| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 27°F | 43°F | 34°F | 5.0" |
| February | 28°F | 46°F | 36°F | 4.8" |
| March | 39°F | 59°F | 48°F | 4.5" |
| April | 48°F | 70°F | 58°F | 10.6" |
| May | 57°F | 78°F | 67°F | 6.0" |
| June | 66°F | 87°F | 76°F | 7.6" |
| July | 67°F | 88°F | 77°F | 6.8" |
| August | 65°F | 87°F | 76°F | 5.4" |
| September | 58°F | 82°F | 69°F | 6.5" |
| October | 47°F | 71°F | 58°F | 3.5" |
| November | 37°F | 58°F | 47°F | 6.2" |
| December | 31°F | 47°F | 38°F | 8.9" |
| Index | Harrisburg | Illinois | National |
| Air quality index | 43 | 41 | 42 |
| Pollution index | 125,359 | 3,148,694 | 2,726,193 |
| Measurement | Total |
| Days measured | 344 |
| Days with good air quality | 269 |
| Days with moderate air quality | 74 |
| Days w/ poor A.Q. for sensitive groups | 1 |
| Days with unhealthy air quality | 0 |
| Pollutant | Total |
| Arsenic | 0.000% |
| Benzene | 0.003% |
| Carbon Tetrachloride | 0.000% |
| Lead | 0.000% |
| Mercury | 0.000% |
| City | Population | Air Quality Index | National Avg. |
| Thompsonville, IL | 527 | 43 | 14.0% |
| Eldorado, IL | 4,000 | 43 | 14.0% |
| Crab Orchard, IL | 222 | 43 | 14.0% |
| Galatia, IL | 1,095 | 43 | 14.0% |
| Broughton, IL | 116 | 43 | 14.0% |
| Harrisburg, IL | 8,578 | 43 | 14.0% |
| City | Population | Air Quality Index | National Avg. |
| Hailey, ID | 8,575 | 20 | 60.0% |
| Richmond Heights, MO | 8,581 | 54 | 8.0% |
| Berthoud, CO | 8,574 | 48 | 4.0% |
| Mount Pleasant, IA | 8,574 | 29 | 42.0% |
| Stony Brook University, NY | 8,574 | 43 | 14.0% |
| Harrisburg, IL | 8,578 | 43 | 14.0% |
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