| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 12°F | 28°F | 20°F | 1.5" |
| February | 14°F | 30°F | 22°F | 2.6" |
| March | 29°F | 50°F | 38°F | 5.4" |
| April | 38°F | 62°F | 50°F | 7.2" |
| May | 51°F | 73°F | 61°F | 8.0" |
| June | 62°F | 83°F | 72°F | 7.9" |
| July | 63°F | 85°F | 74°F | 5.1" |
| August | 62°F | 82°F | 72°F | 6.6" |
| September | 55°F | 79°F | 66°F | 5.4" |
| October | 44°F | 65°F | 54°F | 5.7" |
| November | 28°F | 49°F | 38°F | 2.2" |
| December | 13°F | 26°F | 20°F | 1.8" |
| Index | Union | Iowa | National |
| Air quality index | 37 | 39 | 42 |
| Pollution index | 1,171,930 | 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 | 0.060% |
| Benzene | 0.010% |
| Carbon Tetrachloride | 0.002% |
| Lead | 0.004% |
| Mercury | 0.050% |
| City | Population | Air Quality Index | National Avg. |
| New Providence, IA | 279 | 37 | 26.0% |
| Marshalltown, IA | 26,957 | 37 | 26.0% |
| Clemons, IA | 152 | 37 | 26.0% |
| Owasa, IA | 16 | 37 | 26.0% |
| Eldora, IA | 2,620 | 37 | 26.0% |
| Union, IA | 412 | 37 | 26.0% |
| City | Population | Air Quality Index | National Avg. |
| Damascus, AR | 412 | 45 | 10.0% |
| Lake View, AR | 412 | 43 | 14.0% |
| Titonka, IA | 412 | 15 | 70.0% |
| Loraine, IL | 412 | 41 | 18.0% |
| Betterton, MD | 412 | 46 | 8.0% |
| Union, IA | 412 | 37 | 26.0% |
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