| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 14°F | 37°F | 24°F | 0.6" |
| February | 16°F | 41°F | 27°F | 0.9" |
| March | 29°F | 56°F | 41°F | 1.5" |
| April | 40°F | 67°F | 53°F | 3.8" |
| May | 50°F | 77°F | 63°F | 5.2" |
| June | 62°F | 87°F | 74°F | 4.9" |
| July | 66°F | 91°F | 78°F | 3.1" |
| August | 64°F | 88°F | 75°F | 6.1" |
| September | 55°F | 82°F | 67°F | 3.7" |
| October | 42°F | 69°F | 54°F | 3.0" |
| November | 29°F | 55°F | 41°F | 1.0" |
| December | 17°F | 39°F | 27°F | 1.7" |
| Index | Walnut | Iowa | National |
| Air quality index | 40 | 39 | 42 |
| Pollution index | 4,349,930 | 1,154,535 | 2,726,193 |
| Measurement | Total |
| Days measured | 365 |
| Days with good air quality | 283 |
| Days with moderate air quality | 82 |
| Days w/ poor A.Q. for sensitive groups | 0 |
| Days with unhealthy air quality | 0 |
| Pollutant | Total |
| Arsenic | 0.210% |
| Benzene | 0.040% |
| Carbon Tetrachloride | 0.004% |
| Lead | 0.100% |
| Mercury | 0.160% |
| City | Population | Air Quality Index | National Avg. |
| Harlan, IA | 4,813 | 40 | 20.0% |
| Tennant, IA | 76 | 40 | 20.0% |
| Marne, IA | 131 | 40 | 20.0% |
| Lewis, IA | 513 | 40 | 20.0% |
| Jacksonville, IA | 10 | 40 | 20.0% |
| Walnut, IA | 746 | 40 | 20.0% |
| City | Population | Air Quality Index | National Avg. |
| Watkins, CO | 746 | 58 | 16.0% |
| Corydon, KY | 746 | 45 | 10.0% |
| Hancock, MN | 746 | 32 | 36.0% |
| Holcomb, MO | 746 | 26 | 48.0% |
| Wood Heights, MO | 746 | 47 | 6.0% |
| Walnut, IA | 746 | 40 | 20.0% |
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