| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 8°F | 27°F | 17°F | 0.3" |
| February | 10°F | 30°F | 19°F | 0.5" |
| March | 26°F | 46°F | 34°F | 1.2" |
| April | 36°F | 61°F | 47°F | 2.9" |
| May | 48°F | 73°F | 59°F | 4.0" |
| June | 58°F | 82°F | 70°F | 5.0" |
| July | 62°F | 86°F | 74°F | 3.2" |
| August | 59°F | 82°F | 70°F | 4.3" |
| September | 51°F | 77°F | 63°F | 2.9" |
| October | 39°F | 63°F | 49°F | 2.3" |
| November | 25°F | 48°F | 36°F | 0.9" |
| December | 12°F | 30°F | 21°F | 0.8" |
| Index | Royal | Iowa | National |
| Air quality index | 35 | 39 | 42 |
| Pollution index | 769,697 | 1,154,535 | 2,726,193 |
| Measurement | Total |
| Days measured | 364 |
| Days with good air quality | 322 |
| Days with moderate air quality | 42 |
| Days w/ poor A.Q. for sensitive groups | 0 |
| Days with unhealthy air quality | 0 |
| Pollutant | Total |
| Arsenic | 0.000% |
| Benzene | 0.010% |
| Carbon Tetrachloride | 0.001% |
| Lead | 0.002% |
| Mercury | 0.000% |
| City | Population | Air Quality Index | National Avg. |
| Calumet, IA | 159 | 35 | 30.0% |
| Fostoria, IA | 180 | 35 | 30.0% |
| Greenville, IA | 62 | 35 | 30.0% |
| Peterson, IA | 483 | 35 | 30.0% |
| Larrabee, IA | 142 | 35 | 30.0% |
| Royal, IA | 505 | 35 | 30.0% |
| City | Population | Air Quality Index | National Avg. |
| Galena, AK | 505 | 38 | 24.0% |
| Beulah Valley, CO | 505 | 20 | 60.0% |
| Sumner, GA | 505 | 40 | 20.0% |
| Cisne, IL | 505 | 43 | 14.0% |
| West College Corner, IN | 505 | 6 | 88.0% |
| Royal, IA | 505 | 35 | 30.0% |
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