| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 15°F | 32°F | 23°F | 2.0" |
| February | 16°F | 34°F | 25°F | 2.7" |
| March | 31°F | 50°F | 39°F | 5.0" |
| April | 41°F | 64°F | 51°F | 6.5" |
| May | 51°F | 75°F | 62°F | 4.6" |
| June | 61°F | 83°F | 71°F | 8.9" |
| July | 64°F | 85°F | 74°F | 4.5" |
| August | 61°F | 83°F | 72°F | 4.9" |
| September | 54°F | 78°F | 65°F | 3.6" |
| October | 43°F | 64°F | 53°F | 3.1" |
| November | 32°F | 50°F | 40°F | 2.9" |
| December | 20°F | 35°F | 28°F | 4.0" |
| Index | Lanark | Illinois | National |
| Air quality index | 42 | 41 | 42 |
| Pollution index | 2,097,740 | 3,148,694 | 2,726,193 |
| Measurement | Total |
| Days measured | 364 |
| Days with good air quality | 268 |
| Days with moderate air quality | 96 |
| Days w/ poor A.Q. for sensitive groups | 0 |
| Days with unhealthy air quality | 0 |
| Pollutant | Total |
| Arsenic | 0.001% |
| Benzene | 0.060% |
| Carbon Tetrachloride | 0.000% |
| Lead | 0.001% |
| Mercury | 0.300% |
| City | Population | Air Quality Index | National Avg. |
| Forreston, IL | 1,392 | 42 | 16.0% |
| Coleta, IL | 230 | 42 | 16.0% |
| Pearl City, IL | 831 | 42 | 16.0% |
| Mount Carroll, IL | 1,503 | 42 | 16.0% |
| Milledgeville, IL | 987 | 42 | 16.0% |
| Lanark, IL | 1,453 | 42 | 16.0% |
| City | Population | Air Quality Index | National Avg. |
| Odon, IN | 1,453 | 1 | 98.0% |
| Silver Lake, KS | 1,453 | 41 | 18.0% |
| Morrice, MI | 1,453 | 36 | 28.0% |
| North Bend, NE | 1,453 | 40 | 20.0% |
| Hughestown, PA | 1,453 | 38 | 24.0% |
| Lanark, IL | 1,453 | 42 | 16.0% |
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