| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 15°F | 32°F | 23°F | 3.9" |
| February | 16°F | 34°F | 25°F | 2.5" |
| March | 30°F | 49°F | 38°F | 3.6" |
| April | 40°F | 64°F | 51°F | 5.7" |
| May | 51°F | 75°F | 62°F | 7.1" |
| June | 60°F | 83°F | 71°F | 7.2" |
| July | 63°F | 85°F | 74°F | 7.6" |
| August | 61°F | 83°F | 72°F | 7.2" |
| September | 53°F | 78°F | 65°F | 6.0" |
| October | 43°F | 65°F | 53°F | 4.0" |
| November | 31°F | 51°F | 40°F | 8.0" |
| December | 20°F | 36°F | 28°F | 4.2" |
| Index | Harmon | Illinois | National |
| Air quality index | 42 | 41 | 42 |
| Pollution index | 2,097,740 | 3,148,694 | 2,726,193 |
| Measurement | Total |
| Days measured | 364 |
| Days with good air quality | 268 |
| Days with moderate air quality | 96 |
| Days w/ poor A.Q. for sensitive groups | 0 |
| Days with unhealthy air quality | 0 |
| Pollutant | Total |
| Arsenic | 0.001% |
| Benzene | 0.060% |
| Carbon Tetrachloride | 0.000% |
| Lead | 0.001% |
| Mercury | 0.300% |
| City | Population | Air Quality Index | National Avg. |
| Ohio, IL | 509 | 0 | 100.0% |
| Amboy, IL | 2,453 | 0 | 100.0% |
| La Moille, IL | 801 | 0 | 100.0% |
| New Bedford, IL | 57 | 42 | 16.0% |
| Coleta, IL | 230 | 42 | 16.0% |
| Harmon, IL | 146 | 42 | 16.0% |
| City | Population | Air Quality Index | National Avg. |
| Chinese Camp, CA | 146 | 39 | 22.0% |
| Sharpsburg, IA | 146 | 43 | 14.0% |
| Hettick, IL | 146 | 40 | 20.0% |
| Stoy, IL | 146 | 36 | 28.0% |
| Scotland, IN | 146 | 1 | 98.0% |
| Harmon, IL | 146 | 42 | 16.0% |
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