| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 22°F | 38°F | 30°F | 1.7" |
| February | 25°F | 40°F | 32°F | 2.3" |
| March | 36°F | 54°F | 44°F | 1.1" |
| April | 46°F | 67°F | 55°F | 4.3" |
| May | 55°F | 76°F | 64°F | 0.0" |
| June | 66°F | 86°F | 75°F | 3.0" |
| July | 66°F | 86°F | 75°F | 7.9" |
| August | 66°F | 87°F | 76°F | 4.8" |
| September | 59°F | 81°F | 69°F | 8.6" |
| October | 47°F | 69°F | 57°F | 1.4" |
| November | 36°F | 56°F | 46°F | 11.8" |
| December | 25°F | 38°F | 31°F | 7.1" |
| Index | Mason | Illinois | National |
| Air quality index | 40 | 41 | 42 |
| Pollution index | 959,673 | 3,148,694 | 2,726,193 |
| Measurement | Total |
| Days measured | 245 |
| Days with good air quality | 224 |
| Days with moderate air quality | 21 |
| 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 | 0.002% |
| City | Population | Air Quality Index | National Avg. |
| Louisville, IL | 1,106 | 40 | 20.0% |
| St. Peter, IL | 438 | 40 | 20.0% |
| Effingham, IL | 12,310 | 40 | 20.0% |
| Farina, IL | 749 | 40 | 20.0% |
| Sailor Springs, IL | 114 | 40 | 20.0% |
| Mason, IL | 243 | 40 | 20.0% |
| City | Population | Air Quality Index | National Avg. |
| Koyuk, AK | 243 | 38 | 24.0% |
| Malcolm, AL | 243 | 42 | 16.0% |
| East Globe, AZ | 243 | 60 | 20.0% |
| McNeal, AZ | 243 | 45 | 10.0% |
| Bloomfield, CA | 243 | 34 | 32.0% |
| Mason, IL | 243 | 40 | 20.0% |
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