| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 27°F | 48°F | 37°F | 3.2" |
| February | 30°F | 51°F | 39°F | 2.6" |
| March | 36°F | 58°F | 46°F | 3.5" |
| April | 45°F | 70°F | 57°F | 3.5" |
| May | 55°F | 77°F | 66°F | 3.5" |
| June | 63°F | 86°F | 74°F | 5.0" |
| July | 70°F | 90°F | 79°F | 5.6" |
| August | 66°F | 87°F | 76°F | 7.4" |
| September | 61°F | 82°F | 71°F | 4.9" |
| October | 50°F | 72°F | 60°F | 5.2" |
| November | 37°F | 61°F | 49°F | 1.8" |
| December | 33°F | 54°F | 43°F | 2.9" |
| Index | Mount Vernon | Maryland | National |
| Air quality index | 41 | 43 | 42 |
| Pollution index | 2,538,160 | 5,342,367 | 2,726,193 |
| Measurement | Total |
| Days measured | 365 |
| Days with good air quality | 304 |
| Days with moderate air quality | 58 |
| Days w/ poor A.Q. for sensitive groups | 3 |
| Days with unhealthy air quality | 0 |
| Pollutant | Total |
| Arsenic | 0.002% |
| Benzene | 0.030% |
| Carbon Tetrachloride | 0.004% |
| Lead | 0.010% |
| Mercury | 0.003% |
| City | Population | Air Quality Index | National Avg. |
| Delmar, DE | 2,153 | 36 | 28.0% |
| Delmar, MD | 3,321 | 36 | 28.0% |
| Allen, MD | 419 | 36 | 28.0% |
| Fruitland, MD | 5,281 | 36 | 28.0% |
| Princess Anne, MD | 3,525 | 36 | 28.0% |
| Mount Vernon, MD | 1,016 | 41 | 18.0% |
| City | Population | Air Quality Index | National Avg. |
| Merrill, IA | 1,016 | 35 | 30.0% |
| Burrton, KS | 1,016 | 40 | 20.0% |
| New Franklin, MO | 1,016 | 38 | 24.0% |
| Greene, RI | 1,016 | 44 | 12.0% |
| Hewitt, WI | 1,016 | 34 | 32.0% |
| Mount Vernon, MD | 1,016 | 41 | 18.0% |
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