| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 23°F | 40°F | 31°F | 5.3" |
| February | 24°F | 43°F | 33°F | 3.4" |
| March | 36°F | 57°F | 45°F | 4.0" |
| April | 46°F | 69°F | 56°F | 7.6" |
| May | 56°F | 77°F | 66°F | 6.3" |
| June | 64°F | 86°F | 75°F | 6.7" |
| July | 66°F | 87°F | 76°F | 5.7" |
| August | 63°F | 86°F | 74°F | 4.9" |
| September | 56°F | 82°F | 67°F | 5.9" |
| October | 46°F | 70°F | 57°F | 5.6" |
| November | 35°F | 56°F | 44°F | 5.6" |
| December | 28°F | 44°F | 35°F | 4.9" |
| Index | Flat Rock | Illinois | National |
| Air quality index | 36 | 41 | 50 |
| Pollution index | 500,004 | 3,347,295 | 5,066,862 |
| Measurement | Total |
| Days measured | 359 |
| Days with good air quality | 328 |
| Days with moderate air quality | 31 |
| Days w/ poor A.Q. for sensitive groups | 0 |
| Days with unhealthy air quality | 0 |
| Pollutant | Total |
| Arsenic | 0.001% |
| Benzene | 0.020% |
| Carbon Tetrachloride | 0.000% |
| Lead | 0.000% |
| Mercury | 0.000% |
| City, State | Population | Air Quality Index | National Avg. |
| Oaktown, IN | 690 | 36 | 28% |
| Robinson, IL | 7,447 | 36 | 28% |
| Lawrenceville, IL | 4,464 | 36 | 28% |
| Emison, IN | 127 | 36 | 28% |
| Stoy, IL | 130 | 36 | 28% |
| Flat Rock, IL | 329 | 36 | 28% |
| City, State | Population | Air Quality Index | National Avg. |
| Custer City, OK | 329 | 43 | 14% |
| Kanarraville, UT | 329 | 44 | 12% |
| Lakeland Shores, MN | 329 | 46 | 8% |
| Great Cacapon, WV | 329 | 40 | 20% |
| Florence, SD | 329 | 25 | 50% |
| Flat Rock, IL | 329 | 36 | 28% |
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