| 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 | 33 | 41 | 50 |
| Pollution index | 500,004 | 3,347,295 | 5,066,862 |
| Measurement | Total |
| Days measured | 359 |
| Days with good air quality | 347 |
| Days with moderate air quality | 12 |
| Days w/ poor A.Q. for sensitive groups | |
| Days with unhealthy air quality |
| 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. |
| Oblong, IL | 1,571 | 33 | 34% |
| Stoy, IL | 130 | 33 | 34% |
| Emison, IN | 127 | 33 | 34% |
| Bridgeport, IL | 1,425 | 33 | 34% |
| Palestine, IL | 1,224 | 33 | 34% |
| Flat Rock, IL | 329 | 33 | 34% |
| City, State | Population | Air Quality Index | National Avg. |
| Lakeland Shores, MN | 329 | 44 | 12% |
| Custer City, OK | 329 | 42 | 16% |
| Kanarraville, UT | 329 | 41 | 18% |
| Westside, IA | 329 | 41 | 18% |
| Chandler, MN | 329 | 35 | 30% |
| Flat Rock, IL | 329 | 33 | 34% |