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| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 16°F | 31°F | 23°F | 2.8" |
| February | 17°F | 32°F | 24°F | 2.8" |
| March | 30°F | 47°F | 37°F | 4.6" |
| April | 39°F | 61°F | 49°F | 5.8" |
| May | 50°F | 72°F | 60°F | 5.3" |
| June | 59°F | 80°F | 69°F | 5.6" |
| July | 63°F | 82°F | 72°F | 5.8" |
| August | 61°F | 81°F | 70°F | 5.9" |
| September | 54°F | 75°F | 63°F | 3.7" |
| October | 43°F | 63°F | 52°F | 5.8" |
| November | 32°F | 49°F | 39°F | 4.0" |
| December | 21°F | 34°F | 27°F | 3.1" |
| Index | Cortland | Illinois | National |
| Air quality index | 42 | 41 | 42 |
| Pollution index | 1,874,490 | 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 | 0.002% |
| Benzene | 0.030% |
| Carbon Tetrachloride | 0.010% |
| Lead | 0.010% |
| Mercury | 0.010% |
| City | Population | Air Quality Index | National Avg. |
| Prestbury, IL | 1,978 | 0 | 100.0% |
| Lee, IL | 380 | 0 | 100.0% |
| Sugar Grove, IL | 9,756 | 0 | 100.0% |
| Shabbona, IL | 937 | 0 | 100.0% |
| Kaneville, IL | 518 | 0 | 100.0% |
| Cortland, IL | 4,261 | 42 | 16.0% |
| City | Population | Air Quality Index | National Avg. |
| Highlands-Baywood Park, CA | 4,261 | 43 | 14.0% |
| Centralia, MO | 4,260 | 38 | 24.0% |
| Medina, TN | 4,260 | 27 | 46.0% |
| Morgan, UT | 4,259 | 45 | 10.0% |
| Lyons, GA | 4,263 | 26 | 48.0% |
| Cortland, IL | 4,261 | 42 | 16.0% |
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