| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 22°F | 35°F | 28°F | 4.7" |
| February | 20°F | 34°F | 26°F | 2.5" |
| March | 33°F | 54°F | 42°F | 4.9" |
| April | 42°F | 65°F | 52°F | 4.6" |
| May | 53°F | 77°F | 65°F | 3.7" |
| June | 64°F | 86°F | 74°F | 4.4" |
| July | 65°F | 86°F | 75°F | 4.9" |
| August | 66°F | 88°F | 76°F | 3.8" |
| September | 59°F | 85°F | 70°F | 4.4" |
| October | 48°F | 70°F | 57°F | 5.3" |
| November | 34°F | 53°F | 42°F | 5.3" |
| December | 20°F | 33°F | 26°F | 2.9" |
| Index | Saybrook | 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. |
| Fisher, IL | 1,917 | 42 | 16.0% |
| Bellflower, IL | 430 | 42 | 16.0% |
| Elliott, IL | 310 | 42 | 16.0% |
| Mansfield, IL | 1,006 | 42 | 16.0% |
| Melvin, IL | 397 | 42 | 16.0% |
| Saybrook, IL | 721 | 44 | 12.0% |
| City | Population | Air Quality Index | National Avg. |
| Wallace, CA | 721 | 45 | 10.0% |
| Quasqueton, IA | 721 | 39 | 22.0% |
| Dakota, IL | 721 | 42 | 16.0% |
| St. Bernice, IN | 721 | 45 | 10.0% |
| Fox Chase, KY | 721 | 49 | 2.0% |
| Saybrook, IL | 721 | 44 | 12.0% |
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