| Month | Temp. (min) | Temp. (max) | Temp. (avg) | Precipitation |
| January | 19°F | 39°F | 29°F | 2.7" |
| February | 19°F | 40°F | 29°F | 3.0" |
| March | 28°F | 49°F | 38°F | 3.5" |
| April | 38°F | 62°F | 50°F | 4.0" |
| May | 48°F | 73°F | 60°F | 3.3" |
| June | 57°F | 79°F | 68°F | 5.0" |
| July | 63°F | 85°F | 74°F | 3.9" |
| August | 60°F | 82°F | 71°F | 4.1" |
| September | 54°F | 77°F | 65°F | 3.7" |
| October | 43°F | 66°F | 54°F | 4.4" |
| November | 33°F | 54°F | 43°F | 3.4" |
| December | 26°F | 44°F | 34°F | 4.0" |
| Index | Willimantic | Connecticut | National |
| Air quality index | 33 | 43 | 50 |
| Pollution index | 2,806,150 | 7,127,531 | 5,066,862 |
| Measurement | Total |
| Days measured | 334 |
| Days with good air quality | 319 |
| Days with moderate air quality | 14 |
| Days w/ poor A.Q. for sensitive groups | 1 |
| Days with unhealthy air quality |
| Pollutant | Total |
| Arsenic | 0.004% |
| Benzene | 0.030% |
| Carbon Tetrachloride | 0.010% |
| Lead | 0.040% |
| Mercury | 0.010% |
| City, State | Population | Air Quality Index | National Avg. |
| Mansfield Center, CT | 969 | 33 | 34% |
| Colchester, CT | 4,475 | 33 | 34% |
| South Woodstock, CT | 1,246 | 33 | 34% |
| South Coventry, CT | 1,142 | 33 | 34% |
| Brooklyn, CT | 906 | 33 | 34% |
| Willimantic, CT | 17,339 | 33 | 34% |
| City, State | Population | Air Quality Index | National Avg. |
| Huntington, IN | 17,340 | 31 | 38% |
| Bristol, VA | 17,340 | 45 | 10% |
| Beckley, WV | 17,348 | 295 | 490% |
| Anoka, MN | 17,325 | 46 | 8% |
| Marco Island, FL | 17,361 | 34 | 32% |
| Willimantic, CT | 17,339 | 33 | 34% |
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