| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 13°F | 30°F | 21°F | 0.8" |
| February | 15°F | 33°F | 23°F | 2.0" |
| March | 29°F | 48°F | 38°F | 3.9" |
| April | 40°F | 62°F | 50°F | 4.8" |
| May | 50°F | 74°F | 61°F | 6.0" |
| June | 61°F | 83°F | 71°F | 8.3" |
| July | 64°F | 85°F | 74°F | 7.5" |
| August | 61°F | 82°F | 71°F | 5.6" |
| September | 53°F | 77°F | 64°F | 4.9" |
| October | 42°F | 64°F | 52°F | 7.5" |
| November | 31°F | 49°F | 39°F | 5.2" |
| December | 19°F | 34°F | 26°F | 4.0" |
| Index | Shellsburg | Iowa | National |
| Air quality index | 39 | 39 | 42 |
| Pollution index | 5,075,380 | 1,154,535 | 2,726,193 |
| Measurement | Total |
| Days measured | 365 |
| Days with good air quality | 287 |
| Days with moderate air quality | 78 |
| Days w/ poor A.Q. for sensitive groups | 0 |
| Days with unhealthy air quality | 0 |
| Pollutant | Total |
| Arsenic | 0.070% |
| Benzene | 0.050% |
| Carbon Tetrachloride | 2.000% |
| Lead | 0.040% |
| Mercury | 0.100% |
| City | Population | Air Quality Index | National Avg. |
| Center Point, IA | 2,548 | 39 | 22.0% |
| Marion, IA | 39,910 | 39 | 22.0% |
| Walford, IA | 1,326 | 39 | 22.0% |
| Newhall, IA | 1,096 | 39 | 22.0% |
| Watkins, IA | 84 | 39 | 22.0% |
| Shellsburg, IA | 806 | 39 | 22.0% |
| City | Population | Air Quality Index | National Avg. |
| Offerman, GA | 806 | 36 | 28.0% |
| Ashkum, IL | 806 | 42 | 16.0% |
| Clyde, KS | 806 | 32 | 36.0% |
| Hessmer, LA | 806 | 30 | 40.0% |
| Gibbon, MN | 806 | 32 | 36.0% |
| Shellsburg, IA | 806 | 39 | 22.0% |
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