| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 25°F | 44°F | 34°F | 2.3" |
| February | 27°F | 46°F | 36°F | 2.5" |
| March | 38°F | 60°F | 48°F | 4.8" |
| April | 47°F | 70°F | 58°F | 6.2" |
| May | 56°F | 78°F | 67°F | 4.9" |
| June | 66°F | 88°F | 76°F | 4.1" |
| July | 68°F | 89°F | 78°F | 5.0" |
| August | 66°F | 88°F | 76°F | 3.5" |
| September | 57°F | 81°F | 68°F | 3.4" |
| October | 46°F | 71°F | 58°F | 3.6" |
| November | 36°F | 58°F | 46°F | 3.7" |
| December | 30°F | 47°F | 38°F | 3.7" |
| Index | Royalton | Illinois | National |
| Air quality index | 43 | 41 | 42 |
| Pollution index | 569,178 | 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 | 0.001% |
| Benzene | 0.020% |
| Carbon Tetrachloride | 0.000% |
| Lead | 0.000% |
| Mercury | 0.000% |
| City | Population | Air Quality Index | National Avg. |
| Colp, IL | 234 | 43 | 14.0% |
| Herrin, IL | 12,806 | 43 | 14.0% |
| Pinckneyville, IL | 5,391 | 43 | 14.0% |
| Du Quoin, IL | 5,723 | 43 | 14.0% |
| Murphysboro, IL | 7,727 | 43 | 14.0% |
| Royalton, IL | 1,107 | 43 | 14.0% |
| City | Population | Air Quality Index | National Avg. |
| Douglas, MI | 1,107 | 38 | 24.0% |
| Harbor Springs, MI | 1,107 | 36 | 28.0% |
| Adair Village, OR | 1,107 | 28 | 44.0% |
| Kykotsmovi Village, AZ | 1,106 | 45 | 10.0% |
| Louisville, IL | 1,106 | 40 | 20.0% |
| Royalton, IL | 1,107 | 43 | 14.0% |
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