| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 19°F | 35°F | 27°F | 3.7" |
| February | 20°F | 37°F | 28°F | 2.9" |
| March | 33°F | 52°F | 41°F | 4.7" |
| April | 42°F | 66°F | 53°F | 5.6" |
| May | 54°F | 76°F | 64°F | 6.5" |
| June | 63°F | 84°F | 73°F | 7.6" |
| July | 65°F | 85°F | 75°F | 10.4" |
| August | 62°F | 83°F | 72°F | 7.8" |
| September | 55°F | 79°F | 66°F | 7.6" |
| October | 44°F | 66°F | 54°F | 4.3" |
| November | 34°F | 53°F | 42°F | 5.5" |
| December | 24°F | 39°F | 31°F | 3.5" |
| Index | Forrest | Illinois | National |
| Air quality index | 44 | 41 | 42 |
| Pollution index | 7,745,200 | 3,148,694 | 2,726,193 |
| Measurement | Total |
| Days measured | 364 |
| Days with good air quality | 256 |
| Days with moderate air quality | 108 |
| Days w/ poor A.Q. for sensitive groups | 0 |
| Days with unhealthy air quality | 0 |
| Pollutant | Total |
| Arsenic | 0.030% |
| Benzene | 0.070% |
| Carbon Tetrachloride | 0.020% |
| Lead | 0.860% |
| Mercury | 0.210% |
| City | Population | Air Quality Index | National Avg. |
| Cullom, IL | 501 | 0 | 100.0% |
| Kempton, IL | 317 | 0 | 100.0% |
| Saunemin, IL | 398 | 0 | 100.0% |
| Melvin, IL | 397 | 42 | 16.0% |
| Roberts, IL | 470 | 42 | 16.0% |
| Forrest, IL | 995 | 44 | 12.0% |
| City | Population | Air Quality Index | National Avg. |
| Fair Haven, NY | 995 | 37 | 26.0% |
| North Bonneville, WA | 995 | 38 | 24.0% |
| Sainte-Geneviève-de-Batiscan, QC | 995 | n/a | |
| Saint-Félix-d'Otis, QC | 995 | n/a | |
| Hornepayne, ON | 995 | n/a | |
| Forrest, IL | 995 | 44 | 12.0% |
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