| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 25°F | 41°F | 32°F | 8.1" |
| February | 27°F | 44°F | 34°F | 1.6" |
| March | 39°F | 58°F | 47°F | 5.3" |
| April | 48°F | 69°F | 58°F | 10.0" |
| May | 57°F | 77°F | 66°F | 5.8" |
| June | 66°F | 86°F | 76°F | 3.6" |
| July | 68°F | 87°F | 77°F | 8.5" |
| August | 66°F | 86°F | 75°F | 10.2" |
| September | 59°F | 81°F | 69°F | 6.4" |
| October | 48°F | 70°F | 58°F | 6.6" |
| November | 37°F | 57°F | 46°F | 6.8" |
| December | 30°F | 45°F | 37°F | 4.1" |
| Index | Fairfield | Illinois | National |
| Air quality index | 43 | 41 | 42 |
| Pollution index | 317,387 | 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 | 0.010% |
| Carbon Tetrachloride | 0.000% |
| Lead | 0.000% |
| Mercury | 0.000% |
| City | Population | Air Quality Index | National Avg. |
| Cisne, IL | 505 | 43 | 14.0% |
| Mount Erie, IL | 114 | 43 | 14.0% |
| Burnt Prairie, IL | 89 | 43 | 14.0% |
| Jeffersonville, IL | 333 | 43 | 14.0% |
| Sims, IL | 167 | 43 | 14.0% |
| Fairfield, IL | 4,912 | 43 | 14.0% |
| City | Population | Air Quality Index | National Avg. |
| Demarest, NJ | 4,911 | 51 | 2.0% |
| Northvale, NJ | 4,911 | 51 | 2.0% |
| Pascoag, RI | 4,911 | 44 | 12.0% |
| Falmouth, VA | 4,911 | 47 | 6.0% |
| Eagar, AZ | 4,910 | 44 | 12.0% |
| Fairfield, IL | 4,912 | 43 | 14.0% |
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