| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 18°F | 35°F | 26°F | 3.6" |
| February | 19°F | 38°F | 27°F | 3.0" |
| March | 33°F | 53°F | 42°F | 3.4" |
| April | 43°F | 67°F | 54°F | 5.4" |
| May | 53°F | 76°F | 64°F | 7.4" |
| June | 63°F | 84°F | 73°F | 10.8" |
| July | 65°F | 85°F | 75°F | 8.6" |
| August | 63°F | 84°F | 73°F | 8.8" |
| September | 55°F | 80°F | 66°F | 8.0" |
| October | 44°F | 68°F | 55°F | 8.3" |
| November | 33°F | 54°F | 42°F | 6.4" |
| December | 23°F | 38°F | 30°F | 5.1" |
| Index | Plymouth | Illinois | National |
| Air quality index | 29 | 41 | 42 |
| Pollution index | 802,114 | 3,148,694 | 2,726,193 |
| Measurement | Total |
| Days measured | 120 |
| Days with good air quality | 101 |
| Days with moderate air quality | 19 |
| Days w/ poor A.Q. for sensitive groups | 0 |
| Days with unhealthy air quality | 0 |
| Pollutant | Total |
| Arsenic | 0.001% |
| Benzene | 0.040% |
| Carbon Tetrachloride | 0.000% |
| Lead | 0.000% |
| Mercury | 0.000% |
| City | Population | Air Quality Index | National Avg. |
| Carthage, IL | 2,474 | 29 | 42.0% |
| Macomb, IL | 17,658 | 29 | 42.0% |
| Ferris, IL | 164 | 29 | 42.0% |
| Tennessee, IL | 88 | 29 | 42.0% |
| Basco, IL | 85 | 29 | 42.0% |
| Plymouth, IL | 441 | 29 | 42.0% |
| City | Population | Air Quality Index | National Avg. |
| Norman, AR | 441 | 38 | 24.0% |
| French Gulch, CA | 441 | 42 | 16.0% |
| Sierra Brooks, CA | 441 | 44 | 12.0% |
| Callender, IA | 441 | 37 | 26.0% |
| Farragut, IA | 441 | 40 | 20.0% |
| Plymouth, IL | 441 | 29 | 42.0% |
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