| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 27°F | 44°F | 35°F | 2.5" |
| February | 29°F | 46°F | 37°F | 1.6" |
| March | 40°F | 59°F | 49°F | 4.1" |
| April | 50°F | 70°F | 59°F | 10.7" |
| May | 58°F | 78°F | 68°F | 10.5" |
| June | 67°F | 87°F | 77°F | 13.3" |
| July | 70°F | 88°F | 79°F | 0.5" |
| August | 68°F | 88°F | 77°F | 0.4" |
| September | 61°F | 81°F | 70°F | 1.2" |
| October | 50°F | 70°F | 59°F | 0.8" |
| November | 39°F | 58°F | 48°F | 5.8" |
| December | 32°F | 47°F | 39°F | 1.2" |
| Index | Herrin | Illinois | National |
| Air quality index | 43 | 41 | 42 |
| Pollution index | 125,359 | 3,148,694 | 2,726,193 |
| Measurement | Total |
| Days measured | 344 |
| Days with good air quality | 269 |
| Days with moderate air quality | 74 |
| Days w/ poor A.Q. for sensitive groups | 1 |
| Days with unhealthy air quality | 0 |
| Pollutant | Total |
| Arsenic | 0.000% |
| Benzene | 0.003% |
| Carbon Tetrachloride | 0.000% |
| Lead | 0.000% |
| Mercury | 0.000% |
| City | Population | Air Quality Index | National Avg. |
| Elkville, IL | 796 | 43 | 14.0% |
| Royalton, IL | 1,107 | 43 | 14.0% |
| Cambria, IL | 1,071 | 43 | 14.0% |
| Crainville, IL | 1,372 | 43 | 14.0% |
| Spillertown, IL | 241 | 43 | 14.0% |
| Herrin, IL | 12,806 | 43 | 14.0% |
| City | Population | Air Quality Index | National Avg. |
| Brighton, ON | 12,811 | n/a | |
| Jacksonville, AL | 12,796 | 43 | 14.0% |
| Grass Valley, CA | 12,817 | 44 | 12.0% |
| Lindsay, CA | 12,793 | 66 | 32.0% |
| McComb, MS | 12,788 | 30 | 40.0% |
| Herrin, IL | 12,806 | 43 | 14.0% |
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