| 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 | 42 |
| Pollution index | 2,806,150 | 8,629,883 | 2,726,193 |
| 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 | Population | Air Quality Index | National Avg. |
| Brooklyn, CT | 1,128 | 33 | 34.0% |
| Storrs, CT | 16,392 | 33 | 34.0% |
| South Windham, CT | 1,475 | 33 | 34.0% |
| Coventry Lake, CT | 2,135 | 33 | 34.0% |
| Mansfield Center, CT | 1,367 | 33 | 34.0% |
| Willimantic, CT | 18,669 | 33 | 34.0% |
| City | Population | Air Quality Index | National Avg. |
| Augusta, ME | 18,662 | 28 | 44.0% |
| Cedar Mill, OR | 18,661 | 38 | 24.0% |
| Petawawa, ON | 18,660 | n/a | |
| Homewood, IL | 18,656 | 55 | 10.0% |
| Point Pleasant, NJ | 18,686 | 41 | 18.0% |
| Willimantic, CT | 18,669 | 33 | 34.0% |
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