| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 20°F | 37°F | 28°F | 2.6" |
| February | 21°F | 39°F | 29°F | 3.2" |
| March | 34°F | 55°F | 43°F | 4.1" |
| April | 44°F | 67°F | 55°F | 6.7" |
| May | 54°F | 76°F | 65°F | 4.6" |
| June | 63°F | 85°F | 74°F | 8.5" |
| July | 66°F | 87°F | 76°F | 11.2" |
| August | 64°F | 86°F | 74°F | 8.1" |
| September | 56°F | 80°F | 67°F | 11.5" |
| October | 46°F | 68°F | 56°F | 6.3" |
| November | 34°F | 54°F | 43°F | 5.1" |
| December | 25°F | 40°F | 32°F | 5.3" |
| Index | Delavan | Illinois | National |
| Air quality index | 41 | 41 | 42 |
| Pollution index | 672,031 | 3,148,694 | 2,726,193 |
| Measurement | Total |
| Days measured | 365 |
| Days with good air quality | 285 |
| Days with moderate air quality | 78 |
| Days w/ poor A.Q. for sensitive groups | 2 |
| Days with unhealthy air quality | 0 |
| Pollutant | Total |
| Arsenic | 0.001% |
| Benzene | 0.030% |
| Carbon Tetrachloride | 0.000% |
| Lead | 0.001% |
| Mercury | 0.020% |
| City | Population | Air Quality Index | National Avg. |
| Mason City, IL | 2,342 | 40 | 20.0% |
| New Holland, IL | 257 | 40 | 20.0% |
| Green Valley, IL | 716 | 41 | 18.0% |
| Mackinaw, IL | 1,970 | 41 | 18.0% |
| Kingston Mines, IL | 240 | 41 | 18.0% |
| Delavan, IL | 1,708 | 41 | 18.0% |
| City | Population | Air Quality Index | National Avg. |
| Mancelona, MI | 1,708 | 36 | 28.0% |
| Rawls Springs, MS | 1,708 | 35 | 30.0% |
| Garrison, ND | 1,708 | 36 | 28.0% |
| Santa Clara, NM | 1,708 | 37 | 26.0% |
| Langham, SK | 1,708 | n/a | |
| Delavan, IL | 1,708 | 41 | 18.0% |
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