| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 14°F | 30°F | 22°F | 2.9" |
| February | 15°F | 31°F | 23°F | 2.3" |
| March | 29°F | 47°F | 37°F | 3.6" |
| April | 38°F | 61°F | 48°F | 5.4" |
| May | 49°F | 72°F | 60°F | 5.5" |
| June | 58°F | 80°F | 69°F | 6.3" |
| July | 62°F | 82°F | 72°F | 5.8" |
| August | 60°F | 81°F | 70°F | 7.3" |
| September | 52°F | 75°F | 63°F | 4.7" |
| October | 42°F | 62°F | 51°F | 5.8" |
| November | 30°F | 48°F | 39°F | 4.0" |
| December | 19°F | 33°F | 26°F | 3.4" |
| Index | Forreston | Illinois | National |
| Air quality index | 42 | 41 | 42 |
| Pollution index | 1,767,980 | 3,148,694 | 2,726,193 |
| Measurement | Total |
| Days measured | 341 |
| Days with good air quality | 240 |
| Days with moderate air quality | 100 |
| Days w/ poor A.Q. for sensitive groups | 1 |
| Days with unhealthy air quality | 0 |
| Pollutant | Total |
| Arsenic | 0.001% |
| Benzene | 0.040% |
| Carbon Tetrachloride | 0.010% |
| Lead | 0.001% |
| Mercury | 0.003% |
| City | Population | Air Quality Index | National Avg. |
| German Valley, IL | 565 | 42 | 16.0% |
| Shannon, IL | 745 | 42 | 16.0% |
| Polo, IL | 2,210 | 42 | 16.0% |
| Pearl City, IL | 831 | 42 | 16.0% |
| Lanark, IL | 1,453 | 42 | 16.0% |
| Forreston, IL | 1,392 | 42 | 16.0% |
| City | Population | Air Quality Index | National Avg. |
| Hoopers Creek, NC | 1,392 | 44 | 12.0% |
| Milroy, PA | 1,392 | 44 | 12.0% |
| Charlestown, RI | 1,392 | 37 | 26.0% |
| Jarrell, TX | 1,392 | 42 | 16.0% |
| Macpherson-Grizzly Valley, YT | 1,392 | n/a | |
| Forreston, IL | 1,392 | 42 | 16.0% |
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