| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 12°F | 29°F | 20°F | 0.6" |
| February | 13°F | 32°F | 22°F | 0.8" |
| March | 28°F | 48°F | 37°F | 1.7" |
| April | 39°F | 62°F | 49°F | 4.8" |
| May | 50°F | 73°F | 61°F | 5.8" |
| June | 60°F | 82°F | 71°F | 5.1" |
| July | 63°F | 84°F | 74°F | 4.0" |
| August | 61°F | 82°F | 71°F | 4.9" |
| September | 52°F | 77°F | 64°F | 3.9" |
| October | 41°F | 63°F | 51°F | 2.9" |
| November | 29°F | 49°F | 38°F | 1.5" |
| December | 16°F | 33°F | 24°F | 1.4" |
| Index | Marshalltown | Iowa | National |
| Air quality index | 37 | 39 | 42 |
| Pollution index | 534,730 | 1,154,535 | 2,726,193 |
| Measurement | Total |
| Days measured | 214 |
| Days with good air quality | 210 |
| Days with moderate air quality | 4 |
| Days w/ poor A.Q. for sensitive groups | |
| Days with unhealthy air quality |
| Pollutant | Total |
| Arsenic | 0.000% |
| Benzene | 0.010% |
| Carbon Tetrachloride | 0.001% |
| Lead | 0.000% |
| Mercury | 0.000% |
| City | Population | Air Quality Index | National Avg. |
| State Center, IA | 1,839 | 37 | 26.0% |
| Baxter, IA | 935 | 37 | 26.0% |
| Clemons, IA | 152 | 37 | 26.0% |
| Ferguson, IA | 125 | 37 | 26.0% |
| Liscomb, IA | 381 | 37 | 26.0% |
| Marshalltown, IA | 26,957 | 37 | 26.0% |
| City | Population | Air Quality Index | National Avg. |
| Belmont, CA | 26,963 | 43 | 14.0% |
| Bristol, TN | 26,973 | 45 | 10.0% |
| Eureka, CA | 26,938 | 27 | 46.0% |
| Saint-Bruno-de-Montarville, QC | 26,923 | n/a | |
| Shoreview, MN | 26,922 | 46 | 8.0% |
| Marshalltown, IA | 26,957 | 37 | 26.0% |
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