| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 20°F | 37°F | 28°F | 2.1" |
| February | 21°F | 38°F | 29°F | 2.0" |
| March | 34°F | 54°F | 43°F | 3.0" |
| April | 44°F | 66°F | 55°F | 4.3" |
| May | 54°F | 74°F | 63°F | 3.7" |
| June | 64°F | 84°F | 74°F | 6.0" |
| July | 66°F | 86°F | 76°F | 4.5" |
| August | 66°F | 86°F | 75°F | 4.2" |
| September | 57°F | 79°F | 67°F | 4.3" |
| October | 45°F | 64°F | 54°F | 3.9" |
| November | 36°F | 54°F | 44°F | 2.7" |
| December | 23°F | 38°F | 29°F | 2.4" |
| Index | Hannibal | Missouri | National |
| Air quality index | 41 | 97 | 42 |
| Pollution index | 757,226 | 1,630,255 | 2,726,193 |
| Measurement | Total |
| Days measured | 241 |
| Days with good air quality | 208 |
| Days with moderate air quality | 33 |
| Days w/ poor A.Q. for sensitive groups | 0 |
| Days with unhealthy air quality | 0 |
| Pollutant | Total |
| Arsenic | 0.001% |
| Benzene | 0.010% |
| Carbon Tetrachloride | 0.001% |
| Lead | 0.000% |
| Mercury | 0.000% |
| City | Population | Air Quality Index | National Avg. |
| Payson, IL | 1,398 | 41 | 18.0% |
| New London, MO | 915 | 41 | 18.0% |
| Plainville, IL | 285 | 41 | 18.0% |
| Palmyra, MO | 3,606 | 41 | 18.0% |
| Hull, IL | 390 | 41 | 18.0% |
| Hannibal, MO | 17,312 | 41 | 18.0% |
| City | Population | Air Quality Index | National Avg. |
| Shorewood, IL | 17,303 | 0 | 100.0% |
| Ocean Acres, NJ | 17,325 | 37 | 26.0% |
| Brenham, TX | 17,297 | 52 | 4.0% |
| Estelle, LA | 17,330 | 45 | 10.0% |
| Taylor, TX | 17,291 | 44 | 12.0% |
| Hannibal, MO | 17,312 | 41 | 18.0% |
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