| 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% |
Here are the best places to live in Illinois
These are the largest cities in Illinois
Check out the 10 most dangerous cities in Illinois
See the best places to live in Mason