| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 25°F | 41°F | 32°F | 3.2" |
| February | 25°F | 40°F | 32°F | 3.0" |
| March | 33°F | 52°F | 42°F | 3.2" |
| April | 44°F | 65°F | 54°F | 4.0" |
| May | 52°F | 74°F | 62°F | 4.0" |
| June | 64°F | 83°F | 73°F | 5.1" |
| July | 67°F | 86°F | 76°F | 5.1" |
| August | 67°F | 85°F | 75°F | 3.5" |
| September | 59°F | 77°F | 67°F | 5.2" |
| October | 47°F | 66°F | 55°F | 4.2" |
| November | 38°F | 55°F | 45°F | 3.2" |
| December | 28°F | 42°F | 35°F | 5.1" |
| Index | Hummelstown | Pennsylvania | National |
| Air quality index | 44 | 45 | 42 |
| Pollution index | 11,091,200 | 4,584,805 | 2,726,193 |
| Measurement | Total |
| Days measured | 365 |
| Days with good air quality | 252 |
| Days with moderate air quality | 107 |
| Days w/ poor A.Q. for sensitive groups | 5 |
| Days with unhealthy air quality | 1 |
| Pollutant | Total |
| Arsenic | 0.010% |
| Benzene | 0.140% |
| Carbon Tetrachloride | 0.030% |
| Lead | 0.310% |
| Mercury | 0.004% |
| City | Population | Air Quality Index | National Avg. |
| Elizabethtown, PA | 11,445 | 41 | 18.0% |
| Marietta, PA | 2,616 | 41 | 18.0% |
| Maytown, PA | 4,130 | 41 | 18.0% |
| Wellsville, PA | 257 | 41 | 18.0% |
| York Haven, PA | 819 | 41 | 18.0% |
| Hummelstown, PA | 4,732 | 44 | 12.0% |
| City | Population | Air Quality Index | National Avg. |
| Jackson, AL | 4,731 | 42 | 16.0% |
| Crystal City, MO | 4,733 | 54 | 8.0% |
| Bigfork, MT | 4,733 | 37 | 26.0% |
| Flin Flon (Part), MB | 4,733 | n/a | |
| Hebbronville, TX | 4,730 | 41 | 18.0% |
| Hummelstown, PA | 4,732 | 44 | 12.0% |
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