| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 21°F | 41°F | 31°F | 2.3" |
| February | 21°F | 42°F | 31°F | 2.6" |
| March | 30°F | 54°F | 41°F | 3.0" |
| April | 40°F | 66°F | 53°F | 3.5" |
| May | 50°F | 75°F | 62°F | 4.2" |
| June | 60°F | 82°F | 71°F | 3.7" |
| July | 63°F | 86°F | 75°F | 4.6" |
| August | 61°F | 84°F | 72°F | 4.7" |
| September | 54°F | 78°F | 65°F | 5.1" |
| October | 44°F | 67°F | 55°F | 4.8" |
| November | 33°F | 56°F | 44°F | 2.7" |
| December | 27°F | 46°F | 36°F | 3.3" |
| Index | Westminster | Maryland | National |
| Air quality index | 42 | 43 | 42 |
| Pollution index | 5,553,870 | 5,342,367 | 2,726,193 |
| Measurement | Total |
| Days measured | 365 |
| Days with good air quality | 288 |
| Days with moderate air quality | 77 |
| Days w/ poor A.Q. for sensitive groups | 0 |
| Days with unhealthy air quality | 0 |
| Pollutant | Total |
| Arsenic | 0.004% |
| Benzene | 0.070% |
| Carbon Tetrachloride | 0.010% |
| Lead | 0.410% |
| Mercury | 0.070% |
| City | Population | Air Quality Index | National Avg. |
| New Market, MD | 1,527 | 40 | 20.0% |
| Glen Rock, PA | 2,332 | 41 | 18.0% |
| Middleburg, MD | 0 | 42 | 16.0% |
| Littlestown, PA | 4,494 | 42 | 16.0% |
| Manchester, MD | 4,752 | 42 | 16.0% |
| Westminster, MD | 18,649 | 42 | 16.0% |
| City | Population | Air Quality Index | National Avg. |
| Central Point, OR | 18,648 | 36 | 28.0% |
| Deux-Montagnes, QC | 18,650 | n/a | |
| Homewood, IL | 18,656 | 55 | 10.0% |
| Petawawa, ON | 18,660 | n/a | |
| Cedar Mill, OR | 18,661 | 38 | 24.0% |
| Westminster, MD | 18,649 | 42 | 16.0% |
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