| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 18°F | 33°F | 25°F | 4.0" |
| February | 13°F | 30°F | 22°F | 3.3" |
| March | 25°F | 45°F | 35°F | 3.0" |
| April | 35°F | 60°F | 48°F | 2.7" |
| May | 43°F | 71°F | 57°F | 3.9" |
| June | 55°F | 83°F | 70°F | 5.1" |
| July | 56°F | 83°F | 71°F | 4.4" |
| August | 59°F | 83°F | 71°F | 3.9" |
| September | 49°F | 78°F | 63°F | 1.9" |
| October | 42°F | 66°F | 54°F | 2.3" |
| November | 28°F | 49°F | 39°F | 5.0" |
| December | 19°F | 32°F | 26°F | 3.5" |
| Index | Lake Forest | Illinois | National |
| Air quality index | 55 | 41 | 42 |
| Pollution index | 3,795,410 | 3,148,694 | 2,726,193 |
| Measurement | Total |
| Days measured | 365 |
| Days with good air quality | 121 |
| Days with moderate air quality | 227 |
| Days w/ poor A.Q. for sensitive groups | 16 |
| Days with unhealthy air quality | 1 |
| Pollutant | Total |
| Arsenic | 0.030% |
| Benzene | 0.050% |
| Carbon Tetrachloride | 0.010% |
| Lead | 0.003% |
| Mercury | 0.310% |
| City | Population | Air Quality Index | National Avg. |
| Lake Villa, IL | 8,907 | 39 | 22.0% |
| Round Lake Park, IL | 8,005 | 39 | 22.0% |
| Gages Lake, IL | 10,172 | 39 | 22.0% |
| Third Lake, IL | 1,192 | 39 | 22.0% |
| Old Mill Creek, IL | 205 | 39 | 22.0% |
| Lake Forest, IL | 19,888 | 55 | 10.0% |
| City | Population | Air Quality Index | National Avg. |
| Huntington, NY | 19,888 | 43 | 14.0% |
| Frankfort, IL | 19,886 | 55 | 10.0% |
| Orinda, CA | 19,891 | 40 | 20.0% |
| Plattsburgh, NY | 19,892 | 37 | 26.0% |
| Colchester, Subd. B, NS | 19,895 | n/a | |
| Lake Forest, IL | 19,888 | 55 | 10.0% |