 36 | 28.0% |
| Heber Springs, AR | 7,156 | 45 | 10.0% |
| St. Louis, MI | 7,156 | 26 | 48.0% |
| Knoxville, IA | 7,154 | 39 | 22.0% |
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