| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 14°F | 35°F | 23°F | 0.9" |
| February | 17°F | 39°F | 26°F | 1.2" |
| March | 30°F | 54°F | 41°F | 2.2" |
| April | 41°F | 67°F | 53°F | 4.9" |
| May | 51°F | 76°F | 63°F | 4.9" |
| June | 62°F | 86°F | 74°F | 6.5" |
| July | 66°F | 89°F | 77°F | 4.2" |
| August | 64°F | 87°F | 75°F | 5.1" |
| September | 56°F | 81°F | 68°F | 3.7" |
| October | 43°F | 68°F | 55°F | 3.2" |
| November | 31°F | 54°F | 42°F | 1.8" |
| December | 18°F | 38°F | 27°F | 2.1" |
| Index | Norwalk | Iowa | National |
| Air quality index | 39 | 39 | 42 |
| Pollution index | 9,310,160 | 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 | 0.110% |
| Carbon Tetrachloride | 0.080% |
| Lead | 0.030% |
| Mercury | 0.010% |
| City | Population | Air Quality Index | National Avg. |
| Des Moines, IA | 215,408 | 39 | 22.0% |
| Waukee, IA | 22,598 | 39 | 22.0% |
| St. Marys, IA | 182 | 39 | 22.0% |
| Dallas Center, IA | 1,710 | 39 | 22.0% |
| Indianola, IA | 15,921 | 39 | 22.0% |
| Norwalk, IA | 11,467 | 39 | 22.0% |
| City | Population | Air Quality Index | National Avg. |
| Georgian Bluffs, ON | 11,464 | n/a | |
| Milford, DE | 11,463 | 42 | 16.0% |
| Piedmont, CA | 11,454 | 40 | 20.0% |
| Lyndon, KY | 11,451 | 49 | 2.0% |
| Madison Heights, VA | 11,447 | 26 | 48.0% |
| Norwalk, IA | 11,467 | 39 | 22.0% |
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