| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 20°F | 38°F | 28°F | 1.5" |
| February | 22°F | 40°F | 30°F | 1.8" |
| March | 34°F | 55°F | 43°F | 2.3" |
| April | 44°F | 67°F | 55°F | 4.2" |
| May | 54°F | 76°F | 65°F | 4.6" |
| June | 64°F | 85°F | 74°F | 5.0" |
| July | 66°F | 87°F | 76°F | 3.8" |
| August | 64°F | 86°F | 75°F | 2.7" |
| September | 57°F | 80°F | 68°F | 3.5" |
| October | 46°F | 68°F | 56°F | 3.6" |
| November | 35°F | 54°F | 44°F | 2.7" |
| December | 26°F | 41°F | 33°F | 3.1" |
| Index | Forsyth | Illinois | National |
| Air quality index | 44 | 41 | 42 |
| Pollution index | 703,218 | 3,148,694 | 2,726,193 |
| Measurement | Total |
| Days measured | 365 |
| Days with good air quality | 255 |
| Days with moderate air quality | 110 |
| 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 | Population | Air Quality Index | National Avg. |
| LaPlace, IL | 128 | 44 | 12.0% |
| Illiopolis, IL | 767 | 44 | 12.0% |
| Cisco, IL | 284 | 44 | 12.0% |
| Weldon, IL | 375 | 44 | 12.0% |
| Maroa, IL | 1,982 | 44 | 12.0% |
| Forsyth, IL | 3,474 | 44 | 12.0% |
| City | Population | Air Quality Index | National Avg. |
| East Richmond Heights, CA | 3,473 | 40 | 20.0% |
| Linwood, PA | 3,473 | 51 | 2.0% |
| Black River Falls, WI | 3,473 | 35 | 30.0% |
| Marianna, AR | 3,472 | 45 | 10.0% |
| Sissonville, WV | 3,472 | 40 | 20.0% |
| Forsyth, IL | 3,474 | 44 | 12.0% |
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