| 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 | Colchester | 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. |
| Willimantic, CT | 18,669 | 33 | 34.0% |
| South Windham, CT | 1,475 | 33 | 34.0% |
| Mansfield Center, CT | 1,367 | 33 | 34.0% |
| Coventry Lake, CT | 2,135 | 33 | 34.0% |
| South Coventry, CT | 1,413 | 33 | 34.0% |
| Colchester, CT | 5,147 | 33 | 34.0% |
| City | Population | Air Quality Index | National Avg. |
| Lanoraie, QC | 5,148 | n/a | |
| Johnstown, OH | 5,149 | 46 | 8.0% |
| Sterling, AK | 5,150 | 31 | 38.0% |
| Myrtletown, CA | 5,143 | 27 | 46.0% |
| Philomath, OR | 5,143 | 17 | 66.0% |
| Colchester, CT | 5,147 | 33 | 34.0% |
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