| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 15°F | 32°F | 23°F | 1.5" |
| February | 16°F | 34°F | 24°F | 1.4" |
| March | 29°F | 49°F | 38°F | 2.3" |
| April | 39°F | 62°F | 50°F | 3.9" |
| May | 49°F | 73°F | 61°F | 4.3" |
| June | 59°F | 82°F | 70°F | 4.5" |
| July | 62°F | 84°F | 73°F | 4.3" |
| August | 60°F | 82°F | 71°F | 4.3" |
| September | 53°F | 77°F | 64°F | 2.9" |
| October | 42°F | 64°F | 52°F | 3.6" |
| November | 30°F | 50°F | 40°F | 2.1" |
| December | 20°F | 36°F | 28°F | 2.2" |
| Index | Leland | Illinois | National |
| Air quality index | 0 | 41 | 42 |
| Pollution index | 1,874,490 | 3,148,694 | 2,726,193 |
| Measurement | Total |
| Days measured | 335 |
| Days with good air quality | 334 |
| Days with moderate air quality | 1 |
| Days w/ poor A.Q. for sensitive groups | 0 |
| 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. |
| Troy Grove, IL | 210 | 0 | 100.0% |
| Millington, IL | 662 | 0 | 100.0% |
| Lee, IL | 380 | 0 | 100.0% |
| Shabbona, IL | 937 | 0 | 100.0% |
| Earlville, IL | 2,057 | 0 | 100.0% |
| Leland, IL | 932 | 0 | 100.0% |
| City | Population | Air Quality Index | National Avg. |
| Gowrie, IA | 932 | 37 | 26.0% |
| Black Creek, NC | 932 | 40 | 20.0% |
| Stateline, NV | 932 | 13 | 74.0% |
| Cyril, OK | 932 | 43 | 14.0% |
| Campbell, TX | 932 | 51 | 2.0% |
| Leland, IL | 932 | 0 | 100.0% |
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