| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 19°F | 35°F | 27°F | 3.2" |
| February | 20°F | 38°F | 28°F | 2.3" |
| March | 34°F | 53°F | 42°F | 3.3" |
| April | 44°F | 67°F | 54°F | 5.2" |
| May | 54°F | 77°F | 65°F | 5.3" |
| June | 64°F | 85°F | 74°F | 6.5" |
| July | 66°F | 86°F | 76°F | 5.8" |
| August | 65°F | 85°F | 74°F | 9.3" |
| September | 58°F | 80°F | 68°F | 8.3" |
| October | 46°F | 68°F | 56°F | 4.7" |
| November | 35°F | 54°F | 43°F | 4.6" |
| December | 25°F | 40°F | 32°F | 3.9" |
| Index | Royal | Illinois | National |
| Air quality index | 42 | 41 | 42 |
| Pollution index | 9,793,540 | 3,148,694 | 2,726,193 |
| Measurement | Total |
| Days measured | 365 |
| Days with good air quality | 281 |
| Days with moderate air quality | 84 |
| Days w/ poor A.Q. for sensitive groups | 0 |
| Days with unhealthy air quality | 0 |
| Pollutant | Total |
| Arsenic | 0.010% |
| Benzene | 0.070% |
| Carbon Tetrachloride | 0.010% |
| Lead | 0.001% |
| Mercury | 0.040% |
| City | Population | Air Quality Index | National Avg. |
| Ludlow, IL | 426 | 42 | 16.0% |
| Potomac, IL | 744 | 42 | 16.0% |
| Fithian, IL | 536 | 42 | 16.0% |
| St. Joseph, IL | 4,053 | 42 | 16.0% |
| Savoy, IL | 8,802 | 42 | 16.0% |
| Royal, IL | 370 | 42 | 16.0% |
| City | Population | Air Quality Index | National Avg. |
| Naknek, AK | 370 | 31 | 38.0% |
| Beaverville, IL | 370 | 42 | 16.0% |
| Mount Clare, IL | 370 | 54 | 8.0% |
| Shelby, IN | 370 | 37 | 26.0% |
| Oneida, KY | 370 | 39 | 22.0% |
| Royal, IL | 370 | 42 | 16.0% |
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