| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 25°F | 41°F | 32°F | 2.2" |
| February | 25°F | 43°F | 34°F | 1.8" |
| March | 34°F | 55°F | 44°F | 2.3" |
| April | 44°F | 67°F | 55°F | 3.3" |
| May | 54°F | 76°F | 64°F | 3.9" |
| June | 63°F | 84°F | 73°F | 4.0" |
| July | 67°F | 88°F | 77°F | 3.3" |
| August | 65°F | 85°F | 75°F | 2.8" |
| September | 58°F | 79°F | 68°F | 3.2" |
| October | 48°F | 67°F | 57°F | 3.6" |
| November | 37°F | 55°F | 45°F | 2.1" |
| December | 30°F | 46°F | 37°F | 3.0" |
| Index | Charlton | Maryland | National |
| Air quality index | 40 | 43 | 42 |
| Pollution index | 16,230,700 | 5,342,367 | 2,726,193 |
| Measurement | Total |
| Days measured | 362 |
| Days with good air quality | 306 |
| Days with moderate air quality | 54 |
| Days w/ poor A.Q. for sensitive groups | 2 |
| Days with unhealthy air quality | 0 |
| Pollutant | Total |
| Arsenic | 0.010% |
| Benzene | 0.130% |
| Carbon Tetrachloride | 0.090% |
| Lead | 0.080% |
| Mercury | 0.020% |
| City | Population | Air Quality Index | National Avg. |
| Mercersburg, PA | 1,673 | 36 | 28.0% |
| Needmore, PA | 92 | 36 | 28.0% |
| Eakles Mill, MD | 0 | 40 | 20.0% |
| Sharpsburg, MD | 708 | 40 | 20.0% |
| Clear Spring, MD | 486 | 40 | 20.0% |
| Charlton, MD | 380 | 40 | 20.0% |
| City | Population | Air Quality Index | National Avg. |
| Nances Creek, AL | 380 | 43 | 14.0% |
| Midway town, AR | 380 | 36 | 28.0% |
| Cane Beds, AZ | 380 | 44 | 12.0% |
| El Moro, CO | 380 | 20 | 60.0% |
| Good Hope, GA | 380 | 44 | 12.0% |
| Charlton, MD | 380 | 40 | 20.0% |
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