| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 27°F | 42°F | 34°F | 3.7" |
| February | 22°F | 39°F | 30°F | 2.4" |
| March | 31°F | 51°F | 40°F | 6.5" |
| April | 41°F | 64°F | 52°F | 5.9" |
| May | 50°F | 74°F | 62°F | 3.6" |
| June | 62°F | 84°F | 73°F | 4.8" |
| July | 65°F | 87°F | 76°F | 6.4" |
| August | 66°F | 86°F | 75°F | 8.4" |
| September | 58°F | 81°F | 70°F | 3.6" |
| October | 50°F | 69°F | 59°F | 5.5" |
| November | 36°F | 55°F | 45°F | 4.8" |
| December | 26°F | 40°F | 33°F | 5.1" |
| Index | Halfway | 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% |
| Pondsville, MD | 39 | 40 | 20.0% |
| Smithsburg, MD | 2,977 | 40 | 20.0% |
| Fort Ritchie, MD | 48 | 40 | 20.0% |
| Leitersburg, MD | 386 | 40 | 20.0% |
| Halfway, MD | 10,804 | 40 | 20.0% |
| City | Population | Air Quality Index | National Avg. |
| Saint-Sauveur, QC | 10,802 | n/a | |
| Yazoo City, MS | 10,806 | 44 | 12.0% |
| Tarboro, NC | 10,806 | 36 | 28.0% |
| Saint-Raymond, QC | 10,809 | n/a | |
| Totowa, NJ | 10,810 | 51 | 2.0% |
| Halfway, MD | 10,804 | 40 | 20.0% |
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