| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 17°F | 35°F | 25°F | 1.1" |
| February | 20°F | 38°F | 28°F | 1.5" |
| March | 33°F | 53°F | 41°F | 1.9" |
| April | 43°F | 65°F | 53°F | 3.7" |
| May | 53°F | 75°F | 64°F | 5.1" |
| June | 63°F | 84°F | 73°F | 5.5" |
| July | 66°F | 87°F | 76°F | 4.1" |
| August | 64°F | 84°F | 74°F | 3.3" |
| September | 56°F | 79°F | 67°F | 3.3" |
| October | 45°F | 68°F | 55°F | 3.0" |
| November | 33°F | 53°F | 42°F | 2.4" |
| December | 23°F | 39°F | 31°F | 1.9" |
| Index | La Harpe | Illinois | National |
| Air quality index | 29 | 41 | 42 |
| Pollution index | 802,114 | 3,148,694 | 2,726,193 |
| Measurement | Total |
| Days measured | 120 |
| Days with good air quality | 101 |
| Days with moderate air quality | 19 |
| Days w/ poor A.Q. for sensitive groups | 0 |
| Days with unhealthy air quality | 0 |
| Pollutant | Total |
| Arsenic | 0.001% |
| Benzene | 0.040% |
| Carbon Tetrachloride | 0.000% |
| Lead | 0.000% |
| Mercury | 0.000% |
| City | Population | Air Quality Index | National Avg. |
| Carthage, IL | 2,474 | 29 | 42.0% |
| Pontoosuc, IL | 109 | 29 | 42.0% |
| Lomax, IL | 375 | 29 | 42.0% |
| Stronghurst, IL | 1,066 | 29 | 42.0% |
| Dallas City, IL | 1,064 | 29 | 42.0% |
| La Harpe, IL | 1,318 | 29 | 42.0% |
| City | Population | Air Quality Index | National Avg. |
| Julian, CA | 1,318 | 64 | 28.0% |
| Mulberry, IN | 1,319 | 42 | 16.0% |
| Coal Run Village, KY | 1,319 | 42 | 16.0% |
| Red Lake Falls, MN | 1,319 | 33 | 34.0% |
| Winthrop, MN | 1,317 | 46 | 8.0% |
| La Harpe, IL | 1,318 | 29 | 42.0% |
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