| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 18°F | 37°F | 27°F | 2.8" |
| February | 18°F | 39°F | 28°F | 3.3" |
| March | 26°F | 47°F | 36°F | 4.0" |
| April | 38°F | 62°F | 49°F | 3.9" |
| May | 48°F | 72°F | 59°F | 4.0" |
| June | 57°F | 80°F | 68°F | 5.0" |
| July | 63°F | 86°F | 74°F | 4.0" |
| August | 61°F | 83°F | 72°F | 3.6" |
| September | 54°F | 77°F | 65°F | 3.7" |
| October | 43°F | 65°F | 53°F | 5.8" |
| November | 32°F | 53°F | 42°F | 3.2" |
| December | 23°F | 42°F | 32°F | 4.0" |
| Index | Leominster | Massachusetts | National |
| Air quality index | 40 | 40 | 42 |
| Pollution index | 7,556,320 | 8,399,610 | 2,726,193 |
| Measurement | Total |
| Days measured | 365 |
| Days with good air quality | 312 |
| Days with moderate air quality | 52 |
| Days w/ poor A.Q. for sensitive groups | 1 |
| Days with unhealthy air quality | 0 |
| Pollutant | Total |
| Arsenic | 0.010% |
| Benzene | 0.070% |
| Carbon Tetrachloride | 0.040% |
| Lead | 0.010% |
| Mercury | 0.040% |
| City | Population | Air Quality Index | National Avg. |
| Winchendon, MA | 4,673 | 34 | 32.0% |
| Townsend, MA | 1,276 | 37 | 26.0% |
| Pepperell, MA | 2,685 | 37 | 26.0% |
| East Pepperell, MA | 1,916 | 37 | 26.0% |
| Groton, MA | 1,127 | 37 | 26.0% |
| Leominster, MA | 41,581 | 40 | 20.0% |
| City | Population | Air Quality Index | National Avg. |
| Catonsville, MD | 41,592 | 46 | 8.0% |
| Woonsocket, RI | 41,616 | 44 | 12.0% |
| Charlottetown, PE | 41,537 | n/a | |
| Rancho Palos Verdes, CA | 41,635 | 72 | 44.0% |
| Mableton, GA | 41,647 | 52 | 4.0% |
| Leominster, MA | 41,581 | 40 | 20.0% |
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